Spring Cloud provides tools for developers to quickly build some of the common patterns in distributed systems (e.g. configuration management, service discovery, circuit breakers, intelligent routing, micro-proxy, control bus). Coordination of distributed systems leads to boiler plate patterns, and using Spring Cloud developers can quickly stand up services and applications that implement those patterns. They will work well in any distributed environment, including the developer’s own laptop, bare metal data centres, and managed platforms such as Cloud Foundry.

Release Train Version: 2020.0.0-SNAPSHOT

Supported Boot Version: 2.4.1

1. Features

Spring Cloud focuses on providing good out of box experience for typical use cases and extensibility mechanism to cover others.

  • Distributed/versioned configuration

  • Service registration and discovery

  • Routing

  • Service-to-service calls

  • Load balancing

  • Circuit Breakers

  • Distributed messaging

2. Release Train Versions

Table 1. Release Train Project Versions
Project Name Project Version

spring-boot

2.4.1

spring-cloud-build

3.0.0-SNAPSHOT

spring-cloud-bus

3.0.0-SNAPSHOT

spring-cloud-circuitbreaker

2.0.0-SNAPSHOT

spring-cloud-cli

3.0.0-SNAPSHOT

spring-cloud-cloudfoundry

3.0.0-SNAPSHOT

spring-cloud-commons

3.0.0-SNAPSHOT

spring-cloud-config

3.0.0-SNAPSHOT

spring-cloud-consul

3.0.0-SNAPSHOT

spring-cloud-contract

3.0.0-SNAPSHOT

spring-cloud-function

3.1.0-SNAPSHOT

spring-cloud-gateway

3.0.0-SNAPSHOT

spring-cloud-kubernetes

2.0.0-SNAPSHOT

spring-cloud-netflix

3.0.0-SNAPSHOT

spring-cloud-openfeign

3.0.0-SNAPSHOT

spring-cloud-security

3.0.0-SNAPSHOT

spring-cloud-sleuth

3.0.0-SNAPSHOT

spring-cloud-stream

3.1.0-SNAPSHOT

spring-cloud-task

2.3.0-SNAPSHOT

spring-cloud-vault

3.0.0-SNAPSHOT

spring-cloud-zookeeper

3.0.0-SNAPSHOT

Spring Cloud Build

spring cloud build

Spring Cloud Build is a common utility project for Spring Cloud to use for plugin and dependency management.

3. Building and Deploying

To install locally:

$ mvn install -s .settings.xml

and to deploy snapshots to repo.spring.io:

$ mvn deploy -DaltSnapshotDeploymentRepository=repo.spring.io::default::https://repo.spring.io/snapshot

for a RELEASE build use

$ mvn deploy -DaltReleaseDeploymentRepository=repo.spring.io::default::https://repo.spring.io/release

and for jcenter use

$ mvn deploy -DaltReleaseDeploymentRepository=bintray::default::https://api.bintray.com/maven/spring/jars/org.springframework.cloud:build

and for Maven Central use

$ mvn deploy -P central -DaltReleaseDeploymentRepository=sonatype-nexus-staging::default::https://oss.sonatype.org/service/local/staging/deploy/maven2

(the "central" profile is available for all projects in Spring Cloud and it sets up the gpg jar signing, and the repository has to be specified separately for this project because it is a parent of the starter parent which users in turn have as their own parent).

4. Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.

4.1. Sign the Contributor License Agreement

Before we accept a non-trivial patch or pull request we will need you to sign the Contributor License Agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.

4.2. Code of Conduct

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to [email protected].

4.3. Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.

  • Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the eclipse-code-formatter.xml file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file.

  • Make sure all new .java files to have a simple Javadoc class comment with at least an @author tag identifying you, and preferably at least a paragraph on what the class is for.

  • Add the ASF license header comment to all new .java files (copy from existing files in the project)

  • Add yourself as an @author to the .java files that you modify substantially (more than cosmetic changes).

  • Add some Javadocs and, if you change the namespace, some XSD doc elements.

  • A few unit tests would help a lot as well — someone has to do it.

  • If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).

  • When writing a commit message please follow these conventions, if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).

4.4. Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in the spring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            └── checkstyle.xml (1)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules

4.4.1. Checkstyle configuration

Checkstyle rules are disabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml
<properties>
<maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError> (1)
        <maven-checkstyle-plugin.failsOnViolation>true
        </maven-checkstyle-plugin.failsOnViolation> (2)
        <maven-checkstyle-plugin.includeTestSourceDirectory>true
        </maven-checkstyle-plugin.includeTestSourceDirectory> (3)
</properties>

<build>
        <plugins>
            <plugin> (4)
                <groupId>io.spring.javaformat</groupId>
                <artifactId>spring-javaformat-maven-plugin</artifactId>
            </plugin>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>

    <reporting>
        <plugins>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>
    </reporting>
</build>
1 Fails the build upon Checkstyle errors
2 Fails the build upon Checkstyle violations
3 Checkstyle analyzes also the test sources
4 Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules
5 Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under ${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml
<?xml version="1.0"?>
<!DOCTYPE suppressions PUBLIC
        "-//Puppy Crawl//DTD Suppressions 1.1//EN"
        "https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
<suppressions>
    <suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/>
    <suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/>
</suppressions>

It’s advisable to copy the ${spring-cloud-build.rootFolder}/.editorconfig and ${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig
$ touch .springformat

4.5. IDE setup

4.5.1. Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin. The following files can be found in the Spring Cloud Build project.

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            ├── checkstyle.xml (1)
            └── intellij
                ├── Intellij_Project_Defaults.xml (4)
                └── Intellij_Spring_Boot_Java_Conventions.xml (5)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules
4 Project defaults for Intellij that apply most of Checkstyle rules
5 Project style conventions for Intellij that apply most of Checkstyle rules
Code style
Figure 1. Code style

Go to FileSettingsEditorCode style. There click on the icon next to the Scheme section. There, click on the Import Scheme value and pick the Intellij IDEA code style XML option. Import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Code style
Figure 2. Inspection profiles

Go to FileSettingsEditorInspections. There click on the icon next to the Profile section. There, click on the Import Profile and import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install the Checkstyle plugin. It’s advisable to also install the Assertions2Assertj to automatically convert the JUnit assertions

Checkstyle

Go to FileSettingsOther settingsCheckstyle. There click on the + icon in the Configuration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for the checkstyle.xml : raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

Remember to set the Scan Scope to All sources since we apply checkstyle rules for production and test sources.

5. Flattening the POMs

To avoid propagating build setup that is required to build a Spring Cloud project, we’re using the maven flatten plugin. It has the advantage of letting you use whatever features you need while publishing "clean" pom to the repository.

In order to add it, add the org.codehaus.mojo:flatten-maven-plugin to your pom.xml.

<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>flatten-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

6. Reusing the documentation

Spring Cloud Build publishes its spring-cloud-build-docs module that contains helpful scripts (e.g. README generation ruby script) and css, xslt and images for the Spring Cloud documentation. If you want to follow the same convention approach of generating documentation just add these plugins to your docs module

<properties>
    <upload-docs-zip.phase>deploy</upload-docs-zip.phase> (8)
</properties>
<profiles>
    <profile>
        <id>docs</id>
        <build>
            <plugins>
                <plugin>
                    <groupId>pl.project13.maven</groupId>
                    <artifactId>git-commit-id-plugin</artifactId> (1)
                </plugin>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-dependency-plugin</artifactId> (2)
                </plugin>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-resources-plugin</artifactId> (3)
                </plugin>
                <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>exec-maven-plugin</artifactId> (4)
                </plugin>
                <plugin>
                    <groupId>org.asciidoctor</groupId>
                    <artifactId>asciidoctor-maven-plugin</artifactId> (5)
                </plugin>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-antrun-plugin</artifactId> (6)
                </plugin>
                <plugin>
                    <artifactId>maven-deploy-plugin</artifactId> (7)
                </plugin>
            </plugins>
        </build>
    </profile>
</profiles>
1 This plugin downloads sets up all the git information of the project
2 This plugin downloads the resources of the spring-cloud-build-docs module
3 This plugin unpacks the resources of the spring-cloud-build-docs module
4 This plugin generates an adoc file with all the configuration properties from the classpath
5 This plugin is required to parse the Asciidoctor documentation
6 This plugin is required to copy resources into proper final destinations and to generate main README.adoc and to assert that no files use unresolved links
7 This plugin ensures that the generated zip docs will get published
8 This property turns on the "deploy" phase for <7>
The order of plugin declaration is important!

In order for the build to generate the adoc file with all your configuration properties, your docs module should contain all the dependencies on the classpath, that you would want to scan for configuration properties. The file will be output to ${docsModule}/src/main/asciidoc/_configprops.adoc file (configurable via the configprops.path property).

If you want to modify which of the configuration properties are put in the table, you can tweak the configprops.inclusionPattern pattern to include only a subset of the properties (e.g. <configprops.inclusionPattern>spring.sleuth.*</configprops.inclusionPattern>).

Spring Cloud Build Docs comes with a set of attributes for asciidoctor that you can reuse.

<attributes>
    <docinfo>shared</docinfo>
    <allow-uri-read>true</allow-uri-read>
    <nofooter/>
    <toc>left</toc>
    <toc-levels>4</toc-levels>
    <sectlinks>true</sectlinks>
    <sources-root>${project.basedir}/[email protected]</sources-root>
    <asciidoc-sources-root>${project.basedir}/src/main/[email protected]</asciidoc-sources-root>
    <generated-resources-root>${project.basedir}/target/[email protected]
    </generated-resources-root>
    <!-- Use this attribute the reference code from another module -->
    <!-- Note the @ at the end, lowering the precedence of the attribute -->
    <project-root>${maven.multiModuleProjectDirectory}@</project-root>
    <!-- It's mandatory for you to pass the docs.main property -->
    <github-repo>${docs.main}@</github-repo>
    <github-project>https://github.com/spring-cloud/${docs.main}@</github-project>
    <github-raw>
        https://raw.githubusercontent.com/spring-cloud/${docs.main}/${github-tag}@
    </github-raw>
    <github-code>https://github.com/spring-cloud/${docs.main}/tree/${github-tag}@
    </github-code>
    <github-issues>https://github.com/spring-cloud/${docs.main}/issues/@</github-issues>
    <github-wiki>https://github.com/spring-cloud/${docs.main}/[email protected]</github-wiki>
    <github-master-code>https://github.com/spring-cloud/${docs.main}/tree/[email protected]
    </github-master-code>
    <index-link>${index-link}@</index-link>

    <!-- Spring Cloud specific -->
    <!-- for backward compatibility -->
    <spring-cloud-version>${project.version}@</spring-cloud-version>
    <project-version>${project.version}@</project-version>
    <github-tag>${github-tag}@</github-tag>
    <version-type>${version-type}@</version-type>
    <docs-url>https://docs.spring.io/${docs.main}/docs/${project.version}@</docs-url>
    <raw-docs-url>${github-raw}@</raw-docs-url>
    <project-version>${project.version}@</project-version>
    <project-name>${docs.main}@</project-name>
</attributes>

7. Updating the guides

We assume that your project contains guides under the guides folder.

.
└── guides
    ├── gs-guide1
    ├── gs-guide2
    └── gs-guide3

This means that the project contains 3 guides that would correspond to the following guides in Spring Guides org.

If you deploy your project with the -Pguides profile like this

$ ./mvnw clean deploy -Pguides

what will happen is that for GA project versions, we will clone gs-guide1, gs-guide2 and gs-guide3 and update their contents with the ones being under your guides project.

You can skip this by either not adding the guides profile, or passing the -DskipGuides system property when the profile is turned on.

You can configure the project version passed to guides via the guides-project.version (defaults to ${project.version}). The phase at which guides get updated can be configured by guides-update.phase (defaults to deploy).

Spring Cloud Bus

Spring Cloud Bus links the nodes of a distributed system with a lightweight message broker. This broker can then be used to broadcast state changes (such as configuration changes) or other management instructions. A key idea is that the bus is like a distributed actuator for a Spring Boot application that is scaled out. However, it can also be used as a communication channel between apps. This project provides starters for either an AMQP broker or Kafka as the transport.

Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, please find the source code and issue trackers in the project at github.

8. Quick Start

Spring Cloud Bus works by adding Spring Boot autconfiguration if it detects itself on the classpath. To enable the bus, add spring-cloud-starter-bus-amqp or spring-cloud-starter-bus-kafka to your dependency management. Spring Cloud takes care of the rest. Make sure the broker (RabbitMQ or Kafka) is available and configured. When running on localhost, you need not do anything. If you run remotely, use Spring Cloud Connectors or Spring Boot conventions to define the broker credentials, as shown in the following example for Rabbit:

application.yml
spring:
  rabbitmq:
    host: mybroker.com
    port: 5672
    username: user
    password: secret

The bus currently supports sending messages to all nodes listening or all nodes for a particular service (as defined by Eureka). The /bus/* actuator namespace has some HTTP endpoints. Currently, two are implemented. The first, /bus/env, sends key/value pairs to update each node’s Spring Environment. The second, /bus/refresh, reloads each application’s configuration, as though they had all been pinged on their /refresh endpoint.

The Spring Cloud Bus starters cover Rabbit and Kafka, because those are the two most common implementations. However, Spring Cloud Stream is quite flexible, and the binder works with spring-cloud-bus.

9. Bus Endpoints

Spring Cloud Bus provides two endpoints, /actuator/busrefresh and /actuator/busenv that correspond to individual actuator endpoints in Spring Cloud Commons, /actuator/refresh and /actuator/env respectively.

9.1. Bus Refresh Endpoint

The /actuator/busrefresh endpoint clears the RefreshScope cache and rebinds @ConfigurationProperties. See the Refresh Scope documentation for more information.

To expose the /actuator/busrefresh endpoint, you need to add following configuration to your application:

management.endpoints.web.exposure.include=busrefresh

9.2. Bus Env Endpoint

The /actuator/busenv endpoint updates each instances environment with the specified key/value pair across multiple instances.

To expose the /actuator/busenv endpoint, you need to add following configuration to your application:

management.endpoints.web.exposure.include=busenv

The /actuator/busenv endpoint accepts POST requests with the following shape:

{
    "name": "key1",
    "value": "value1"
}

10. Addressing an Instance

Each instance of the application has a service ID, whose value can be set with spring.cloud.bus.id and whose value is expected to be a colon-separated list of identifiers, in order from least specific to most specific. The default value is constructed from the environment as a combination of the spring.application.name and server.port (or spring.application.index, if set). The default value of the ID is constructed in the form of app:index:id, where:

  • app is the vcap.application.name, if it exists, or spring.application.name

  • index is the vcap.application.instance_index, if it exists, spring.application.index, local.server.port, server.port, or 0 (in that order).

  • id is the vcap.application.instance_id, if it exists, or a random value.

The HTTP endpoints accept a “destination” path parameter, such as /busrefresh/customers:9000, where destination is a service ID. If the ID is owned by an instance on the bus, it processes the message, and all other instances ignore it.

11. Addressing All Instances of a Service

The “destination” parameter is used in a Spring PathMatcher (with the path separator as a colon — :) to determine if an instance processes the message. Using the example from earlier, /busenv/customers:** targets all instances of the “customers” service regardless of the rest of the service ID.

12. Service ID Must Be Unique

The bus tries twice to eliminate processing an event — once from the original ApplicationEvent and once from the queue. To do so, it checks the sending service ID against the current service ID. If multiple instances of a service have the same ID, events are not processed. When running on a local machine, each service is on a different port, and that port is part of the ID. Cloud Foundry supplies an index to differentiate. To ensure that the ID is unique outside Cloud Foundry, set spring.application.index to something unique for each instance of a service.

13. Customizing the Message Broker

Spring Cloud Bus uses Spring Cloud Stream to broadcast the messages. So, to get messages to flow, you need only include the binder implementation of your choice in the classpath. There are convenient starters for the bus with AMQP (RabbitMQ) and Kafka (spring-cloud-starter-bus-[amqp|kafka]). Generally speaking, Spring Cloud Stream relies on Spring Boot autoconfiguration conventions for configuring middleware. For instance, the AMQP broker address can be changed with spring.rabbitmq.* configuration properties. Spring Cloud Bus has a handful of native configuration properties in spring.cloud.bus.* (for example, spring.cloud.bus.destination is the name of the topic to use as the external middleware). Normally, the defaults suffice.

To learn more about how to customize the message broker settings, consult the Spring Cloud Stream documentation.

14. Tracing Bus Events

Bus events (subclasses of RemoteApplicationEvent) can be traced by setting spring.cloud.bus.trace.enabled=true. If you do so, the Spring Boot TraceRepository (if it is present) shows each event sent and all the acks from each service instance. The following example comes from the /trace endpoint:

{
  "timestamp": "2015-11-26T10:24:44.411+0000",
  "info": {
    "signal": "spring.cloud.bus.ack",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "stores:8081",
    "destination": "*:**"
  }
  },
  {
  "timestamp": "2015-11-26T10:24:41.864+0000",
  "info": {
    "signal": "spring.cloud.bus.sent",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "customers:9000",
    "destination": "*:**"
  }
  },
  {
  "timestamp": "2015-11-26T10:24:41.862+0000",
  "info": {
    "signal": "spring.cloud.bus.ack",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "customers:9000",
    "destination": "*:**"
  }
}

The preceding trace shows that a RefreshRemoteApplicationEvent was sent from customers:9000, broadcast to all services, and received (acked) by customers:9000 and stores:8081.

To handle the ack signals yourself, you could add an @EventListener for the AckRemoteApplicationEvent and SentApplicationEvent types to your app (and enable tracing). Alternatively, you could tap into the TraceRepository and mine the data from there.

Any Bus application can trace acks. However, sometimes, it is useful to do this in a central service that can do more complex queries on the data or forward it to a specialized tracing service.

15. Broadcasting Your Own Events

The Bus can carry any event of type RemoteApplicationEvent. The default transport is JSON, and the deserializer needs to know which types are going to be used ahead of time. To register a new type, you must put it in a subpackage of org.springframework.cloud.bus.event.

To customise the event name, you can use @JsonTypeName on your custom class or rely on the default strategy, which is to use the simple name of the class.

Both the producer and the consumer need access to the class definition.

15.1. Registering events in custom packages

If you cannot or do not want to use a subpackage of org.springframework.cloud.bus.event for your custom events, you must specify which packages to scan for events of type RemoteApplicationEvent by using the @RemoteApplicationEventScan annotation. Packages specified with @RemoteApplicationEventScan include subpackages.

For example, consider the following custom event, called MyEvent:

package com.acme;

public class MyEvent extends RemoteApplicationEvent {
    ...
}

You can register that event with the deserializer in the following way:

package com.acme;

@Configuration
@RemoteApplicationEventScan
public class BusConfiguration {
    ...
}

Without specifying a value, the package of the class where @RemoteApplicationEventScan is used is registered. In this example, com.acme is registered by using the package of BusConfiguration.

You can also explicitly specify the packages to scan by using the value, basePackages or basePackageClasses properties on @RemoteApplicationEventScan, as shown in the following example:

package com.acme;

@Configuration
//@RemoteApplicationEventScan({"com.acme", "foo.bar"})
//@RemoteApplicationEventScan(basePackages = {"com.acme", "foo.bar", "fizz.buzz"})
@RemoteApplicationEventScan(basePackageClasses = BusConfiguration.class)
public class BusConfiguration {
    ...
}

All of the preceding examples of @RemoteApplicationEventScan are equivalent, in that the com.acme package is registered by explicitly specifying the packages on @RemoteApplicationEventScan.

You can specify multiple base packages to scan.

16. Configuration properties

To see the list of all Bus related configuration properties please check the Appendix page.

Spring Cloud Circuit Breaker

2020.0.0-SNAPSHOT

.1. Configuring Resilience4J Circuit Breakers

.1.1. Starters

There are two starters for the Resilience4J implementations, one for reactive applications and one for non-reactive applications.

  • org.springframework.cloud:spring-cloud-starter-circuitbreaker-resilience4j - non-reactive applications

  • org.springframework.cloud:spring-cloud-starter-circuitbreaker-reactor-resilience4j - reactive applications

.1.2. Auto-Configuration

You can disable the Resilience4J auto-configuration by setting spring.cloud.circuitbreaker.resilience4j.enabled to false.

.1.3. Default Configuration

To provide a default configuration for all of your circuit breakers create a Customize bean that is passed a Resilience4JCircuitBreakerFactory or ReactiveResilience4JCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> defaultCustomizer() {
    return factory -> factory.configureDefault(id -> new Resilience4JConfigBuilder(id)
            .timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(4)).build())
            .circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
            .build());
}
Reactive Example
@Bean
public Customizer<ReactiveResilience4JCircuitBreakerFactory> defaultCustomizer() {
    return factory -> factory.configureDefault(id -> new Resilience4JConfigBuilder(id)
            .circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
            .timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(4)).build()).build());
}

.1.4. Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customize bean this is passed a Resilience4JCircuitBreakerFactory or ReactiveResilience4JCircuitBreakerFactory.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> slowCustomizer() {
    return factory -> factory.configure(builder -> builder.circuitBreakerConfig(CircuitBreakerConfig.ofDefaults())
            .timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(2)).build()), "slow");
}

In addition to configuring the circuit breaker that is created you can also customize the circuit breaker after it has been created but before it is returned to the caller. To do this you can use the addCircuitBreakerCustomizer method. This can be useful for adding event handlers to Resilience4J circuit breakers.

@Bean
public Customizer<Resilience4JCircuitBreakerFactory> slowCustomizer() {
    return factory -> factory.addCircuitBreakerCustomizer(circuitBreaker -> circuitBreaker.getEventPublisher()
    .onError(normalFluxErrorConsumer).onSuccess(normalFluxSuccessConsumer), "normalflux");
}
Reactive Example
@Bean
public Customizer<ReactiveResilience4JCircuitBreakerFactory> slowCustomizer() {
    return factory -> {
        factory.configure(builder -> builder
        .timeLimiterConfig(TimeLimiterConfig.custom().timeoutDuration(Duration.ofSeconds(2)).build())
        .circuitBreakerConfig(CircuitBreakerConfig.ofDefaults()), "slow", "slowflux");
        factory.addCircuitBreakerCustomizer(circuitBreaker -> circuitBreaker.getEventPublisher()
            .onError(normalFluxErrorConsumer).onSuccess(normalFluxSuccessConsumer), "normalflux");
     };
}

.1.5. Collecting Metrics

Spring Cloud Circuit Breaker Resilience4j includes auto-configuration to setup metrics collection as long as the right dependencies are on the classpath. To enable metric collection you must include org.springframework.boot:spring-boot-starter-actuator, and io.github.resilience4j:resilience4j-micrometer. For more information on the metrics that get produced when these dependencies are present, see the Resilience4j documentation.

You don’t have to include micrometer-core directly as it is brought in by spring-boot-starter-actuator

.2. Configuring Spring Retry Circuit Breakers

Spring Retry provides declarative retry support for Spring applications. A subset of the project includes the ability to implement circuit breaker functionality. Spring Retry provides a circuit breaker implementation via a combination of it’s CircuitBreakerRetryPolicy and a stateful retry. All circuit breakers created using Spring Retry will be created using the CircuitBreakerRetryPolicy and a DefaultRetryState. Both of these classes can be configured using SpringRetryConfigBuilder.

.2.1. Default Configuration

To provide a default configuration for all of your circuit breakers create a Customize bean that is passed a SpringRetryCircuitBreakerFactory. The configureDefault method can be used to provide a default configuration.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> defaultCustomizer() {
    return factory -> factory.configureDefault(id -> new SpringRetryConfigBuilder(id)
        .retryPolicy(new TimeoutRetryPolicy()).build());
}

.2.2. Specific Circuit Breaker Configuration

Similarly to providing a default configuration, you can create a Customize bean this is passed a SpringRetryCircuitBreakerFactory.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> slowCustomizer() {
    return factory -> factory.configure(builder -> builder.retryPolicy(new SimpleRetryPolicy(1)).build(), "slow");
}

In addition to configuring the circuit breaker that is created you can also customize the circuit breaker after it has been created but before it is returned to the caller. To do this you can use the addRetryTemplateCustomizers method. This can be useful for adding event handlers to the RetryTemplate.

@Bean
public Customizer<SpringRetryCircuitBreakerFactory> slowCustomizer() {
    return factory -> factory.addRetryTemplateCustomizers(retryTemplate -> retryTemplate.registerListener(new RetryListener() {

        @Override
        public <T, E extends Throwable> boolean open(RetryContext context, RetryCallback<T, E> callback) {
            return false;
        }

        @Override
        public <T, E extends Throwable> void close(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {

        }

        @Override
        public <T, E extends Throwable> void onError(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {

        }
    }));
}

17. Building

17.1. Basic Compile and Test

To build the source you will need to install JDK 1.8.

Spring Cloud uses Maven for most build-related activities, and you should be able to get off the ground quite quickly by cloning the project you are interested in and typing

$ ./mvnw install
You can also install Maven (>=3.3.3) yourself and run the mvn command in place of ./mvnw in the examples below. If you do that you also might need to add -P spring if your local Maven settings do not contain repository declarations for spring pre-release artifacts.
Be aware that you might need to increase the amount of memory available to Maven by setting a MAVEN_OPTS environment variable with a value like -Xmx512m -XX:MaxPermSize=128m. We try to cover this in the .mvn configuration, so if you find you have to do it to make a build succeed, please raise a ticket to get the settings added to source control.

For hints on how to build the project look in .travis.yml if there is one. There should be a "script" and maybe "install" command. Also look at the "services" section to see if any services need to be running locally (e.g. mongo or rabbit). Ignore the git-related bits that you might find in "before_install" since they’re related to setting git credentials and you already have those.

The projects that require middleware generally include a docker-compose.yml, so consider using Docker Compose to run the middeware servers in Docker containers. See the README in the scripts demo repository for specific instructions about the common cases of mongo, rabbit and redis.

If all else fails, build with the command from .travis.yml (usually ./mvnw install).

17.2. Documentation

The spring-cloud-build module has a "docs" profile, and if you switch that on it will try to build asciidoc sources from src/main/asciidoc. As part of that process it will look for a README.adoc and process it by loading all the includes, but not parsing or rendering it, just copying it to ${main.basedir} (defaults to $/opt/jenkins/data/workspace/spring-cloud-release-train-docs-master-ci/train-docs/target/unpacked-docs, i.e. the root of the project). If there are any changes in the README it will then show up after a Maven build as a modified file in the correct place. Just commit it and push the change.

17.3. Working with the code

If you don’t have an IDE preference we would recommend that you use Spring Tools Suite or Eclipse when working with the code. We use the m2eclipse eclipse plugin for maven support. Other IDEs and tools should also work without issue as long as they use Maven 3.3.3 or better.

17.3.1. Activate the Spring Maven profile

Spring Cloud projects require the 'spring' Maven profile to be activated to resolve the spring milestone and snapshot repositories. Use your preferred IDE to set this profile to be active, or you may experience build errors.

17.3.2. Importing into eclipse with m2eclipse

We recommend the m2eclipse eclipse plugin when working with eclipse. If you don’t already have m2eclipse installed it is available from the "eclipse marketplace".

Older versions of m2e do not support Maven 3.3, so once the projects are imported into Eclipse you will also need to tell m2eclipse to use the right profile for the projects. If you see many different errors related to the POMs in the projects, check that you have an up to date installation. If you can’t upgrade m2e, add the "spring" profile to your settings.xml. Alternatively you can copy the repository settings from the "spring" profile of the parent pom into your settings.xml.

17.3.3. Importing into eclipse without m2eclipse

If you prefer not to use m2eclipse you can generate eclipse project metadata using the following command:

$ ./mvnw eclipse:eclipse

The generated eclipse projects can be imported by selecting import existing projects from the file menu.

18. Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.

18.1. Sign the Contributor License Agreement

Before we accept a non-trivial patch or pull request we will need you to sign the Contributor License Agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.

18.2. Code of Conduct

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to [email protected].

18.3. Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.

  • Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the eclipse-code-formatter.xml file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file.

  • Make sure all new .java files to have a simple Javadoc class comment with at least an @author tag identifying you, and preferably at least a paragraph on what the class is for.

  • Add the ASF license header comment to all new .java files (copy from existing files in the project)

  • Add yourself as an @author to the .java files that you modify substantially (more than cosmetic changes).

  • Add some Javadocs and, if you change the namespace, some XSD doc elements.

  • A few unit tests would help a lot as well — someone has to do it.

  • If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).

  • When writing a commit message please follow these conventions, if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).

18.4. Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in the spring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            └── checkstyle.xml (1)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules

18.4.1. Checkstyle configuration

Checkstyle rules are disabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml
<properties>
<maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError> (1)
        <maven-checkstyle-plugin.failsOnViolation>true
        </maven-checkstyle-plugin.failsOnViolation> (2)
        <maven-checkstyle-plugin.includeTestSourceDirectory>true
        </maven-checkstyle-plugin.includeTestSourceDirectory> (3)
</properties>

<build>
        <plugins>
            <plugin> (4)
                <groupId>io.spring.javaformat</groupId>
                <artifactId>spring-javaformat-maven-plugin</artifactId>
            </plugin>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>

    <reporting>
        <plugins>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>
    </reporting>
</build>
1 Fails the build upon Checkstyle errors
2 Fails the build upon Checkstyle violations
3 Checkstyle analyzes also the test sources
4 Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules
5 Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under ${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml
<?xml version="1.0"?>
<!DOCTYPE suppressions PUBLIC
        "-//Puppy Crawl//DTD Suppressions 1.1//EN"
        "https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
<suppressions>
    <suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/>
    <suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/>
</suppressions>

It’s advisable to copy the ${spring-cloud-build.rootFolder}/.editorconfig and ${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig
$ touch .springformat

18.5. IDE setup

18.5.1. Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin. The following files can be found in the Spring Cloud Build project.

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            ├── checkstyle.xml (1)
            └── intellij
                ├── Intellij_Project_Defaults.xml (4)
                └── Intellij_Spring_Boot_Java_Conventions.xml (5)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules
4 Project defaults for Intellij that apply most of Checkstyle rules
5 Project style conventions for Intellij that apply most of Checkstyle rules
Code style
Figure 3. Code style

Go to FileSettingsEditorCode style. There click on the icon next to the Scheme section. There, click on the Import Scheme value and pick the Intellij IDEA code style XML option. Import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Code style
Figure 4. Inspection profiles

Go to FileSettingsEditorInspections. There click on the icon next to the Profile section. There, click on the Import Profile and import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install the Checkstyle plugin. It’s advisable to also install the Assertions2Assertj to automatically convert the JUnit assertions

Checkstyle

Go to FileSettingsOther settingsCheckstyle. There click on the + icon in the Configuration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for the checkstyle.xml : raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

Remember to set the Scan Scope to All sources since we apply checkstyle rules for production and test sources.

Spring Boot Cloud CLI

Spring Boot CLI provides Spring Boot command line features for Spring Cloud. You can write Groovy scripts to run Spring Cloud component applications (e.g. @EnableEurekaServer). You can also easily do things like encryption and decryption to support Spring Cloud Config clients with secret configuration values. With the Launcher CLI you can launch services like Eureka, Zipkin, Config Server conveniently all at once from the command line (very useful at development time).

Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, please find the source code and issue trackers in the project at github.

19. Installation

To install, make sure you have Spring Boot CLI (2.0.0 or better):

$ spring version
Spring CLI v2.2.0.BUILD-SNAPSHOT

E.g. for SDKMan users

$ sdk install springboot 2.2.0.BUILD-SNAPSHOT
$ sdk use springboot 2.2.0.BUILD-SNAPSHOT

and install the Spring Cloud plugin

$ mvn install
$ spring install org.springframework.cloud:spring-cloud-cli:2.2.0.BUILD-SNAPSHOT
Prerequisites: to use the encryption and decryption features you need the full-strength JCE installed in your JVM (it’s not there by default). You can download the "Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files" from Oracle, and follow instructions for installation (essentially replace the 2 policy files in the JRE lib/security directory with the ones that you downloaded).

20. Running Spring Cloud Services in Development

The Launcher CLI can be used to run common services like Eureka, Config Server etc. from the command line. To list the available services you can do spring cloud --list, and to launch a default set of services just spring cloud. To choose the services to deploy, just list them on the command line, e.g.

$ spring cloud eureka configserver h2 kafka stubrunner zipkin

Summary of supported deployables:

Service Name Address Description

eureka

Eureka Server

localhost:8761

Eureka server for service registration and discovery. All the other services show up in its catalog by default.

configserver

Config Server

localhost:8888

Spring Cloud Config Server running in the "native" profile and serving configuration from the local directory ./launcher

h2

H2 Database

localhost:9095 (console), jdbc:h2:tcp://localhost:9096/{data}

Relation database service. Use a file path for {data} (e.g. ./target/test) when you connect. Remember that you can add ;MODE=MYSQL or ;MODE=POSTGRESQL to connect with compatibility to other server types.

kafka

Kafka Broker

localhost:9091 (actuator endpoints), localhost:9092

dataflow

Dataflow Server

localhost:9393

Spring Cloud Dataflow server with UI at /admin-ui. Connect the Dataflow shell to target at root path.

zipkin

Zipkin Server

localhost:9411

Zipkin Server with UI for visualizing traces. Stores span data in memory and accepts them via HTTP POST of JSON data.

stubrunner

Stub Runner Boot

localhost:8750

Downloads WireMock stubs, starts WireMock and feeds the started servers with stored stubs. Pass stubrunner.ids to pass stub coordinates and then go to localhost:8750/stubs.

Each of these apps can be configured using a local YAML file with the same name (in the current working directory or a subdirectory called "config" or in ~/.spring-cloud). E.g. in configserver.yml you might want to do something like this to locate a local git repository for the backend:

configserver.yml
spring:
  profiles:
    active: git
  cloud:
    config:
      server:
        git:
          uri: file://${user.home}/dev/demo/config-repo

E.g. in Stub Runner app you could fetch stubs from your local .m2 in the following way.

stubrunner.yml
stubrunner:
  workOffline: true
  ids:
    - com.example:beer-api-producer:+:9876

20.1. Adding Additional Applications

Additional applications can be added to ./config/cloud.yml (not ./config.yml because that would replace the defaults), e.g. with

config/cloud.yml
spring:
  cloud:
    launcher:
      deployables:
        source:
          coordinates: maven://com.example:source:0.0.1-SNAPSHOT
          port: 7000
        sink:
          coordinates: maven://com.example:sink:0.0.1-SNAPSHOT
          port: 7001

when you list the apps:

$ spring cloud --list
source sink configserver dataflow eureka h2 kafka stubrunner zipkin

(notice the additional apps at the start of the list).

21. Writing Groovy Scripts and Running Applications

Spring Cloud CLI has support for most of the Spring Cloud declarative features, such as the @Enable* class of annotations. For example, here is a fully functional Eureka server

app.groovy
@EnableEurekaServer
class Eureka {}

which you can run from the command line like this

$ spring run app.groovy

To include additional dependencies, often it suffices just to add the appropriate feature-enabling annotation, e.g. @EnableConfigServer, @EnableOAuth2Sso or @EnableEurekaClient. To manually include a dependency you can use a @Grab with the special "Spring Boot" short style artifact co-ordinates, i.e. with just the artifact ID (no need for group or version information), e.g. to set up a client app to listen on AMQP for management events from the Spring CLoud Bus:

app.groovy
@Grab('spring-cloud-starter-bus-amqp')
@RestController
class Service {
  @RequestMapping('/')
  def home() { [message: 'Hello'] }
}

22. Encryption and Decryption

The Spring Cloud CLI comes with an "encrypt" and a "decrypt" command. Both accept arguments in the same form with a key specified as a mandatory "--key", e.g.

$ spring encrypt mysecret --key foo
682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
$ spring decrypt --key foo 682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
mysecret

To use a key in a file (e.g. an RSA public key for encyption) prepend the key value with "@" and provide the file path, e.g.

$ spring encrypt mysecret --key @${HOME}/.ssh/id_rsa.pub
AQAjPgt3eFZQXwt8tsHAVv/QHiY5sI2dRcR+...

Spring Cloud for Cloud Foundry

Spring Cloud for Cloudfoundry makes it easy to run Spring Cloud apps in Cloud Foundry (the Platform as a Service). Cloud Foundry has the notion of a "service", which is middlware that you "bind" to an app, essentially providing it with an environment variable containing credentials (e.g. the location and username to use for the service).

The spring-cloud-cloudfoundry-commons module configures the Reactor-based Cloud Foundry Java client, v 3.0, and can be used standalone.

The spring-cloud-cloudfoundry-web project provides basic support for some enhanced features of webapps in Cloud Foundry: binding automatically to single-sign-on services and optionally enabling sticky routing for discovery.

The spring-cloud-cloudfoundry-discovery project provides an implementation of Spring Cloud Commons DiscoveryClient so you can @EnableDiscoveryClient and provide your credentials as spring.cloud.cloudfoundry.discovery.[username,password] (also *.url if you are not connecting to Pivotal Web Services) and then you can use the DiscoveryClient directly or via a LoadBalancerClient.

The first time you use it the discovery client might be slow owing to the fact that it has to get an access token from Cloud Foundry.

23. Discovery

Here’s a Spring Cloud app with Cloud Foundry discovery:

app.groovy
@Grab('org.springframework.cloud:spring-cloud-cloudfoundry')
@RestController
@EnableDiscoveryClient
class Application {

  @Autowired
  DiscoveryClient client

  @RequestMapping('/')
  String home() {
    'Hello from ' + client.getLocalServiceInstance()
  }

}

If you run it without any service bindings:

$ spring jar app.jar app.groovy
$ cf push -p app.jar

It will show its app name in the home page.

The DiscoveryClient can lists all the apps in a space, according to the credentials it is authenticated with, where the space defaults to the one the client is running in (if any). If neither org nor space are configured, they default per the user’s profile in Cloud Foundry.

24. Single Sign On

All of the OAuth2 SSO and resource server features moved to Spring Boot in version 1.3. You can find documentation in the Spring Boot user guide.

This project provides automatic binding from CloudFoundry service credentials to the Spring Boot features. If you have a CloudFoundry service called "sso", for instance, with credentials containing "client_id", "client_secret" and "auth_domain", it will bind automatically to the Spring OAuth2 client that you enable with @EnableOAuth2Sso (from Spring Boot). The name of the service can be parameterized using spring.oauth2.sso.serviceId.

25. Configuration

To see the list of all Spring Cloud Sloud Foundry related configuration properties please check the Appendix page.

Cloud Native Applications

Cloud Native is a style of application development that encourages easy adoption of best practices in the areas of continuous delivery and value-driven development. A related discipline is that of building 12-factor Applications, in which development practices are aligned with delivery and operations goals — for instance, by using declarative programming and management and monitoring. Spring Cloud facilitates these styles of development in a number of specific ways. The starting point is a set of features to which all components in a distributed system need easy access.

Many of those features are covered by Spring Boot, on which Spring Cloud builds. Some more features are delivered by Spring Cloud as two libraries: Spring Cloud Context and Spring Cloud Commons. Spring Cloud Context provides utilities and special services for the ApplicationContext of a Spring Cloud application (bootstrap context, encryption, refresh scope, and environment endpoints). Spring Cloud Commons is a set of abstractions and common classes used in different Spring Cloud implementations (such as Spring Cloud Netflix and Spring Cloud Consul).

If you get an exception due to "Illegal key size" and you use Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information:

Extract the files into the JDK/jre/lib/security folder for whichever version of JRE/JDK x64/x86 you use.

Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, you can find the source code and issue trackers for the project at github.

26. Spring Cloud Context: Application Context Services

Spring Boot has an opinionated view of how to build an application with Spring. For instance, it has conventional locations for common configuration files and has endpoints for common management and monitoring tasks. Spring Cloud builds on top of that and adds a few features that many components in a system would use or occasionally need.

26.1. The Bootstrap Application Context

A Spring Cloud application operates by creating a “bootstrap” context, which is a parent context for the main application. This context is responsible for loading configuration properties from the external sources and for decrypting properties in the local external configuration files. The two contexts share an Environment, which is the source of external properties for any Spring application. By default, bootstrap properties (not bootstrap.properties but properties that are loaded during the bootstrap phase) are added with high precedence, so they cannot be overridden by local configuration.

The bootstrap context uses a different convention for locating external configuration than the main application context. Instead of application.yml (or .properties), you can use bootstrap.yml, keeping the external configuration for bootstrap and main context nicely separate. The following listing shows an example:

Example 1. bootstrap.yml
spring:
  application:
    name: foo
  cloud:
    config:
      uri: ${SPRING_CONFIG_URI:http://localhost:8888}

If your application needs any application-specific configuration from the server, it is a good idea to set the spring.application.name (in bootstrap.yml or application.yml). For the property spring.application.name to be used as the application’s context ID, you must set it in bootstrap.[properties | yml].

If you want to retrieve specific profile configuration, you should also set spring.profiles.active in bootstrap.[properties | yml].

You can disable the bootstrap process completely by setting spring.cloud.bootstrap.enabled=false (for example, in system properties).

26.2. Application Context Hierarchies

If you build an application context from SpringApplication or SpringApplicationBuilder, the Bootstrap context is added as a parent to that context. It is a feature of Spring that child contexts inherit property sources and profiles from their parent, so the “main” application context contains additional property sources, compared to building the same context without Spring Cloud Config. The additional property sources are:

  • “bootstrap”: If any PropertySourceLocators are found in the bootstrap context and if they have non-empty properties, an optional CompositePropertySource appears with high priority. An example would be properties from the Spring Cloud Config Server. See “Customizing the Bootstrap Property Sources” for how to customize the contents of this property source.

  • “applicationConfig: [classpath:bootstrap.yml]” (and related files if Spring profiles are active): If you have a bootstrap.yml (or .properties), those properties are used to configure the bootstrap context. Then they get added to the child context when its parent is set. They have lower precedence than the application.yml (or .properties) and any other property sources that are added to the child as a normal part of the process of creating a Spring Boot application. See “Changing the Location of Bootstrap Properties” for how to customize the contents of these property sources.

Because of the ordering rules of property sources, the “bootstrap” entries take precedence. However, note that these do not contain any data from bootstrap.yml, which has very low precedence but can be used to set defaults.

You can extend the context hierarchy by setting the parent context of any ApplicationContext you create — for example, by using its own interface or with the SpringApplicationBuilder convenience methods (parent(), child() and sibling()). The bootstrap context is the parent of the most senior ancestor that you create yourself. Every context in the hierarchy has its own “bootstrap” (possibly empty) property source to avoid promoting values inadvertently from parents down to their descendants. If there is a config server, every context in the hierarchy can also (in principle) have a different spring.application.name and, hence, a different remote property source. Normal Spring application context behavior rules apply to property resolution: properties from a child context override those in the parent, by name and also by property source name. (If the child has a property source with the same name as the parent, the value from the parent is not included in the child).

Note that the SpringApplicationBuilder lets you share an Environment amongst the whole hierarchy, but that is not the default. Thus, sibling contexts (in particular) do not need to have the same profiles or property sources, even though they may share common values with their parent.

26.3. Changing the Location of Bootstrap Properties

The bootstrap.yml (or .properties) location can be specified by setting spring.cloud.bootstrap.name (default: bootstrap), spring.cloud.bootstrap.location (default: empty) or spring.cloud.bootstrap.additional-location (default: empty) — for example, in System properties.

Those properties behave like the spring.config.* variants with the same name. With spring.cloud.bootstrap.location the default locations are replaced and only the specified ones are used. To add locations to the list of default ones, spring.cloud.bootstrap.additional-location could be used. In fact, they are used to set up the bootstrap ApplicationContext by setting those properties in its Environment. If there is an active profile (from spring.profiles.active or through the Environment API in the context you are building), properties in that profile get loaded as well, the same as in a regular Spring Boot app — for example, from bootstrap-development.properties for a development profile.

26.4. Overriding the Values of Remote Properties

The property sources that are added to your application by the bootstrap context are often “remote” (from example, from Spring Cloud Config Server). By default, they cannot be overridden locally. If you want to let your applications override the remote properties with their own system properties or config files, the remote property source has to grant it permission by setting spring.cloud.config.allowOverride=true (it does not work to set this locally). Once that flag is set, two finer-grained settings control the location of the remote properties in relation to system properties and the application’s local configuration:

  • spring.cloud.config.overrideNone=true: Override from any local property source.

  • spring.cloud.config.overrideSystemProperties=false: Only system properties, command line arguments, and environment variables (but not the local config files) should override the remote settings.

26.5. Customizing the Bootstrap Configuration

The bootstrap context can be set to do anything you like by adding entries to /META-INF/spring.factories under a key named org.springframework.cloud.bootstrap.BootstrapConfiguration. This holds a comma-separated list of Spring @Configuration classes that are used to create the context. Any beans that you want to be available to the main application context for autowiring can be created here. There is a special contract for @Beans of type ApplicationContextInitializer. If you want to control the startup sequence, you can mark classes with the @Order annotation (the default order is last).

When adding custom BootstrapConfiguration, be careful that the classes you add are not @ComponentScanned by mistake into your “main” application context, where they might not be needed. Use a separate package name for boot configuration classes and make sure that name is not already covered by your @ComponentScan or @SpringBootApplication annotated configuration classes.

The bootstrap process ends by injecting initializers into the main SpringApplication instance (which is the normal Spring Boot startup sequence, whether it runs as a standalone application or is deployed in an application server). First, a bootstrap context is created from the classes found in spring.factories. Then, all @Beans of type ApplicationContextInitializer are added to the main SpringApplication before it is started.

26.6. Customizing the Bootstrap Property Sources

The default property source for external configuration added by the bootstrap process is the Spring Cloud Config Server, but you can add additional sources by adding beans of type PropertySourceLocator to the bootstrap context (through spring.factories). For instance, you can insert additional properties from a different server or from a database.

As an example, consider the following custom locator:

@Configuration
public class CustomPropertySourceLocator implements PropertySourceLocator {

    @Override
    public PropertySource<?> locate(Environment environment) {
        return new MapPropertySource("customProperty",
                Collections.<String, Object>singletonMap("property.from.sample.custom.source", "worked as intended"));
    }

}

The Environment that is passed in is the one for the ApplicationContext about to be created — in other words, the one for which we supply additional property sources. It already has its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this Environment (for example, by keying it on spring.application.name, as is done in the default Spring Cloud Config Server property source locator).

If you create a jar with this class in it and then add a META-INF/spring.factories containing the following setting, the customProperty PropertySource appears in any application that includes that jar on its classpath:

org.springframework.cloud.bootstrap.BootstrapConfiguration=sample.custom.CustomPropertySourceLocator

26.7. Logging Configuration

If you use Spring Boot to configure log settings, you should place this configuration in bootstrap.[yml | properties] if you would like it to apply to all events.

For Spring Cloud to initialize logging configuration properly, you cannot use a custom prefix. For example, using custom.loggin.logpath is not recognized by Spring Cloud when initializing the logging system.

26.8. Environment Changes

The application listens for an EnvironmentChangeEvent and reacts to the change in a couple of standard ways (additional ApplicationListeners can be added as @Beans in the normal way). When an EnvironmentChangeEvent is observed, it has a list of key values that have changed, and the application uses those to:

  • Re-bind any @ConfigurationProperties beans in the context.

  • Set the logger levels for any properties in logging.level.*.

Note that the Spring Cloud Config Client does not, by default, poll for changes in the Environment. Generally, we would not recommend that approach for detecting changes (although you could set it up with a @Scheduled annotation). If you have a scaled-out client application, it is better to broadcast the EnvironmentChangeEvent to all the instances instead of having them polling for changes (for example, by using the Spring Cloud Bus).

The EnvironmentChangeEvent covers a large class of refresh use cases, as long as you can actually make a change to the Environment and publish the event. Note that those APIs are public and part of core Spring). You can verify that the changes are bound to @ConfigurationProperties beans by visiting the /configprops endpoint (a standard Spring Boot Actuator feature). For instance, a DataSource can have its maxPoolSize changed at runtime (the default DataSource created by Spring Boot is a @ConfigurationProperties bean) and grow capacity dynamically. Re-binding @ConfigurationProperties does not cover another large class of use cases, where you need more control over the refresh and where you need a change to be atomic over the whole ApplicationContext. To address those concerns, we have @RefreshScope.

26.9. Refresh Scope

When there is a configuration change, a Spring @Bean that is marked as @RefreshScope gets special treatment. This feature addresses the problem of stateful beans that get their configuration injected only when they are initialized. For instance, if a DataSource has open connections when the database URL is changed through the Environment, you probably want the holders of those connections to be able to complete what they are doing. Then, the next time something borrows a connection from the pool, it gets one with the new URL.

Sometimes, it might even be mandatory to apply the @RefreshScope annotation on some beans that can be only initialized once. If a bean is “immutable”, you have to either annotate the bean with @RefreshScope or specify the classname under the property key: spring.cloud.refresh.extra-refreshable.

If you hava a DataSource bean that is a HikariDataSource, it can not be refreshed. It is the default value for spring.cloud.refresh.never-refreshable. Choose a different DataSource implementation if you need it to be refreshed.

Refresh scope beans are lazy proxies that initialize when they are used (that is, when a method is called), and the scope acts as a cache of initialized values. To force a bean to re-initialize on the next method call, you must invalidate its cache entry.

The RefreshScope is a bean in the context and has a public refreshAll() method to refresh all beans in the scope by clearing the target cache. The /refresh endpoint exposes this functionality (over HTTP or JMX). To refresh an individual bean by name, there is also a refresh(String) method.

To expose the /refresh endpoint, you need to add following configuration to your application:

management:
  endpoints:
    web:
      exposure:
        include: refresh
@RefreshScope works (technically) on a @Configuration class, but it might lead to surprising behavior. For example, it does not mean that all the @Beans defined in that class are themselves in @RefreshScope. Specifically, anything that depends on those beans cannot rely on them being updated when a refresh is initiated, unless it is itself in @RefreshScope. In that case, it is rebuilt on a refresh and its dependencies are re-injected. At that point, they are re-initialized from the refreshed @Configuration).

26.10. Encryption and Decryption

Spring Cloud has an Environment pre-processor for decrypting property values locally. It follows the same rules as the Spring Cloud Config Server and has the same external configuration through encrypt.*. Thus, you can use encrypted values in the form of {cipher}*, and, as long as there is a valid key, they are decrypted before the main application context gets the Environment settings. To use the encryption features in an application, you need to include Spring Security RSA in your classpath (Maven co-ordinates: org.springframework.security:spring-security-rsa), and you also need the full strength JCE extensions in your JVM.

If you get an exception due to "Illegal key size" and you use Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information:

Extract the files into the JDK/jre/lib/security folder for whichever version of JRE/JDK x64/x86 you use.

26.11. Endpoints

For a Spring Boot Actuator application, some additional management endpoints are available. You can use:

  • POST to /actuator/env to update the Environment and rebind @ConfigurationProperties and log levels.

  • /actuator/refresh to re-load the boot strap context and refresh the @RefreshScope beans.

  • /actuator/restart to close the ApplicationContext and restart it (disabled by default).

  • /actuator/pause and /actuator/resume for calling the Lifecycle methods (stop() and start() on the ApplicationContext).

If you disable the /actuator/restart endpoint then the /actuator/pause and /actuator/resume endpoints will also be disabled since they are just a special case of /actuator/restart.

27. Spring Cloud Commons: Common Abstractions

Patterns such as service discovery, load balancing, and circuit breakers lend themselves to a common abstraction layer that can be consumed by all Spring Cloud clients, independent of the implementation (for example, discovery with Eureka or Consul).

27.1. The @EnableDiscoveryClient Annotation

Spring Cloud Commons provides the @EnableDiscoveryClient annotation. This looks for implementations of the DiscoveryClient and ReactiveDiscoveryClient interfaces with META-INF/spring.factories. Implementations of the discovery client add a configuration class to spring.factories under the org.springframework.cloud.client.discovery.EnableDiscoveryClient key. Examples of DiscoveryClient implementations include Spring Cloud Netflix Eureka, Spring Cloud Consul Discovery, and Spring Cloud Zookeeper Discovery.

Spring Cloud will provide both the blocking and reactive service discovery clients by default. You can disable the blocking and/or reactive clients easily by setting spring.cloud.discovery.blocking.enabled=false or spring.cloud.discovery.reactive.enabled=false. To completely disable service discovery you just need to set spring.cloud.discovery.enabled=false.

By default, implementations of DiscoveryClient auto-register the local Spring Boot server with the remote discovery server. This behavior can be disabled by setting autoRegister=false in @EnableDiscoveryClient.

@EnableDiscoveryClient is no longer required. You can put a DiscoveryClient implementation on the classpath to cause the Spring Boot application to register with the service discovery server.

27.1.1. Health Indicator

Commons creates a Spring Boot HealthIndicator that DiscoveryClient implementations can participate in by implementing DiscoveryHealthIndicator. To disable the composite HealthIndicator, set spring.cloud.discovery.client.composite-indicator.enabled=false. A generic HealthIndicator based on DiscoveryClient is auto-configured (DiscoveryClientHealthIndicator). To disable it, set spring.cloud.discovery.client.health-indicator.enabled=false. To disable the description field of the DiscoveryClientHealthIndicator, set spring.cloud.discovery.client.health-indicator.include-description=false. Otherwise, it can bubble up as the description of the rolled up HealthIndicator.

27.1.2. Ordering DiscoveryClient instances

DiscoveryClient interface extends Ordered. This is useful when using multiple discovery clients, as it allows you to define the order of the returned discovery clients, similar to how you can order the beans loaded by a Spring application. By default, the order of any DiscoveryClient is set to 0. If you want to set a different order for your custom DiscoveryClient implementations, you just need to override the getOrder() method so that it returns the value that is suitable for your setup. Apart from this, you can use properties to set the order of the DiscoveryClient implementations provided by Spring Cloud, among others ConsulDiscoveryClient, EurekaDiscoveryClient and ZookeeperDiscoveryClient. In order to do it, you just need to set the spring.cloud.{clientIdentifier}.discovery.order (or eureka.client.order for Eureka) property to the desired value.

27.1.3. SimpleDiscoveryClient

If there is no Service-Registry-backed DiscoveryClient in the classpath, SimpleDiscoveryClient instance, that uses properties to get information on service and instances, will be used.

The information about the available instances should be passed to via properties in the following format: spring.cloud.discovery.client.simple.instances.service1[0].uri=http://s11:8080, where spring.cloud.discovery.client.simple.instances is the common prefix, then service1 stands for the ID of the service in question, while [0] indicates the index number of the instance (as visible in the example, indexes start with 0), and then the value of uri is the actual URI under which the instance is available.

27.2. ServiceRegistry

Commons now provides a ServiceRegistry interface that provides methods such as register(Registration) and deregister(Registration), which let you provide custom registered services. Registration is a marker interface.

The following example shows the ServiceRegistry in use:

@Configuration
@EnableDiscoveryClient(autoRegister=false)
public class MyConfiguration {
    private ServiceRegistry registry;

    public MyConfiguration(ServiceRegistry registry) {
        this.registry = registry;
    }

    // called through some external process, such as an event or a custom actuator endpoint
    public void register() {
        Registration registration = constructRegistration();
        this.registry.register(registration);
    }
}

Each ServiceRegistry implementation has its own Registry implementation.

  • ZookeeperRegistration used with ZookeeperServiceRegistry

  • EurekaRegistration used with EurekaServiceRegistry

  • ConsulRegistration used with ConsulServiceRegistry

If you are using the ServiceRegistry interface, you are going to need to pass the correct Registry implementation for the ServiceRegistry implementation you are using.

27.2.1. ServiceRegistry Auto-Registration

By default, the ServiceRegistry implementation auto-registers the running service. To disable that behavior, you can set: * @EnableDiscoveryClient(autoRegister=false) to permanently disable auto-registration. * spring.cloud.service-registry.auto-registration.enabled=false to disable the behavior through configuration.

ServiceRegistry Auto-Registration Events

There are two events that will be fired when a service auto-registers. The first event, called InstancePreRegisteredEvent, is fired before the service is registered. The second event, called InstanceRegisteredEvent, is fired after the service is registered. You can register an ApplicationListener(s) to listen to and react to these events.

These events will not be fired if the spring.cloud.service-registry.auto-registration.enabled property is set to false.

27.2.2. Service Registry Actuator Endpoint

Spring Cloud Commons provides a /service-registry actuator endpoint. This endpoint relies on a Registration bean in the Spring Application Context. Calling /service-registry with GET returns the status of the Registration. Using POST to the same endpoint with a JSON body changes the status of the current Registration to the new value. The JSON body has to include the status field with the preferred value. Please see the documentation of the ServiceRegistry implementation you use for the allowed values when updating the status and the values returned for the status. For instance, Eureka’s supported statuses are UP, DOWN, OUT_OF_SERVICE, and UNKNOWN.

27.3. Spring RestTemplate as a Load Balancer Client

You can configure a RestTemplate to use a Load-balancer client. To create a load-balanced RestTemplate, create a RestTemplate @Bean and use the @LoadBalanced qualifier, as the following example shows:

@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
    @Autowired
    private RestTemplate restTemplate;

    public String doOtherStuff() {
        String results = restTemplate.getForObject("http://stores/stores", String.class);
        return results;
    }
}
A RestTemplate bean is no longer created through auto-configuration. Individual applications must create it.

The URI needs to use a virtual host name (that is, a service name, not a host name). The BlockingLoadBalancerClient is used to create a full physical address.

To use a load-balanced RestTemplate, you need to have a load-balancer implementation in your classpath. Add Spring Cloud LoadBalancer starter to your project in order to use it.

27.4. Spring WebClient as a Load Balancer Client

You can configure WebClient to automatically use a load-balancer client. To create a load-balanced WebClient, create a WebClient.Builder @Bean and use the @LoadBalanced qualifier, as follows:

@Configuration
public class MyConfiguration {

    @Bean
    @LoadBalanced
    public WebClient.Builder loadBalancedWebClientBuilder() {
        return WebClient.builder();
    }
}

public class MyClass {
    @Autowired
    private WebClient.Builder webClientBuilder;

    public Mono<String> doOtherStuff() {
        return webClientBuilder.build().get().uri("http://stores/stores")
                        .retrieve().bodyToMono(String.class);
    }
}

The URI needs to use a virtual host name (that is, a service name, not a host name). The Spring Cloud LoadBalancer is used to create a full physical address.

If you want to use a @LoadBalanced WebClient.Builder, you need to have a load balancer implementation in the classpath. We recommend that you add the Spring Cloud LoadBalancer starter to your project. Then, ReactiveLoadBalancer is used underneath.

27.4.1. Retrying Failed Requests

A load-balanced RestTemplate can be configured to retry failed requests. By default, this logic is disabled. For the non-reactive version (with RestTemplate), you can enable it by adding Spring Retry to your application’s classpath. For the reactive version (with WebTestClient), you need to set `spring.cloud.loadbalancer.retry.enabled=false.

If you would like to disable the retry logic with Spring Retry or Reactive Retry on the classpath, you can set spring.cloud.loadbalancer.retry.enabled=false.

For the non-reactive implementation, if you would like to implement a BackOffPolicy in your retries, you need to create a bean of type LoadBalancedRetryFactory and override the createBackOffPolicy() method.

For the reactive implementation, you just need to enable it by setting spring.cloud.loadbalancer.retry.backoff.enabled to false.

You can set:

  • spring.cloud.loadbalancer.retry.maxRetriesOnSameServiceInstance - indicates how many times a request should be retried on the same ServiceInstance (counted separately for every selected instance)

  • spring.cloud.loadbalancer.retry.maxRetriesOnNextServiceInstance - indicates how many times a request should be retried a newly selected ServiceInstance

  • spring.cloud.loadbalancer.retry.retryableStatusCodes - the status codes on which to always retry a failed request.

For the reactive implementation, you can additionally set: - spring.cloud.loadbalancer.retry.backoff.minBackoff - Sets the minimum backoff duration (by default, 5 milliseconds) - spring.cloud.loadbalancer.retry.backoff.maxBackoff - Sets the maximum backoff duration (by default, max long value of milliseconds) - spring.cloud.loadbalancer.retry.backoff.jitter - Sets the jitter used for calculationg the actual backoff duration for each call (by default, 0.5).

For the reactive implementation, you can also implement your own LoadBalancerRetryPolicy to have more detailed control over the load-balanced call retries.

For load-balanced retries, by default, we wrap the ServiceInstanceListSupplier bean with RetryAwareServiceInstanceListSupplier to select a different instance from the one previously chosen, if available. You can disable this behavior by setting the value of spring.cloud.loadbalancer.retry.avoidPreviousInstance to false.
@Configuration
public class MyConfiguration {
    @Bean
    LoadBalancedRetryFactory retryFactory() {
        return new LoadBalancedRetryFactory() {
            @Override
            public BackOffPolicy createBackOffPolicy(String service) {
                return new ExponentialBackOffPolicy();
            }
        };
    }
}

If you want to add one or more RetryListener implementations to your retry functionality, you need to create a bean of type LoadBalancedRetryListenerFactory and return the RetryListener array you would like to use for a given service, as the following example shows:

@Configuration
public class MyConfiguration {
    @Bean
    LoadBalancedRetryListenerFactory retryListenerFactory() {
        return new LoadBalancedRetryListenerFactory() {
            @Override
            public RetryListener[] createRetryListeners(String service) {
                return new RetryListener[]{new RetryListener() {
                    @Override
                    public <T, E extends Throwable> boolean open(RetryContext context, RetryCallback<T, E> callback) {
                        //TODO Do you business...
                        return true;
                    }

                    @Override
                     public <T, E extends Throwable> void close(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {
                        //TODO Do you business...
                    }

                    @Override
                    public <T, E extends Throwable> void onError(RetryContext context, RetryCallback<T, E> callback, Throwable throwable) {
                        //TODO Do you business...
                    }
                }};
            }
        };
    }
}

27.5. Multiple RestTemplate Objects

If you want a RestTemplate that is not load-balanced, create a RestTemplate bean and inject it. To access the load-balanced RestTemplate, use the @LoadBalanced qualifier when you create your @Bean, as the following example shows:

@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    RestTemplate loadBalanced() {
        return new RestTemplate();
    }

    @Primary
    @Bean
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

public class MyClass {
@Autowired
private RestTemplate restTemplate;

    @Autowired
    @LoadBalanced
    private RestTemplate loadBalanced;

    public String doOtherStuff() {
        return loadBalanced.getForObject("http://stores/stores", String.class);
    }

    public String doStuff() {
        return restTemplate.getForObject("http://example.com", String.class);
    }
}
Notice the use of the @Primary annotation on the plain RestTemplate declaration in the preceding example to disambiguate the unqualified @Autowired injection.
If you see errors such as java.lang.IllegalArgumentException: Can not set org.springframework.web.client.RestTemplate field com.my.app.Foo.restTemplate to com.sun.proxy.$Proxy89, try injecting RestOperations or setting spring.aop.proxyTargetClass=true.

27.6. Multiple WebClient Objects

If you want a WebClient that is not load-balanced, create a WebClient bean and inject it. To access the load-balanced WebClient, use the @LoadBalanced qualifier when you create your @Bean, as the following example shows:

@Configuration
public class MyConfiguration {

    @LoadBalanced
    @Bean
    WebClient.Builder loadBalanced() {
        return WebClient.builder();
    }

    @Primary
    @Bean
    WebClient.Builder webClient() {
        return WebClient.builder();
    }
}

public class MyClass {
    @Autowired
    private WebClient.Builder webClientBuilder;

    @Autowired
    @LoadBalanced
    private WebClient.Builder loadBalanced;

    public Mono<String> doOtherStuff() {
        return loadBalanced.build().get().uri("http://stores/stores")
                        .retrieve().bodyToMono(String.class);
    }

    public Mono<String> doStuff() {
        return webClientBuilder.build().get().uri("http://example.com")
                        .retrieve().bodyToMono(String.class);
    }
}

27.7. Spring WebFlux WebClient as a Load Balancer Client

The Spring WebFlux can work with both reactive and non-reactive WebClient configurations, as the topics describe:

27.7.1. Spring WebFlux WebClient with ReactorLoadBalancerExchangeFilterFunction

You can configure WebClient to use the ReactiveLoadBalancer. If you add Spring Cloud LoadBalancer starter to your project and if spring-webflux is on the classpath, ReactorLoadBalancerExchangeFilterFunction is auto-configured. The following example shows how to configure a WebClient to use reactive load-balancer:

public class MyClass {
    @Autowired
    private ReactorLoadBalancerExchangeFilterFunction lbFunction;

    public Mono<String> doOtherStuff() {
        return WebClient.builder().baseUrl("http://stores")
            .filter(lbFunction)
            .build()
            .get()
            .uri("/stores")
            .retrieve()
            .bodyToMono(String.class);
    }
}

The URI needs to use a virtual host name (that is, a service name, not a host name). The ReactorLoadBalancer is used to create a full physical address.

27.7.2. Spring WebFlux WebClient with a Non-reactive Load Balancer Client

If spring-webflux is on the classpath, LoadBalancerExchangeFilterFunction is auto-configured. Note, however, that this uses a non-reactive client under the hood. The following example shows how to configure a WebClient to use load-balancer:

public class MyClass {
    @Autowired
    private LoadBalancerExchangeFilterFunction lbFunction;

    public Mono<String> doOtherStuff() {
        return WebClient.builder().baseUrl("http://stores")
            .filter(lbFunction)
            .build()
            .get()
            .uri("/stores")
            .retrieve()
            .bodyToMono(String.class);
    }
}

The URI needs to use a virtual host name (that is, a service name, not a host name). The LoadBalancerClient is used to create a full physical address.

WARN: This approach is now deprecated. We suggest that you use WebFlux with reactive Load-Balancer instead.

27.8. Ignore Network Interfaces

Sometimes, it is useful to ignore certain named network interfaces so that they can be excluded from Service Discovery registration (for example, when running in a Docker container). A list of regular expressions can be set to cause the desired network interfaces to be ignored. The following configuration ignores the docker0 interface and all interfaces that start with veth:

Example 2. application.yml
spring:
  cloud:
    inetutils:
      ignoredInterfaces:
        - docker0
        - veth.*

You can also force the use of only specified network addresses by using a list of regular expressions, as the following example shows:

Example 3. bootstrap.yml
spring:
  cloud:
    inetutils:
      preferredNetworks:
        - 192.168
        - 10.0

You can also force the use of only site-local addresses, as the following example shows:

Example 4. application.yml
spring:
  cloud:
    inetutils:
      useOnlySiteLocalInterfaces: true

See Inet4Address.html.isSiteLocalAddress() for more details about what constitutes a site-local address.

27.9. HTTP Client Factories

Spring Cloud Commons provides beans for creating both Apache HTTP clients (ApacheHttpClientFactory) and OK HTTP clients (OkHttpClientFactory). The OkHttpClientFactory bean is created only if the OK HTTP jar is on the classpath. In addition, Spring Cloud Commons provides beans for creating the connection managers used by both clients: ApacheHttpClientConnectionManagerFactory for the Apache HTTP client and OkHttpClientConnectionPoolFactory for the OK HTTP client. If you would like to customize how the HTTP clients are created in downstream projects, you can provide your own implementation of these beans. In addition, if you provide a bean of type HttpClientBuilder or OkHttpClient.Builder, the default factories use these builders as the basis for the builders returned to downstream projects. You can also disable the creation of these beans by setting spring.cloud.httpclientfactories.apache.enabled or spring.cloud.httpclientfactories.ok.enabled to false.

27.10. Enabled Features

Spring Cloud Commons provides a /features actuator endpoint. This endpoint returns features available on the classpath and whether they are enabled. The information returned includes the feature type, name, version, and vendor.

27.10.1. Feature types

There are two types of 'features': abstract and named.

Abstract features are features where an interface or abstract class is defined and that an implementation the creates, such as DiscoveryClient, LoadBalancerClient, or LockService. The abstract class or interface is used to find a bean of that type in the context. The version displayed is bean.getClass().getPackage().getImplementationVersion().

Named features are features that do not have a particular class they implement. These features include “Circuit Breaker”, “API Gateway”, “Spring Cloud Bus”, and others. These features require a name and a bean type.

27.10.2. Declaring features

Any module can declare any number of HasFeature beans, as the following examples show:

@Bean
public HasFeatures commonsFeatures() {
  return HasFeatures.abstractFeatures(DiscoveryClient.class, LoadBalancerClient.class);
}

@Bean
public HasFeatures consulFeatures() {
  return HasFeatures.namedFeatures(
    new NamedFeature("Spring Cloud Bus", ConsulBusAutoConfiguration.class),
    new NamedFeature("Circuit Breaker", HystrixCommandAspect.class));
}

@Bean
HasFeatures localFeatures() {
  return HasFeatures.builder()
      .abstractFeature(Something.class)
      .namedFeature(new NamedFeature("Some Other Feature", Someother.class))
      .abstractFeature(Somethingelse.class)
      .build();
}

Each of these beans should go in an appropriately guarded @Configuration.

27.11. Spring Cloud Compatibility Verification

Due to the fact that some users have problem with setting up Spring Cloud application, we’ve decided to add a compatibility verification mechanism. It will break if your current setup is not compatible with Spring Cloud requirements, together with a report, showing what exactly went wrong.

At the moment we verify which version of Spring Boot is added to your classpath.

Example of a report

***************************
APPLICATION FAILED TO START
***************************

Description:

Your project setup is incompatible with our requirements due to following reasons:

- Spring Boot [2.1.0.RELEASE] is not compatible with this Spring Cloud release train


Action:

Consider applying the following actions:

- Change Spring Boot version to one of the following versions [1.2.x, 1.3.x] .
You can find the latest Spring Boot versions here [https://spring.io/projects/spring-boot#learn].
If you want to learn more about the Spring Cloud Release train compatibility, you can visit this page [https://spring.io/projects/spring-cloud#overview] and check the [Release Trains] section.

In order to disable this feature, set spring.cloud.compatibility-verifier.enabled to false. If you want to override the compatible Spring Boot versions, just set the spring.cloud.compatibility-verifier.compatible-boot-versions property with a comma separated list of compatible Spring Boot versions.

28. Spring Cloud LoadBalancer

Spring Cloud provides its own client-side load-balancer abstraction and implementation. For the load-balancing mechanism, ReactiveLoadBalancer interface has been added and a Round-Robin-based and Random implementations have been provided for it. In order to get instances to select from reactive ServiceInstanceListSupplier is used. Currently we support a service-discovery-based implementation of ServiceInstanceListSupplier that retrieves available instances from Service Discovery using a Discovery Client available in the classpath.

28.1. Switching between the load-balancing algorithms

The ReactiveLoadBalancer implementation that is used by default is RoundRobinLoadBalancer. To switch to a different implementation, either for selected services or all of them, you can use the custom LoadBalancer configurations mechanism.

For example, the following configuration can be passed via @LoadBalancerClient annotation to switch to using the RandomLoadBalancer:

public class CustomLoadBalancerConfiguration {

    @Bean
    ReactorLoadBalancer<ServiceInstance> randomLoadBalancer(Environment environment,
            LoadBalancerClientFactory loadBalancerClientFactory) {
        String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
        return new RandomLoadBalancer(loadBalancerClientFactory
                .getLazyProvider(name, ServiceInstanceListSupplier.class),
                name);
    }
}

28.2. Spring Cloud LoadBalancer integrations

In order to make it easy to use Spring Cloud LoadBalancer, we provide ReactorLoadBalancerExchangeFilterFunction that can be used with WebClient and BlockingLoadBalancerClient that works with RestTemplate. You can see more information and examples of usage in the following sections:

28.3. Spring Cloud LoadBalancer Caching

Apart from the basic ServiceInstanceListSupplier implementation that retrieves instances via DiscoveryClient each time it has to choose an instance, we provide two caching implementations.

28.3.1. Caffeine-backed LoadBalancer Cache Implementation

If you have com.github.ben-manes.caffeine:caffeine in the classpath, Caffeine-based implementation will be used. See the LoadBalancerCacheConfiguration section for information on how to configure it.

If you are using Caffeine, you can also override the default Caffeine Cache setup for the LoadBalancer by passing your own Caffeine Specification in the spring.cloud.loadbalancer.cache.caffeine.spec property.

WARN: Passing your own Caffeine specification will override any other LoadBalancerCache settings, including General LoadBalancer Cache Configuration fields, such as ttl and capacity.

28.3.2. Default LoadBalancer Cache Implementation

If you do not have Caffeine in the classpath, the DefaultLoadBalancerCache, which comes automatically with spring-cloud-starter-loadbalancer, will be used. See the LoadBalancerCacheConfiguration section for information on how to configure it.

To use Caffeine instead of the default cache, add the com.github.ben-manes.caffeine:caffeine dependency to classpath.

28.3.3. LoadBalancer Cache Configuration

You can set your own ttl value (the time after write after which entries should be expired), expressed as Duration, by passing a String compliant with the Spring Boot String to Duration converter syntax. as the value of the spring.cloud.loadbalancer.cache.ttl property. You can also set your own LoadBalancer cache initial capacity by setting the value of the spring.cloud.loadbalancer.cache.capacity property.

The default setup includes ttl set to 35 seconds and the default initialCapacity is 256.

You can also altogether disable loadBalancer caching by setting the value of spring.cloud.loadbalancer.cache.enabled to false.

Although the basic, non-cached, implementation is useful for prototyping and testing, it’s much less efficient than the cached versions, so we recommend always using the cached version in production.

28.4. Zone-Based Load-Balancing

To enable zone-based load-balancing, we provide the ZonePreferenceServiceInstanceListSupplier. We use DiscoveryClient-specific zone configuration (for example, eureka.instance.metadata-map.zone) to pick the zone that the client tries to filter available service instances for.

You can also override DiscoveryClient-specific zone setup by setting the value of spring.cloud.loadbalancer.zone property.
For the time being, only Eureka Discovery Client is instrumented to set the LoadBalancer zone. For other discovery client, set the spring.cloud.loadbalancer.zone property. More instrumentations coming shortly.
To determine the zone of a retrieved ServiceInstance, we check the value under the "zone" key in its metadata map.

The ZonePreferenceServiceInstanceListSupplier filters retrieved instances and only returns the ones within the same zone. If the zone is null or there are no instances within the same zone, it returns all the retrieved instances.

In order to use the zone-based load-balancing approach, you will have to instantiate a ZonePreferenceServiceInstanceListSupplier bean in a custom configuration.

We use delegates to work with ServiceInstanceListSupplier beans. We suggest passing a DiscoveryClientServiceInstanceListSupplier delegate in the constructor of ZonePreferenceServiceInstanceListSupplier and, in turn, wrapping the latter with a CachingServiceInstanceListSupplier to leverage LoadBalancer caching mechanism.

You could use this sample configuration to set it up:

public class CustomLoadBalancerConfiguration {

    @Bean
    public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
            ConfigurableApplicationContext context) {
        return ServiceInstanceListSuppliers.builder()
                    .withDiscoveryClient()
                    .withZonePreference()
                    .withCaching()
                    .build(context);
    }
}

28.5. Instance Health-Check for LoadBalancer

It is possible to enable a scheduled HealthCheck for the LoadBalancer. The HealthCheckServiceInstanceListSupplier is provided for that. It regularly verifies if the instances provided by a delegate ServiceInstanceListSupplier are still alive and only returns the healthy instances, unless there are none - then it returns all the retrieved instances.

This mechanism is particularly helpful while using the SimpleDiscoveryClient. For the clients backed by an actual Service Registry, it’s not necessary to use, as we already get healthy instances after querying the external ServiceDiscovery.
This supplier is also recommended for setups with a small number of instances per service in order to avoid retrying calls on a failing instance.
If using any of the Service Discovery-backed suppliers, adding this health-check mechanism is usually not necessary, as we retrieve the health state of the instances directly from the Service Registry.
The HealthCheckServiceInstanceListSupplier relies on having updated instances provided by a delegate flux. In the rare cases when you want to use a delegate that does not refresh the instances, even though the list of instances may change (such as the ReactiveDiscoveryClientServiceInstanceListSupplier provided by us), you can set spring.cloud.loadbalancer.health-check.refetch-instances to true to have the instance list refreshed by the HealthCheckServiceInstanceListSupplier. You can then also adjust the refretch intervals by modifying the value of spring.cloud.loadbalancer.health-check.refetch-instances-interval and opt to disable the additional healthcheck repetitions by setting spring.cloud.loadbalancer.repeat-health-check to fasle as every instances refetch will also trigger a healthcheck.

HealthCheckServiceInstanceListSupplier uses properties prefixed with spring.cloud.loadbalancer.health-check. You can set the initialDelay and interval for the scheduler. You can set the default path for the healthcheck URL by setting the value of the spring.cloud.loadbalancer.health-check.path.default property. You can also set a specific value for any given service by setting the value of the spring.cloud.loadbalancer.health-check.path.[SERVICE_ID] property, substituting [SERVICE_ID] with the correct ID of your service. If the path is not set, /actuator/health is used by default.

If you rely on the default path (/actuator/health), make sure you add spring-boot-starter-actuator to your collaborator’s dependencies, unless you are planning to add such an endpoint on your own.

In order to use the health-check scheduler approach, you will have to instantiate a HealthCheckServiceInstanceListSupplier bean in a custom configuration.

We use delegates to work with ServiceInstanceListSupplier beans. We suggest passing a DiscoveryClientServiceInstanceListSupplier delegate in the constructor of HealthCheckServiceInstanceListSupplier.

You could use this sample configuration to set it up:

public class CustomLoadBalancerConfiguration {

    @Bean
    public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
            ConfigurableApplicationContext context) {
        return ServiceInstanceListSupplier.builder()
                    .withDiscoveryClient()
                    .withHealthChecks()
                    .build(context);
        }
    }
For the non-reactive stack, create this supplier with the withBlockingHealthChecks(). You can also pass your own WebClient or RestTemplate instance to be used for the checks.
HealthCheckServiceInstanceListSupplier has its own caching mechanism based on Reactor Flux replay(). Therefore, if it’s being used, you may want to skip wrapping that supplier with CachingServiceInstanceListSupplier.

28.6. Same instance preference for LoadBalancer

You can set up the LoadBalancer in such a way that it prefers the instance that was previously selected, if that instance is available.

For that, you need to use SameInstancePreferenceServiceInstanceListSupplier. You can configure it either by setting the value of spring.cloud.loadbalancer.configurations to same-instance-preference or by providing your own ServiceInstanceListSupplier bean — for example:

public class CustomLoadBalancerConfiguration {

    @Bean
    public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
            ConfigurableApplicationContext context) {
        return ServiceInstanceListSupplier.builder()
                    .withDiscoveryClient()
                    .withSameInstancePreference()
                    .build(context);
        }
    }
This is also a replacement for Zookeeper StickyRule.

28.7. Request-based Sticky Session for LoadBalancer

You can set up the LoadBalancer in such a way that it prefers the instance with instanceId provided in a request cookie. We currently support this if the request is being passed to the LoadBalancer through either ClientRequestContext or ServerHttpRequestContext, which are used by the SC LoadBalancer exchange filter functions and filters.

For that, you need to use the RequestBasedStickySessionServiceInstanceListSupplier. You can configure it either by setting the value of spring.cloud.loadbalancer.configurations to request-based-sticky-session or by providing your own ServiceInstanceListSupplier bean — for example:

public class CustomLoadBalancerConfiguration {

    @Bean
    public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
            ConfigurableApplicationContext context) {
        return ServiceInstanceListSupplier.builder()
                    .withDiscoveryClient()
                    .withRequestBasedStickySession()
                    .build(context);
        }
    }

For that functionality, it is useful to have the selected service instance (which can be different from the one in the original request cookie if that one is not available) to be updated before sending the request forward. To do that, set the value of spring.cloud.loadbalancer.sticky-session.add-service-instance-cookie to true.

By default, the name of the cookie is sc-lb-instance-id. You can modify it by changing the value of the spring.cloud.loadbalancer.instance-id-cookie-name property.

Passing the selected service instance id as a cookie is currently only supported for the reactive @LoadBalanced Webclient, ReactorLoadBalancerExchangeFilterFunction and RetryableLoadBalancerExchangeFilterFunction.

28.8. Spring Cloud LoadBalancer Hints

Spring Cloud LoadBalancer lets you set String hints that are passed to the LoadBalancer within the Request object and that can later be used in ReactiveLoadBalancer implementations that can handle them.

You can set a default hint for all services by setting the value of the spring.cloud.loadbalancer.hint.default property. You can also set a specific value for any given service by setting the value of the spring.cloud.loadbalancer.hint.[SERVICE_ID] property, substituting [SERVICE_ID] with the correct ID of your service. If the hint is not set by the user, default is used.

28.9. Spring Cloud LoadBalancer Starter

We also provide a starter that allows you to easily add Spring Cloud LoadBalancer in a Spring Boot app. In order to use it, just add org.springframework.cloud:spring-cloud-starter-loadbalancer to your Spring Cloud dependencies in your build file.

Spring Cloud LoadBalancer starter includes Spring Boot Caching and Evictor.

28.10. Passing Your Own Spring Cloud LoadBalancer Configuration

You can also use the @LoadBalancerClient annotation to pass your own load-balancer client configuration, passing the name of the load-balancer client and the configuration class, as follows:

@Configuration
@LoadBalancerClient(value = "stores", configuration = CustomLoadBalancerConfiguration.class)
public class MyConfiguration {

    @Bean
    @LoadBalanced
    public WebClient.Builder loadBalancedWebClientBuilder() {
        return WebClient.builder();
    }
}
TIP

In order to make working on your own LoadBalancer configuration easier, we have added a builder() method to the ServiceInstanceListSupplier class.

TIP

You can also use our alternative predefined configurations in place of the default ones by setting the value of spring.cloud.loadbalancer.configurations property to zone-preference to use ZonePreferenceServiceInstanceListSupplier with caching or to health-check to use HealthCheckServiceInstanceListSupplier with caching.

You can use this feature to instantiate different implementations of ServiceInstanceListSupplier or ReactorLoadBalancer, either written by you, or provided by us as alternatives (for example ZonePreferenceServiceInstanceListSupplier) to override the default setup.

You can see an example of a custom configuration here.

The annotation value arguments (stores in the example above) specifies the service id of the service that we should send the requests to with the given custom configuration.

You can also pass multiple configurations (for more than one load-balancer client) through the @LoadBalancerClients annotation, as the following example shows:

@Configuration
@LoadBalancerClients({@LoadBalancerClient(value = "stores", configuration = StoresLoadBalancerClientConfiguration.class), @LoadBalancerClient(value = "customers", configuration = CustomersLoadBalancerClientConfiguration.class)})
public class MyConfiguration {

    @Bean
    @LoadBalanced
    public WebClient.Builder loadBalancedWebClientBuilder() {
        return WebClient.builder();
    }
}

28.11. Spring Cloud LoadBalancer Lifecycle

One type of bean that it may be useful to register using Custom LoadBalancer configuration is LoadBalancerLifecycle.

The LoadBalancerLifecycle beans provide callback methods, named onStart(Request<RC> request), onStartRequest(Request<RC> request, Response<T> lbResponse) and onComplete(CompletionContext<RES, T, RC> completionContext), that you should implement to specify what actions should take place before and after load-balancing.

onStart(Request<RC> request) takes a Request object as a parameter. It contains data that is used to select an appropriate instance, including the downstream client request and hint. onStartRequest also takes the Request object and, additionally, the Response<T> object as parameters. On the other hand, a CompletionContext object is provided to the onComplete(CompletionContext<RES, T, RC> completionContext) method. It contains the LoadBalancer Response, including the selected service instance, the Status of the request executed against that service instance and (if available) the response returned to the downstream client, and (if an exception has occurred) the corresponding Throwable.

The supports(Class requestContextClass, Class responseClass, Class serverTypeClass) method can be used to determine whether the processor in question handles objects of provided types. If not overridden by the user, it returns true.

In the preceding method calls, RC means RequestContext type, RES means client response type, and T means returned server type.

28.12. Spring Cloud LoadBalancer Statistics

We provide a LoadBalancerLifecycle bean called MicrometerStatsLoadBalancerLifecycle, which uses Micrometer to provide statistics for load-balanced calls.

In order to get this bean added to your application context, set the value of the spring.cloud.loadbalancer.stats.micrometer.enabled to true and have a MeterRegistry available (for example, by adding Spring Boot Actuator to your project).

MicrometerStatsLoadBalancerLifecycle registers the following meters in MeterRegistry:

  • loadbalancer.requests.active: A gauge that allows you to monitor the number of currently active requests for any service instance (service instance data available via tags);

  • loadbalancer.requests.success: A timer that measures the time of execution of any load-balanced requests that have ended in passing a response on to the underlying client;

  • loadbalancer.requests.failed: A timer that measures the time of execution of any load-balanced requests that have ended with an exception;

  • loadbalancer.requests.discard: A counter that measures the number of discarded load-balanced requests, i.e. requests where a service instance to run the request on has not been retrieved by the LoadBalancer.

Additional information regarding the service instances, request data, and response data is added to metrics via tags whenever available.

For some implementations, such as BlockingLoadBalancerClient, request and response data might not be available, as we establish generic types from arguments and might not be able to determine the types and read the data.
The meters are registered in the registry when at least one record is added for a given meter.
You can further configure the behavior of those metrics (for example, add publishing percentiles and histograms) by adding MeterFilters.

29. Spring Cloud Circuit Breaker

29.1. Introduction

Spring Cloud Circuit breaker provides an abstraction across different circuit breaker implementations. It provides a consistent API to use in your applications, letting you, the developer, choose the circuit breaker implementation that best fits your needs for your application.

29.1.1. Supported Implementations

Spring Cloud supports the following circuit-breaker implementations:

29.2. Core Concepts

To create a circuit breaker in your code, you can use the CircuitBreakerFactory API. When you include a Spring Cloud Circuit Breaker starter on your classpath, a bean that implements this API is automatically created for you. The following example shows a simple example of how to use this API:

@Service
public static class DemoControllerService {
    private RestTemplate rest;
    private CircuitBreakerFactory cbFactory;

    public DemoControllerService(RestTemplate rest, CircuitBreakerFactory cbFactory) {
        this.rest = rest;
        this.cbFactory = cbFactory;
    }

    public String slow() {
        return cbFactory.create("slow").run(() -> rest.getForObject("/slow", String.class), throwable -> "fallback");
    }

}

The CircuitBreakerFactory.create API creates an instance of a class called CircuitBreaker. The run method takes a Supplier and a Function. The Supplier is the code that you are going to wrap in a circuit breaker. The Function is the fallback that is run if the circuit breaker is tripped. The function is passed the Throwable that caused the fallback to be triggered. You can optionally exclude the fallback if you do not want to provide one.

29.2.1. Circuit Breakers In Reactive Code

If Project Reactor is on the class path, you can also use ReactiveCircuitBreakerFactory for your reactive code. The following example shows how to do so:

@Service
public static class DemoControllerService {
    private ReactiveCircuitBreakerFactory cbFactory;
    private WebClient webClient;


    public DemoControllerService(WebClient webClient, ReactiveCircuitBreakerFactory cbFactory) {
        this.webClient = webClient;
        this.cbFactory = cbFactory;
    }

    public Mono<String> slow() {
        return webClient.get().uri("/slow").retrieve().bodyToMono(String.class).transform(
        it -> cbFactory.create("slow").run(it, throwable -> return Mono.just("fallback")));
    }
}

The ReactiveCircuitBreakerFactory.create API creates an instance of a class called ReactiveCircuitBreaker. The run method takes a Mono or a Flux and wraps it in a circuit breaker. You can optionally profile a fallback Function, which will be called if the circuit breaker is tripped and is passed the Throwable that caused the failure.

29.3. Configuration

You can configure your circuit breakers by creating beans of type Customizer. The Customizer interface has a single method (called customize) that takes the Object to customize.

For detailed information on how to customize a given implementation see the following documentation:

Some CircuitBreaker implementations such as Resilience4JCircuitBreaker call customize method every time CircuitBreaker#run is called. It can be inefficient. In that case, you can use CircuitBreaker#once method. It is useful where calling customize many times doesn’t make sense, for example, in case of consuming Resilience4j’s events.

The following example shows the way for each io.github.resilience4j.circuitbreaker.CircuitBreaker to consume events.

Customizer.once(circuitBreaker -> {
  circuitBreaker.getEventPublisher()
    .onStateTransition(event -> log.info("{}: {}", event.getCircuitBreakerName(), event.getStateTransition()));
}, CircuitBreaker::getName)

30. CachedRandomPropertySource

Spring Cloud Context provides a PropertySource that caches random values based on a key. Outside of the caching functionality it works the same as Spring Boot’s RandomValuePropertySource. This random value might be useful in the case where you want a random value that is consistent even after the Spring Application context restarts. The property value takes the form of cachedrandom.[yourkey].[type] where yourkey is the key in the cache. The type value can be any type supported by Spring Boot’s RandomValuePropertySource.

myrandom=${cachedrandom.appname.value}

31. Security

31.1. Single Sign On

All of the OAuth2 SSO and resource server features moved to Spring Boot in version 1.3. You can find documentation in the Spring Boot user guide.

31.1.1. Client Token Relay

If your app is a user facing OAuth2 client (i.e. has declared @EnableOAuth2Sso or @EnableOAuth2Client) then it has an OAuth2ClientContext in request scope from Spring Boot. You can create your own OAuth2RestTemplate from this context and an autowired OAuth2ProtectedResourceDetails, and then the context will always forward the access token downstream, also refreshing the access token automatically if it expires. (These are features of Spring Security and Spring Boot.)

31.1.2. Resource Server Token Relay

If your app has @EnableResourceServer you might want to relay the incoming token downstream to other services. If you use a RestTemplate to contact the downstream services then this is just a matter of how to create the template with the right context.

If your service uses UserInfoTokenServices to authenticate incoming tokens (i.e. it is using the security.oauth2.user-info-uri configuration), then you can simply create an OAuth2RestTemplate using an autowired OAuth2ClientContext (it will be populated by the authentication process before it hits the backend code). Equivalently (with Spring Boot 1.4), you could inject a UserInfoRestTemplateFactory and grab its OAuth2RestTemplate in your configuration. For example:

MyConfiguration.java
@Bean
public OAuth2RestTemplate restTemplate(UserInfoRestTemplateFactory factory) {
    return factory.getUserInfoRestTemplate();
}

This rest template will then have the same OAuth2ClientContext (request-scoped) that is used by the authentication filter, so you can use it to send requests with the same access token.

If your app is not using UserInfoTokenServices but is still a client (i.e. it declares @EnableOAuth2Client or @EnableOAuth2Sso), then with Spring Security Cloud any OAuth2RestOperations that the user creates from an @Autowired OAuth2Context will also forward tokens. This feature is implemented by default as an MVC handler interceptor, so it only works in Spring MVC. If you are not using MVC you could use a custom filter or AOP interceptor wrapping an AccessTokenContextRelay to provide the same feature.

Here’s a basic example showing the use of an autowired rest template created elsewhere ("foo.com" is a Resource Server accepting the same tokens as the surrounding app):

MyController.java
@Autowired
private OAuth2RestOperations restTemplate;

@RequestMapping("/relay")
public String relay() {
    ResponseEntity<String> response =
      restTemplate.getForEntity("https://foo.com/bar", String.class);
    return "Success! (" + response.getBody() + ")";
}

If you don’t want to forward tokens (and that is a valid choice, since you might want to act as yourself, rather than the client that sent you the token), then you only need to create your own OAuth2Context instead of autowiring the default one.

Feign clients will also pick up an interceptor that uses the OAuth2ClientContext if it is available, so they should also do a token relay anywhere where a RestTemplate would.

32. Configuration Properties

To see the list of all Spring Cloud Commons related configuration properties please check the Appendix page.

Spring Cloud Config

2020.0.0-SNAPSHOT

Spring Cloud Config provides server-side and client-side support for externalized configuration in a distributed system. With the Config Server, you have a central place to manage external properties for applications across all environments. The concepts on both client and server map identically to the Spring Environment and PropertySource abstractions, so they fit very well with Spring applications but can be used with any application running in any language. As an application moves through the deployment pipeline from dev to test and into production, you can manage the configuration between those environments and be certain that applications have everything they need to run when they migrate. The default implementation of the server storage backend uses git, so it easily supports labelled versions of configuration environments as well as being accessible to a wide range of tooling for managing the content. It is easy to add alternative implementations and plug them in with Spring configuration.

33. Quick Start

This quick start walks through using both the server and the client of Spring Cloud Config Server.

First, start the server, as follows:

$ cd spring-cloud-config-server
$ ../mvnw spring-boot:run

The server is a Spring Boot application, so you can run it from your IDE if you prefer to do so (the main class is ConfigServerApplication).

Next try out a client, as follows:

$ curl localhost:8888/foo/development
{"name":"foo","label":"master","propertySources":[
  {"name":"https://github.com/scratches/config-repo/foo-development.properties","source":{"bar":"spam"}},
  {"name":"https://github.com/scratches/config-repo/foo.properties","source":{"foo":"bar"}}
]}

The default strategy for locating property sources is to clone a git repository (at spring.cloud.config.server.git.uri) and use it to initialize a mini SpringApplication. The mini-application’s Environment is used to enumerate property sources and publish them at a JSON endpoint.

The HTTP service has resources in the following form:

/{application}/{profile}[/{label}]
/{application}-{profile}.yml
/{label}/{application}-{profile}.yml
/{application}-{profile}.properties
/{label}/{application}-{profile}.properties

where application is injected as the spring.config.name in the SpringApplication (what is normally application in a regular Spring Boot app), profile is an active profile (or comma-separated list of properties), and label is an optional git label (defaults to master.)

Spring Cloud Config Server pulls configuration for remote clients from various sources. The following example gets configuration from a git repository (which must be provided), as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo

Other sources are any JDBC compatible database, Subversion, Hashicorp Vault, Credhub and local filesystems.

33.1. Client Side Usage

To use these features in an application, you can build it as a Spring Boot application that depends on spring-cloud-config-client (for an example, see the test cases for the config-client or the sample application). The most convenient way to add the dependency is with a Spring Boot starter org.springframework.cloud:spring-cloud-starter-config. There is also a parent pom and BOM (spring-cloud-starter-parent) for Maven users and a Spring IO version management properties file for Gradle and Spring CLI users. The following example shows a typical Maven configuration:

pom.xml
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>{spring-boot-docs-version}</version>
    <relativePath /> <!-- lookup parent from repository -->
</parent>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>{spring-cloud-version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-config</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

<!-- repositories also needed for snapshots and milestones -->

Now you can create a standard Spring Boot application, such as the following HTTP server:

@SpringBootApplication
@RestController
public class Application {

    @RequestMapping("/")
    public String home() {
        return "Hello World!";
    }

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}

When this HTTP server runs, it picks up the external configuration from the default local config server (if it is running) on port 8888. To modify the startup behavior, you can change the location of the config server by using application.properties as shown in the following example:

spring.config.import=optional:configserver:http://myconfigserver.com

By default, if no application name is set, application will be used. To modify the name, the following property can be added to the application.properties file:

spring.application.name: myapp
When setting the property ${spring.application.name} do not prefix your app name with the reserved word application- to prevent issues resolving the correct property source.

The Config Server properties show up in the /env endpoint as a high-priority property source, as shown in the following example.

$ curl localhost:8080/env
{
  "activeProfiles": [],
  {
    "name": "servletContextInitParams",
    "properties": {}
  },
  {
    "name": "configserver:https://github.com/spring-cloud-samples/config-repo/foo.properties",
    "properties": {
      "foo": {
        "value": "bar",
        "origin": "Config Server https://github.com/spring-cloud-samples/config-repo/foo.properties:2:12"
      }
    }
  },
  ...
}

A property source called configserver:<URL of remote repository>/<file name> contains the foo property with a value of bar.

The URL in the property source name is the git repository, not the config server URL.

34. Spring Cloud Config Server

Spring Cloud Config Server provides an HTTP resource-based API for external configuration (name-value pairs or equivalent YAML content). The server is embeddable in a Spring Boot application, by using the @EnableConfigServer annotation. Consequently, the following application is a config server:

ConfigServer.java
@SpringBootApplication
@EnableConfigServer
public class ConfigServer {
  public static void main(String[] args) {
    SpringApplication.run(ConfigServer.class, args);
  }
}

Like all Spring Boot applications, it runs on port 8080 by default, but you can switch it to the more conventional port 8888 in various ways. The easiest, which also sets a default configuration repository, is by launching it with spring.config.name=configserver (there is a configserver.yml in the Config Server jar). Another is to use your own application.properties, as shown in the following example:

application.properties
server.port: 8888
spring.cloud.config.server.git.uri: file://${user.home}/config-repo

where ${user.home}/config-repo is a git repository containing YAML and properties files.

On Windows, you need an extra "/" in the file URL if it is absolute with a drive prefix (for example,/${user.home}/config-repo).

The following listing shows a recipe for creating the git repository in the preceding example:

$ cd $HOME
$ mkdir config-repo
$ cd config-repo
$ git init .
$ echo info.foo: bar > application.properties
$ git add -A .
$ git commit -m "Add application.properties"
Using the local filesystem for your git repository is intended for testing only. You should use a server to host your configuration repositories in production.
The initial clone of your configuration repository can be quick and efficient if you keep only text files in it. If you store binary files, especially large ones, you may experience delays on the first request for configuration or encounter out of memory errors in the server.

34.1. Environment Repository

Where should you store the configuration data for the Config Server? The strategy that governs this behaviour is the EnvironmentRepository, serving Environment objects. This Environment is a shallow copy of the domain from the Spring Environment (including propertySources as the main feature). The Environment resources are parametrized by three variables:

  • {application}, which maps to spring.application.name on the client side.

  • {profile}, which maps to spring.profiles.active on the client (comma-separated list).

  • {label}, which is a server side feature labelling a "versioned" set of config files.

Repository implementations generally behave like a Spring Boot application, loading configuration files from a spring.config.name equal to the {application} parameter, and spring.profiles.active equal to the {profiles} parameter. Precedence rules for profiles are also the same as in a regular Spring Boot application: Active profiles take precedence over defaults, and, if there are multiple profiles, the last one wins (similar to adding entries to a Map).

The following sample client application has this bootstrap configuration:

spring:
  application:
    name: foo
  profiles:
    active: dev,mysql

(As usual with a Spring Boot application, these properties could also be set by environment variables or command line arguments).

If the repository is file-based, the server creates an Environment from application.yml (shared between all clients) and foo.yml (with foo.yml taking precedence). If the YAML files have documents inside them that point to Spring profiles, those are applied with higher precedence (in order of the profiles listed). If there are profile-specific YAML (or properties) files, these are also applied with higher precedence than the defaults. Higher precedence translates to a PropertySource listed earlier in the Environment. (These same rules apply in a standalone Spring Boot application.)

You can set spring.cloud.config.server.accept-empty to false so that Server would return a HTTP 404 status, if the application is not found.By default, this flag is set to true.

34.1.1. Git Backend

The default implementation of EnvironmentRepository uses a Git backend, which is very convenient for managing upgrades and physical environments and for auditing changes. To change the location of the repository, you can set the spring.cloud.config.server.git.uri configuration property in the Config Server (for example in application.yml). If you set it with a file: prefix, it should work from a local repository so that you can get started quickly and easily without a server. However, in that case, the server operates directly on the local repository without cloning it (it does not matter if it is not bare because the Config Server never makes changes to the "remote" repository). To scale the Config Server up and make it highly available, you need to have all instances of the server pointing to the same repository, so only a shared file system would work. Even in that case, it is better to use the ssh: protocol for a shared filesystem repository, so that the server can clone it and use a local working copy as a cache.

This repository implementation maps the {label} parameter of the HTTP resource to a git label (commit id, branch name, or tag). If the git branch or tag name contains a slash (/), then the label in the HTTP URL should instead be specified with the special string (_) (to avoid ambiguity with other URL paths). For example, if the label is foo/bar, replacing the slash would result in the following label: foo(_)bar. The inclusion of the special string (_) can also be applied to the {application} parameter. If you use a command-line client such as curl, be careful with the brackets in the URL — you should escape them from the shell with single quotes ('').

Skipping SSL Certificate Validation

The configuration server’s validation of the Git server’s SSL certificate can be disabled by setting the git.skipSslValidation property to true (default is false).

spring:
  cloud:
    config:
      server:
        git:
          uri: https://example.com/my/repo
          skipSslValidation: true
Setting HTTP Connection Timeout

You can configure the time, in seconds, that the configuration server will wait to acquire an HTTP connection. Use the git.timeout property.

spring:
  cloud:
    config:
      server:
        git:
          uri: https://example.com/my/repo
          timeout: 4
Placeholders in Git URI

Spring Cloud Config Server supports a git repository URL with placeholders for the {application} and {profile} (and {label} if you need it, but remember that the label is applied as a git label anyway). So you can support a “one repository per application” policy by using a structure similar to the following:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/myorg/{application}

You can also support a “one repository per profile” policy by using a similar pattern but with {profile}.

Additionally, using the special string "(_)" within your {application} parameters can enable support for multiple organizations, as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/{application}

where {application} is provided at request time in the following format: organization(_)application.

Pattern Matching and Multiple Repositories

Spring Cloud Config also includes support for more complex requirements with pattern matching on the application and profile name. The pattern format is a comma-separated list of {application}/{profile} names with wildcards (note that a pattern beginning with a wildcard may need to be quoted), as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          repos:
            simple: https://github.com/simple/config-repo
            special:
              pattern: special*/dev*,*special*/dev*
              uri: https://github.com/special/config-repo
            local:
              pattern: local*
              uri: file:/home/configsvc/config-repo

If {application}/{profile} does not match any of the patterns, it uses the default URI defined under spring.cloud.config.server.git.uri. In the above example, for the “simple” repository, the pattern is simple/* (it only matches one application named simple in all profiles). The “local” repository matches all application names beginning with local in all profiles (the /* suffix is added automatically to any pattern that does not have a profile matcher).

The “one-liner” short cut used in the “simple” example can be used only if the only property to be set is the URI. If you need to set anything else (credentials, pattern, and so on) you need to use the full form.

The pattern property in the repo is actually an array, so you can use a YAML array (or [0], [1], etc. suffixes in properties files) to bind to multiple patterns. You may need to do so if you are going to run apps with multiple profiles, as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          repos:
            development:
              pattern:
                - '*/development'
                - '*/staging'
              uri: https://github.com/development/config-repo
            staging:
              pattern:
                - '*/qa'
                - '*/production'
              uri: https://github.com/staging/config-repo
Spring Cloud guesses that a pattern containing a profile that does not end in * implies that you actually want to match a list of profiles starting with this pattern (so */staging is a shortcut for ["*/staging", "*/staging,*"], and so on). This is common where, for instance, you need to run applications in the “development” profile locally but also the “cloud” profile remotely.

Every repository can also optionally store config files in sub-directories, and patterns to search for those directories can be specified as searchPaths. The following example shows a config file at the top level:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          searchPaths: foo,bar*

In the preceding example, the server searches for config files in the top level and in the foo/ sub-directory and also any sub-directory whose name begins with bar.

By default, the server clones remote repositories when configuration is first requested. The server can be configured to clone the repositories at startup, as shown in the following top-level example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://git/common/config-repo.git
          repos:
            team-a:
                pattern: team-a-*
                cloneOnStart: true
                uri: https://git/team-a/config-repo.git
            team-b:
                pattern: team-b-*
                cloneOnStart: false
                uri: https://git/team-b/config-repo.git
            team-c:
                pattern: team-c-*
                uri: https://git/team-a/config-repo.git

In the preceding example, the server clones team-a’s config-repo on startup, before it accepts any requests. All other repositories are not cloned until configuration from the repository is requested.

Setting a repository to be cloned when the Config Server starts up can help to identify a misconfigured configuration source (such as an invalid repository URI) quickly, while the Config Server is starting up. With cloneOnStart not enabled for a configuration source, the Config Server may start successfully with a misconfigured or invalid configuration source and not detect an error until an application requests configuration from that configuration source.
Authentication

To use HTTP basic authentication on the remote repository, add the username and password properties separately (not in the URL), as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          username: trolley
          password: strongpassword

If you do not use HTTPS and user credentials, SSH should also work out of the box when you store keys in the default directories (~/.ssh) and the URI points to an SSH location, such as [email protected]:configuration/cloud-configuration. It is important that an entry for the Git server be present in the ~/.ssh/known_hosts file and that it is in ssh-rsa format. Other formats (such as ecdsa-sha2-nistp256) are not supported. To avoid surprises, you should ensure that only one entry is present in the known_hosts file for the Git server and that it matches the URL you provided to the config server. If you use a hostname in the URL, you want to have exactly that (not the IP) in the known_hosts file. The repository is accessed by using JGit, so any documentation you find on that should be applicable. HTTPS proxy settings can be set in ~/.git/config or (in the same way as for any other JVM process) with system properties (-Dhttps.proxyHost and -Dhttps.proxyPort).

If you do not know where your ~/.git directory is, use git config --global to manipulate the settings (for example, git config --global http.sslVerify false).

JGit requires RSA keys in PEM format. Below is an example ssh-keygen (from openssh) command that will generate a key in the corect format:

ssh-keygen -m PEM -t rsa -b 4096 -f ~/config_server_deploy_key.rsa

Warning: When working with SSH keys, the expected ssh private-key must begin with -----BEGIN RSA PRIVATE KEY-----. If the key starts with -----BEGIN OPENSSH PRIVATE KEY----- then the RSA key will not load when spring-cloud-config server is started. The error looks like:

- Error in object 'spring.cloud.config.server.git': codes [PrivateKeyIsValid.spring.cloud.config.server.git,PrivateKeyIsValid]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [spring.cloud.config.server.git.,]; arguments []; default message []]; default message [Property 'spring.cloud.config.server.git.privateKey' is not a valid private key]

To correct the above error the RSA key must be converted to PEM format. An example using openssh is provided above for generating a new key in the appropriate format.

Authentication with AWS CodeCommit

Spring Cloud Config Server also supports AWS CodeCommit authentication. AWS CodeCommit uses an authentication helper when using Git from the command line. This helper is not used with the JGit library, so a JGit CredentialProvider for AWS CodeCommit is created if the Git URI matches the AWS CodeCommit pattern. AWS CodeCommit URIs follow this pattern://git-codecommit.${AWS_REGION}.amazonaws.com/${repopath}.

If you provide a username and password with an AWS CodeCommit URI, they must be the AWS accessKeyId and secretAccessKey that provide access to the repository. If you do not specify a username and password, the accessKeyId and secretAccessKey are retrieved by using the AWS Default Credential Provider Chain.

If your Git URI matches the CodeCommit URI pattern (shown earlier), you must provide valid AWS credentials in the username and password or in one of the locations supported by the default credential provider chain. AWS EC2 instances may use IAM Roles for EC2 Instances.

The aws-java-sdk-core jar is an optional dependency. If the aws-java-sdk-core jar is not on your classpath, the AWS Code Commit credential provider is not created, regardless of the git server URI.
Authentication with Google Cloud Source

Spring Cloud Config Server also supports authenticating against Google Cloud Source repositories.

If your Git URI uses the http or https protocol and the domain name is source.developers.google.com, the Google Cloud Source credentials provider will be used. A Google Cloud Source repository URI has the format source.developers.google.com/p/${GCP_PROJECT}/r/${REPO}. To obtain the URI for your repository, click on "Clone" in the Google Cloud Source UI, and select "Manually generated credentials". Do not generate any credentials, simply copy the displayed URI.

The Google Cloud Source credentials provider will use Google Cloud Platform application default credentials. See Google Cloud SDK documentation on how to create application default credentials for a system. This approach will work for user accounts in dev environments and for service accounts in production environments.

com.google.auth:google-auth-library-oauth2-http is an optional dependency. If the google-auth-library-oauth2-http jar is not on your classpath, the Google Cloud Source credential provider is not created, regardless of the git server URI.
Git SSH configuration using properties

By default, the JGit library used by Spring Cloud Config Server uses SSH configuration files such as ~/.ssh/known_hosts and /etc/ssh/ssh_config when connecting to Git repositories by using an SSH URI. In cloud environments such as Cloud Foundry, the local filesystem may be ephemeral or not easily accessible. For those cases, SSH configuration can be set by using Java properties. In order to activate property-based SSH configuration, the spring.cloud.config.server.git.ignoreLocalSshSettings property must be set to true, as shown in the following example:

  spring:
    cloud:
      config:
        server:
          git:
            uri: [email protected]:team/repo1.git
            ignoreLocalSshSettings: true
            hostKey: someHostKey
            hostKeyAlgorithm: ssh-rsa
            privateKey: |
                         -----BEGIN RSA PRIVATE KEY-----
                         MIIEpgIBAAKCAQEAx4UbaDzY5xjW6hc9jwN0mX33XpTDVW9WqHp5AKaRbtAC3DqX
                         IXFMPgw3K45jxRb93f8tv9vL3rD9CUG1Gv4FM+o7ds7FRES5RTjv2RT/JVNJCoqF
                         ol8+ngLqRZCyBtQN7zYByWMRirPGoDUqdPYrj2yq+ObBBNhg5N+hOwKjjpzdj2Ud
                         1l7R+wxIqmJo1IYyy16xS8WsjyQuyC0lL456qkd5BDZ0Ag8j2X9H9D5220Ln7s9i
                         oezTipXipS7p7Jekf3Ywx6abJwOmB0rX79dV4qiNcGgzATnG1PkXxqt76VhcGa0W
                         DDVHEEYGbSQ6hIGSh0I7BQun0aLRZojfE3gqHQIDAQABAoIBAQCZmGrk8BK6tXCd
                         fY6yTiKxFzwb38IQP0ojIUWNrq0+9Xt+NsypviLHkXfXXCKKU4zUHeIGVRq5MN9b
                         BO56/RrcQHHOoJdUWuOV2qMqJvPUtC0CpGkD+valhfD75MxoXU7s3FK7yjxy3rsG
                         EmfA6tHV8/4a5umo5TqSd2YTm5B19AhRqiuUVI1wTB41DjULUGiMYrnYrhzQlVvj
                         5MjnKTlYu3V8PoYDfv1GmxPPh6vlpafXEeEYN8VB97e5x3DGHjZ5UrurAmTLTdO8
                         +AahyoKsIY612TkkQthJlt7FJAwnCGMgY6podzzvzICLFmmTXYiZ/28I4BX/mOSe
                         pZVnfRixAoGBAO6Uiwt40/PKs53mCEWngslSCsh9oGAaLTf/XdvMns5VmuyyAyKG
                         ti8Ol5wqBMi4GIUzjbgUvSUt+IowIrG3f5tN85wpjQ1UGVcpTnl5Qo9xaS1PFScQ
                         xrtWZ9eNj2TsIAMp/svJsyGG3OibxfnuAIpSXNQiJPwRlW3irzpGgVx/AoGBANYW
                         dnhshUcEHMJi3aXwR12OTDnaLoanVGLwLnkqLSYUZA7ZegpKq90UAuBdcEfgdpyi
                         PhKpeaeIiAaNnFo8m9aoTKr+7I6/uMTlwrVnfrsVTZv3orxjwQV20YIBCVRKD1uX
                         VhE0ozPZxwwKSPAFocpyWpGHGreGF1AIYBE9UBtjAoGBAI8bfPgJpyFyMiGBjO6z
                         FwlJc/xlFqDusrcHL7abW5qq0L4v3R+FrJw3ZYufzLTVcKfdj6GelwJJO+8wBm+R
                         gTKYJItEhT48duLIfTDyIpHGVm9+I1MGhh5zKuCqIhxIYr9jHloBB7kRm0rPvYY4
                         VAykcNgyDvtAVODP+4m6JvhjAoGBALbtTqErKN47V0+JJpapLnF0KxGrqeGIjIRV
                         cYA6V4WYGr7NeIfesecfOC356PyhgPfpcVyEztwlvwTKb3RzIT1TZN8fH4YBr6Ee
                         KTbTjefRFhVUjQqnucAvfGi29f+9oE3Ei9f7wA+H35ocF6JvTYUsHNMIO/3gZ38N
                         CPjyCMa9AoGBAMhsITNe3QcbsXAbdUR00dDsIFVROzyFJ2m40i4KCRM35bC/BIBs
                         q0TY3we+ERB40U8Z2BvU61QuwaunJ2+uGadHo58VSVdggqAo0BSkH58innKKt96J
                         69pcVH/4rmLbXdcmNYGm6iu+MlPQk4BUZknHSmVHIFdJ0EPupVaQ8RHT
                         -----END RSA PRIVATE KEY-----

The following table describes the SSH configuration properties.

Table 2. SSH Configuration Properties
Property Name Remarks

ignoreLocalSshSettings

If true, use property-based instead of file-based SSH config. Must be set at as spring.cloud.config.server.git.ignoreLocalSshSettings, not inside a repository definition.

privateKey

Valid SSH private key. Must be set if ignoreLocalSshSettings is true and Git URI is SSH format.

hostKey

Valid SSH host key. Must be set if hostKeyAlgorithm is also set.

hostKeyAlgorithm

One of ssh-dss, ssh-rsa, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, or ecdsa-sha2-nistp521. Must be set if hostKey is also set.

strictHostKeyChecking

true or false. If false, ignore errors with host key.

knownHostsFile

Location of custom .known_hosts file.

preferredAuthentications

Override server authentication method order. This should allow for evading login prompts if server has keyboard-interactive authentication before the publickey method.

Placeholders in Git Search Paths

Spring Cloud Config Server also supports a search path with placeholders for the {application} and {profile} (and {label} if you need it), as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          searchPaths: '{application}'

The preceding listing causes a search of the repository for files in the same name as the directory (as well as the top level). Wildcards are also valid in a search path with placeholders (any matching directory is included in the search).

Force pull in Git Repositories

As mentioned earlier, Spring Cloud Config Server makes a clone of the remote git repository in case the local copy gets dirty (for example, folder content changes by an OS process) such that Spring Cloud Config Server cannot update the local copy from remote repository.

To solve this issue, there is a force-pull property that makes Spring Cloud Config Server force pull from the remote repository if the local copy is dirty, as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          force-pull: true

If you have a multiple-repositories configuration, you can configure the force-pull property per repository, as shown in the following example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://git/common/config-repo.git
          force-pull: true
          repos:
            team-a:
                pattern: team-a-*
                uri: https://git/team-a/config-repo.git
                force-pull: true
            team-b:
                pattern: team-b-*
                uri: https://git/team-b/config-repo.git
                force-pull: true
            team-c:
                pattern: team-c-*
                uri: https://git/team-a/config-repo.git
The default value for force-pull property is false.
Deleting untracked branches in Git Repositories

As Spring Cloud Config Server has a clone of the remote git repository after check-outing branch to local repo (e.g fetching properties by label) it will keep this branch forever or till the next server restart (which creates new local repo). So there could be a case when remote branch is deleted but local copy of it is still available for fetching. And if Spring Cloud Config Server client service starts with --spring.cloud.config.label=deletedRemoteBranch,master it will fetch properties from deletedRemoteBranch local branch, but not from master.

In order to keep local repository branches clean and up to remote - deleteUntrackedBranches property could be set. It will make Spring Cloud Config Server force delete untracked branches from local repository. Example:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          deleteUntrackedBranches: true
The default value for deleteUntrackedBranches property is false.
Git Refresh Rate

You can control how often the config server will fetch updated configuration data from your Git backend by using spring.cloud.config.server.git.refreshRate. The value of this property is specified in seconds. By default the value is 0, meaning the config server will fetch updated configuration from the Git repo every time it is requested.

34.1.2. Version Control Backend Filesystem Use

With VCS-based backends (git, svn), files are checked out or cloned to the local filesystem. By default, they are put in the system temporary directory with a prefix of config-repo-. On linux, for example, it could be /tmp/config-repo-<randomid>. Some operating systems routinely clean out temporary directories. This can lead to unexpected behavior, such as missing properties. To avoid this problem, change the directory that Config Server uses by setting spring.cloud.config.server.git.basedir or spring.cloud.config.server.svn.basedir to a directory that does not reside in the system temp structure.

34.1.3. File System Backend

There is also a “native” profile in the Config Server that does not use Git but loads the config files from the local classpath or file system (any static URL you want to point to with spring.cloud.config.server.native.searchLocations). To use the native profile, launch the Config Server with spring.profiles.active=native.

Remember to use the file: prefix for file resources (the default without a prefix is usually the classpath). As with any Spring Boot configuration, you can embed ${}-style environment placeholders, but remember that absolute paths in Windows require an extra / (for example, /${user.home}/config-repo).
The default value of the searchLocations is identical to a local Spring Boot application (that is, [classpath:/, classpath:/config, file:./, file:./config]). This does not expose the application.properties from the server to all clients, because any property sources present in the server are removed before being sent to the client.
A filesystem backend is great for getting started quickly and for testing. To use it in production, you need to be sure that the file system is reliable and shared across all instances of the Config Server.

The search locations can contain placeholders for {application}, {profile}, and {label}. In this way, you can segregate the directories in the path and choose a strategy that makes sense for you (such as subdirectory per application or subdirectory per profile).

If you do not use placeholders in the search locations, this repository also appends the {label} parameter of the HTTP resource to a suffix on the search path, so properties files are loaded from each search location and a subdirectory with the same name as the label (the labelled properties take precedence in the Spring Environment). Thus, the default behaviour with no placeholders is the same as adding a search location ending with /{label}/. For example, file:/tmp/config is the same as file:/tmp/config,file:/tmp/config/{label}. This behavior can be disabled by setting spring.cloud.config.server.native.addLabelLocations=false.

34.1.4. Vault Backend

Spring Cloud Config Server also supports Vault as a backend.

Vault is a tool for securely accessing secrets. A secret is anything that to which you want to tightly control access, such as API keys, passwords, certificates, and other sensitive information. Vault provides a unified interface to any secret while providing tight access control and recording a detailed audit log.

For more information on Vault, see the Vault quick start guide.

To enable the config server to use a Vault backend, you can run your config server with the vault profile. For example, in your config server’s application.properties, you can add spring.profiles.active=vault.

By default, the config server assumes that your Vault server runs at 127.0.0.1:8200. It also assumes that the name of backend is secret and the key is application. All of these defaults can be configured in your config server’s application.properties. The following table describes configurable Vault properties:

Name Default Value

host

127.0.0.1

port

8200

scheme

http

backend

secret

defaultKey

application

profileSeparator

,

kvVersion

1

skipSslValidation

false

timeout

5

namespace

null

All of the properties in the preceding table must be prefixed with spring.cloud.config.server.vault or placed in the correct Vault section of a composite configuration.

All configurable properties can be found in org.springframework.cloud.config.server.environment.VaultEnvironmentProperties.

Vault 0.10.0 introduced a versioned key-value backend (k/v backend version 2) that exposes a different API than earlier versions, it now requires a data/ between the mount path and the actual context path and wraps secrets in a data object. Setting spring.cloud.config.server.vault.kv-version=2 will take this into account.

Optionally, there is support for the Vault Enterprise X-Vault-Namespace header. To have it sent to Vault set the namespace property.

With your config server running, you can make HTTP requests to the server to retrieve values from the Vault backend. To do so, you need a token for your Vault server.

First, place some data in you Vault, as shown in the following example:

$ vault kv put secret/application foo=bar baz=bam
$ vault kv put secret/myapp foo=myappsbar

Second, make an HTTP request to your config server to retrieve the values, as shown in the following example:

$ curl -X "GET" "http://localhost:8888/myapp/default" -H "X-Config-Token: yourtoken"

You should see a response similar to the following:

{
   "name":"myapp",
   "profiles":[
      "default"
   ],
   "label":null,
   "version":null,
   "state":null,
   "propertySources":[
      {
         "name":"vault:myapp",
         "source":{
            "foo":"myappsbar"
         }
      },
      {
         "name":"vault:application",
         "source":{
            "baz":"bam",
            "foo":"bar"
         }
      }
   ]
}

The default way for a client to provide the necessary authentication to let Config Server talk to Vault is to set the X-Config-Token header. However, you can instead omit the header and configure the authentication in the server, by setting the same configuration properties as Spring Cloud Vault. The property to set is spring.cloud.config.server.vault.authentication. It should be set to one of the supported authentication methods. You may also need to set other properties specific to the authentication method you use, by using the same property names as documented for spring.cloud.vault but instead using the spring.cloud.config.server.vault prefix. See the Spring Cloud Vault Reference Guide for more detail.

If you omit the X-Config-Token header and use a server property to set the authentication, the Config Server application needs an additional dependency on Spring Vault to enable the additional authentication options. See the Spring Vault Reference Guide for how to add that dependency.
Multiple Properties Sources

When using Vault, you can provide your applications with multiple properties sources. For example, assume you have written data to the following paths in Vault:

secret/myApp,dev
secret/myApp
secret/application,dev
secret/application

Properties written to secret/application are available to all applications using the Config Server. An application with the name, myApp, would have any properties written to secret/myApp and secret/application available to it. When myApp has the dev profile enabled, properties written to all of the above paths would be available to it, with properties in the first path in the list taking priority over the others.

34.1.5. Accessing Backends Through a Proxy

The configuration server can access a Git or Vault backend through an HTTP or HTTPS proxy. This behavior is controlled for either Git or Vault by settings under proxy.http and proxy.https. These settings are per repository, so if you are using a composite environment repository you must configure proxy settings for each backend in the composite individually. If using a network which requires separate proxy servers for HTTP and HTTPS URLs, you can configure both the HTTP and the HTTPS proxy settings for a single backend.

The following table describes the proxy configuration properties for both HTTP and HTTPS proxies. All of these properties must be prefixed by proxy.http or proxy.https.

Table 3. Proxy Configuration Properties
Property Name Remarks

host

The host of the proxy.

port

The port with which to access the proxy.

nonProxyHosts

Any hosts which the configuration server should access outside the proxy. If values are provided for both proxy.http.nonProxyHosts and proxy.https.nonProxyHosts, the proxy.http value will be used.

username

The username with which to authenticate to the proxy. If values are provided for both proxy.http.username and proxy.https.username, the proxy.http value will be used.

password

The password with which to authenticate to the proxy. If values are provided for both proxy.http.password and proxy.https.password, the proxy.http value will be used.

The following configuration uses an HTTPS proxy to access a Git repository.

spring:
  profiles:
    active: git
  cloud:
    config:
      server:
        git:
          uri: https://github.com/spring-cloud-samples/config-repo
          proxy:
            https:
              host: my-proxy.host.io
              password: myproxypassword
              port: '3128'
              username: myproxyusername
              nonProxyHosts: example.com

34.1.6. Sharing Configuration With All Applications

Sharing configuration between all applications varies according to which approach you take, as described in the following topics:

File Based Repositories

With file-based (git, svn, and native) repositories, resources with file names in application* (application.properties, application.yml, application-*.properties, and so on) are shared between all client applications. You can use resources with these file names to configure global defaults and have them be overridden by application-specific files as necessary.

The property overrides feature can also be used for setting global defaults, with placeholders applications allowed to override them locally.

With the “native” profile (a local file system backend) , you should use an explicit search location that is not part of the server’s own configuration. Otherwise, the application* resources in the default search locations get removed because they are part of the server.
Vault Server

When using Vault as a backend, you can share configuration with all applications by placing configuration in secret/application. For example, if you run the following Vault command, all applications using the config server will have the properties foo and baz available to them:

$ vault write secret/application foo=bar baz=bam
CredHub Server

When using CredHub as a backend, you can share configuration with all applications by placing configuration in /application/ or by placing it in the default profile for the application. For example, if you run the following CredHub command, all applications using the config server will have the properties shared.color1 and shared.color2 available to them:

credhub set --name "/application/profile/master/shared" --type=json
value: {"shared.color1": "blue", "shared.color": "red"}
credhub set --name "/my-app/default/master/more-shared" --type=json
value: {"shared.word1": "hello", "shared.word2": "world"}

34.1.7. JDBC Backend

Spring Cloud Config Server supports JDBC (relational database) as a backend for configuration properties. You can enable this feature by adding spring-jdbc to the classpath and using the jdbc profile or by adding a bean of type JdbcEnvironmentRepository. If you include the right dependencies on the classpath (see the user guide for more details on that), Spring Boot configures a data source.

You can disable autoconfiguration for JdbcEnvironmentRepository by setting the spring.cloud.config.server.jdbc.enabled property to false.

The database needs to have a table called PROPERTIES with columns called APPLICATION, PROFILE, and LABEL (with the usual Environment meaning), plus KEY and VALUE for the key and value pairs in Properties style. All fields are of type String in Java, so you can make them VARCHAR of whatever length you need. Property values behave in the same way as they would if they came from Spring Boot properties files named {application}-{profile}.properties, including all the encryption and decryption, which will be applied as post-processing steps (that is, not in the repository implementation directly).

34.1.8. Redis Backend

Spring Cloud Config Server supports Redis as a backend for configuration properties. You can enable this feature by adding a dependency to Spring Data Redis.

pom.xml
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-redis</artifactId>
    </dependency>
</dependencies>

The following configuration uses Spring Data RedisTemplate to access a Redis. We can use spring.redis.* properties to override default connection settings.

spring:
  profiles:
    active: redis
  redis:
    host: redis
    port: 16379

The properties should be stored as fields in a hash. The name of hash should be the same as spring.application.name property or conjunction of spring.application.name and spring.profiles.active[n].

HMSET sample-app server.port "8100" sample.topic.name "test" test.property1 "property1"

After running the command visible above a hash should contain the following keys with values:

HGETALL sample-app
{
  "server.port": "8100",
  "sample.topic.name": "test",
  "test.property1": "property1"
}
When no profile is specified default will be used.

34.1.9. AWS S3 Backend

Spring Cloud Config Server supports AWS S3 as a backend for configuration properties. You can enable this feature by adding a dependency to the AWS Java SDK For Amazon S3.

pom.xml
<dependencies>
    <dependency>
        <groupId>com.amazonaws</groupId>
        <artifactId>aws-java-sdk-s3</artifactId>
    </dependency>
</dependencies>

The following configuration uses the AWS S3 client to access configuration files. We can use spring.awss3.* properties to select the bucket where your configuration is stored.

spring:
  profiles:
    active: awss3
  cloud:
    config:
      server:
        awss3:
          region: us-east-1
          bucket: bucket1

It is also possible to specify an AWS URL to override the standard endpoint of your S3 service with spring.awss3.endpoint. This allows support for beta regions of S3, and other S3 compatible storage APIs.

Credentials are found using the Default AWS Credential Provider Chain. Versioned and encrypted buckets are supported without further configuration.

Configuration files are stored in your bucket as {application}-{profile}.properties, {application}-{profile}.yml or {application}-{profile}.json. An optional label can be provided to specify a directory path to the file.

When no profile is specified default will be used.

34.1.10. CredHub Backend

Spring Cloud Config Server supports CredHub as a backend for configuration properties. You can enable this feature by adding a dependency to Spring CredHub.

pom.xml
<dependencies>
    <dependency>
        <groupId>org.springframework.credhub</groupId>
        <artifactId>spring-credhub-starter</artifactId>
    </dependency>
</dependencies>

The following configuration uses mutual TLS to access a CredHub:

spring:
  profiles:
    active: credhub
  cloud:
    config:
      server:
        credhub:
          url: https://credhub:8844

The properties should be stored as JSON, such as:

credhub set --name "/demo-app/default/master/toggles" --type=json
value: {"toggle.button": "blue", "toggle.link": "red"}
credhub set --name "/demo-app/default/master/abs" --type=json
value: {"marketing.enabled": true, "external.enabled": false}

All client applications with the name spring.cloud.config.name=demo-app will have the following properties available to them:

{
    toggle.button: "blue",
    toggle.link: "red",
    marketing.enabled: true,
    external.enabled: false
}
When no profile is specified default will be used and when no label is specified master will be used as a default value. NOTE: Values added to application will be shared by all the applications.
OAuth 2.0

You can authenticate with OAuth 2.0 using UAA as a provider.

pom.xml
<dependencies>
    <dependency>
        <groupId>org.springframework.security</groupId>
        <artifactId>spring-security-config</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.security</groupId>
        <artifactId>spring-security-oauth2-client</artifactId>
    </dependency>
</dependencies>

The following configuration uses OAuth 2.0 and UAA to access a CredHub:

spring:
  profiles:
    active: credhub
  cloud:
    config:
      server:
        credhub:
          url: https://credhub:8844
          oauth2:
            registration-id: credhub-client
  security:
    oauth2:
      client:
        registration:
          credhub-client:
            provider: uaa
            client-id: credhub_config_server
            client-secret: asecret
            authorization-grant-type: client_credentials
        provider:
          uaa:
            token-uri: https://uaa:8443/oauth/token
The used UAA client-id should have credhub.read as scope.

34.1.11. Composite Environment Repositories

In some scenarios, you may wish to pull configuration data from multiple environment repositories. To do so, you can enable the composite profile in your configuration server’s application properties or YAML file. If, for example, you want to pull configuration data from a Subversion repository as well as two Git repositories, you can set the following properties for your configuration server:

spring:
  profiles:
    active: composite
  cloud:
    config:
      server:
        composite:
        -
          type: svn
          uri: file:///path/to/svn/repo
        -
          type: git
          uri: file:///path/to/rex/git/repo
        -
          type: git
          uri: file:///path/to/walter/git/repo

Using this configuration, precedence is determined by the order in which repositories are listed under the composite key. In the above example, the Subversion repository is listed first, so a value found in the Subversion repository will override values found for the same property in one of the Git repositories. A value found in the rex Git repository will be used before a value found for the same property in the walter Git repository.

If you want to pull configuration data only from repositories that are each of distinct types, you can enable the corresponding profiles, rather than the composite profile, in your configuration server’s application properties or YAML file. If, for example, you want to pull configuration data from a single Git repository and a single HashiCorp Vault server, you can set the following properties for your configuration server:

spring:
  profiles:
    active: git, vault
  cloud:
    config:
      server:
        git:
          uri: file:///path/to/git/repo
          order: 2
        vault:
          host: 127.0.0.1
          port: 8200
          order: 1

Using this configuration, precedence can be determined by an order property. You can use the order property to specify the priority order for all your repositories. The lower the numerical value of the order property, the higher priority it has. The priority order of a repository helps resolve any potential conflicts between repositories that contain values for the same properties.

If your composite environment includes a Vault server as in the previous example, you must include a Vault token in every request made to the configuration server. See Vault Backend.
Any type of failure when retrieving values from an environment repository results in a failure for the entire composite environment.
When using a composite environment, it is important that all repositories contain the same labels. If you have an environment similar to those in the preceding examples and you request configuration data with the master label but the Subversion repository does not contain a branch called master, the entire request fails.
Custom Composite Environment Repositories

In addition to using one of the environment repositories from Spring Cloud, you can also provide your own EnvironmentRepository bean to be included as part of a composite environment. To do so, your bean must implement the EnvironmentRepository interface. If you want to control the priority of your custom EnvironmentRepository within the composite environment, you should also implement the Ordered interface and override the getOrdered method. If you do not implement the Ordered interface, your EnvironmentRepository is given the lowest priority.

34.1.12. Property Overrides

The Config Server has an “overrides” feature that lets the operator provide configuration properties to all applications. The overridden properties cannot be accidentally changed by the application with the normal Spring Boot hooks. To declare overrides, add a map of name-value pairs to spring.cloud.config.server.overrides, as shown in the following example:

spring:
  cloud:
    config:
      server:
        overrides:
          foo: bar

The preceding examples causes all applications that are config clients to read foo=bar, independent of their own configuration.

A configuration system cannot force an application to use configuration data in any particular way. Consequently, overrides are not enforceable. However, they do provide useful default behavior for Spring Cloud Config clients.
Normally, Spring environment placeholders with ${} can be escaped (and resolved on the client) by using backslash (\) to escape the $ or the {. For example, \${app.foo:bar} resolves to bar, unless the app provides its own app.foo.
In YAML, you do not need to escape the backslash itself. However, in properties files, you do need to escape the backslash, when you configure the overrides on the server.

You can change the priority of all overrides in the client to be more like default values, letting applications supply their own values in environment variables or System properties, by setting the spring.cloud.config.overrideNone=true flag (the default is false) in the remote repository.

34.2. Health Indicator

Config Server comes with a Health Indicator that checks whether the configured EnvironmentRepository is working. By default, it asks the EnvironmentRepository for an application named app, the default profile, and the default label provided by the EnvironmentRepository implementation.

You can configure the Health Indicator to check more applications along with custom profiles and custom labels, as shown in the following example:

spring:
  cloud:
    config:
      server:
        health:
          repositories:
            myservice:
              label: mylabel
            myservice-dev:
              name: myservice
              profiles: development

You can disable the Health Indicator by setting management.health.config.enabled=false.

34.3. Security

You can secure your Config Server in any way that makes sense to you (from physical network security to OAuth2 bearer tokens), because Spring Security and Spring Boot offer support for many security arrangements.

To use the default Spring Boot-configured HTTP Basic security, include Spring Security on the classpath (for example, through spring-boot-starter-security). The default is a username of user and a randomly generated password. A random password is not useful in practice, so we recommend you configure the password (by setting spring.security.user.password) and encrypt it (see below for instructions on how to do that).

34.4. Encryption and Decryption

To use the encryption and decryption features you need the full-strength JCE installed in your JVM (it is not included by default). You can download the “Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files” from Oracle and follow the installation instructions (essentially, you need to replace the two policy files in the JRE lib/security directory with the ones that you downloaded).

If the remote property sources contain encrypted content (values starting with {cipher}), they are decrypted before sending to clients over HTTP. The main advantage of this setup is that the property values need not be in plain text when they are “at rest” (for example, in a git repository). If a value cannot be decrypted, it is removed from the property source and an additional property is added with the same key but prefixed with invalid and a value that means “not applicable” (usually <n/a>). This is largely to prevent cipher text being used as a password and accidentally leaking.

If you set up a remote config repository for config client applications, it might contain an application.yml similar to the following:

application.yml
spring:
  datasource:
    username: dbuser
    password: '{cipher}FKSAJDFGYOS8F7GLHAKERGFHLSAJ'

Encrypted values in a .properties file must not be wrapped in quotes. Otherwise, the value is not decrypted. The following example shows values that would work:

application.properties
spring.datasource.username: dbuser
spring.datasource.password: {cipher}FKSAJDFGYOS8F7GLHAKERGFHLSAJ

You can safely push this plain text to a shared git repository, and the secret password remains protected.

The server also exposes /encrypt and /decrypt endpoints (on the assumption that these are secured and only accessed by authorized agents). If you edit a remote config file, you can use the Config Server to encrypt values by POSTing to the /encrypt endpoint, as shown in the following example:

$ curl localhost:8888/encrypt -s -d mysecret
682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
If you are testing with curl, then use --data-urlencode (instead of -d) and prefix the value to encrypt with = (curl requires this) or set an explicit Content-Type: text/plain to make sure curl encodes the data correctly when there are special characters ('+' is particularly tricky).
Be sure not to include any of the curl command statistics in the encrypted value, this is why the examples use the -s option to silence them. Outputting the value to a file can help avoid this problem.

The inverse operation is also available through /decrypt (provided the server is configured with a symmetric key or a full key pair), as shown in the following example:

$ curl localhost:8888/decrypt -s -d 682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
mysecret

Take the encrypted value and add the {cipher} prefix before you put it in the YAML or properties file and before you commit and push it to a remote (potentially insecure) store.

The /encrypt and /decrypt endpoints also both accept paths in the form of /*/{application}/{profiles}, which can be used to control cryptography on a per-application (name) and per-profile basis when clients call into the main environment resource.

To control the cryptography in this granular way, you must also provide a @Bean of type TextEncryptorLocator that creates a different encryptor per name and profiles. The one that is provided by default does not do so (all encryptions use the same key).

The spring command line client (with Spring Cloud CLI extensions installed) can also be used to encrypt and decrypt, as shown in the following example:

$ spring encrypt mysecret --key foo
682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
$ spring decrypt --key foo 682bc583f4641835fa2db009355293665d2647dade3375c0ee201de2a49f7bda
mysecret

To use a key in a file (such as an RSA public key for encryption), prepend the key value with "@" and provide the file path, as shown in the following example:

$ spring encrypt mysecret --key @${HOME}/.ssh/id_rsa.pub
AQAjPgt3eFZQXwt8tsHAVv/QHiY5sI2dRcR+...
The --key argument is mandatory (despite having a -- prefix).

34.5. Key Management

The Config Server can use a symmetric (shared) key or an asymmetric one (RSA key pair). The asymmetric choice is superior in terms of security, but it is often more convenient to use a symmetric key since it is a single property value to configure in the bootstrap.properties.

To configure a symmetric key, you need to set encrypt.key to a secret String (or use the ENCRYPT_KEY environment variable to keep it out of plain-text configuration files).

You cannot configure an asymmetric key using encrypt.key.

To configure an asymmetric key use a keystore (e.g. as created by the keytool utility that comes with the JDK). The keystore properties are encrypt.keyStore.* with * equal to

Property Description

encrypt.keyStore.location

Contains a Resource location

encrypt.keyStore.password

Holds the password that unlocks the keystore

encrypt.keyStore.alias

Identifies which key in the store to use

encrypt.keyStore.type

The type of KeyStore to create. Defaults to jks.

The encryption is done with the public key, and a private key is needed for decryption. Thus, in principle, you can configure only the public key in the server if you want to only encrypt (and are prepared to decrypt the values yourself locally with the private key). In practice, you might not want to do decrypt locally, because it spreads the key management process around all the clients, instead of concentrating it in the server. On the other hand, it can be a useful option if your config server is relatively insecure and only a handful of clients need the encrypted properties.

34.6. Creating a Key Store for Testing

To create a keystore for testing, you can use a command resembling the following:

$ keytool -genkeypair -alias mytestkey -keyalg RSA \
  -dname "CN=Web Server,OU=Unit,O=Organization,L=City,S=State,C=US" \
  -keypass changeme -keystore server.jks -storepass letmein
When using JDK 11 or above you may get the following warning when using the command above. In this case you probably want to make sure the keypass and storepass values match.
Warning:  Different store and key passwords not supported for PKCS12 KeyStores. Ignoring user-specified -keypass value.

Put the server.jks file in the classpath (for instance) and then, in your bootstrap.yml, for the Config Server, create the following settings:

encrypt:
  keyStore:
    location: classpath:/server.jks
    password: letmein
    alias: mytestkey
    secret: changeme

34.7. Using Multiple Keys and Key Rotation

In addition to the {cipher} prefix in encrypted property values, the Config Server looks for zero or more {name:value} prefixes before the start of the (Base64 encoded) cipher text. The keys are passed to a TextEncryptorLocator, which can do whatever logic it needs to locate a TextEncryptor for the cipher. If you have configured a keystore (encrypt.keystore.location), the default locator looks for keys with aliases supplied by the key prefix, with a cipher text like resembling the following:

foo:
  bar: `{cipher}{key:testkey}...`

The locator looks for a key named "testkey". A secret can also be supplied by using a {secret:…​} value in the prefix. However, if it is not supplied, the default is to use the keystore password (which is what you get when you build a keystore and do not specify a secret). If you do supply a secret, you should also encrypt the secret using a custom SecretLocator.

When the keys are being used only to encrypt a few bytes of configuration data (that is, they are not being used elsewhere), key rotation is hardly ever necessary on cryptographic grounds. However, you might occasionally need to change the keys (for example, in the event of a security breach). In that case, all the clients would need to change their source config files (for example, in git) and use a new {key:…​} prefix in all the ciphers. Note that the clients need to first check that the key alias is available in the Config Server keystore.

If you want to let the Config Server handle all encryption as well as decryption, the {name:value} prefixes can also be added as plain text posted to the /encrypt endpoint, .

34.8. Serving Encrypted Properties

Sometimes you want the clients to decrypt the configuration locally, instead of doing it in the server. In that case, if you provide the encrypt.* configuration to locate a key, you can still have /encrypt and /decrypt endpoints, but you need to explicitly switch off the decryption of outgoing properties by placing spring.cloud.config.server.encrypt.enabled=false in bootstrap.[yml|properties]. If you do not care about the endpoints, it should work if you do not configure either the key or the enabled flag.

35. Serving Alternative Formats

The default JSON format from the environment endpoints is perfect for consumption by Spring applications, because it maps directly onto the Environment abstraction. If you prefer, you can consume the same data as YAML or Java properties by adding a suffix (".yml", ".yaml" or ".properties") to the resource path. This can be useful for consumption by applications that do not care about the structure of the JSON endpoints or the extra metadata they provide (for example, an application that is not using Spring might benefit from the simplicity of this approach).

The YAML and properties representations have an additional flag (provided as a boolean query parameter called resolvePlaceholders) to signal that placeholders in the source documents (in the standard Spring ${…​} form) should be resolved in the output before rendering, where possible. This is a useful feature for consumers that do not know about the Spring placeholder conventions.

There are limitations in using the YAML or properties formats, mainly in relation to the loss of metadata. For example, the JSON is structured as an ordered list of property sources, with names that correlate with the source. The YAML and properties forms are coalesced into a single map, even if the origin of the values has multiple sources, and the names of the original source files are lost. Also, the YAML representation is not necessarily a faithful representation of the YAML source in a backing repository either. It is constructed from a list of flat property sources, and assumptions have to be made about the form of the keys.

36. Serving Plain Text

Instead of using the Environment abstraction (or one of the alternative representations of it in YAML or properties format), your applications might need generic plain-text configuration files that are tailored to their environment. The Config Server provides these through an additional endpoint at /{application}/{profile}/{label}/{path}, where application, profile, and label have the same meaning as the regular environment endpoint, but path is a path to a file name (such as log.xml). The source files for this endpoint are located in the same way as for the environment endpoints. The same search path is used for properties and YAML files. However, instead of aggregating all matching resources, only the first one to match is returned.

After a resource is located, placeholders in the normal format (${…​}) are resolved by using the effective Environment for the supplied application name, profile, and label. In this way, the resource endpoint is tightly integrated with the environment endpoints.

As with the source files for environment configuration, the profile is used to resolve the file name. So, if you want a profile-specific file, /*/development/*/logback.xml can be resolved by a file called logback-development.xml (in preference to logback.xml).
If you do not want to supply the label and let the server use the default label, you can supply a useDefaultLabel request parameter. Consequently, the preceding example for the default profile could be /sample/default/nginx.conf?useDefaultLabel.

At present, Spring Cloud Config can serve plaintext for git, SVN, native backends, and AWS S3. The support for git, SVN, and native backends is identical. AWS S3 works a bit differently. The following sections show how each one works:

36.1. Git, SVN, and Native Backends

Consider the following example for a GIT or SVN repository or a native backend:

application.yml
nginx.conf

The nginx.conf might resemble the following listing:

server {
    listen              80;
    server_name         ${nginx.server.name};
}

application.yml might resemble the following listing:

nginx:
  server:
    name: example.com
---
spring:
  profiles: development
nginx:
  server:
    name: develop.com

The /sample/default/master/nginx.conf resource might be as follows:

server {
    listen              80;
    server_name         example.com;
}

/sample/development/master/nginx.conf might be as follows:

server {
    listen              80;
    server_name         develop.com;
}

36.2. AWS S3

To enable serving plain text for AWS s3, the Config Server application needs to include a dependency on Spring Cloud AWS. For details on how to set up that dependency, see the Spring Cloud AWS Reference Guide. Then you need to configure Spring Cloud AWS, as described in the Spring Cloud AWS Reference Guide.

36.3. Decrypting Plain Text

By default, encrypted values in plain text files are not decrypted. In order to enable decryption for plain text files, set spring.cloud.config.server.encrypt.enabled=true and spring.cloud.config.server.encrypt.plainTextEncrypt=true in bootstrap.[yml|properties]

Decrypting plain text files is only supported for YAML, JSON, and properties file extensions.

If this feature is enabled, and an unsupported file extention is requested, any encrypted values in the file will not be decrypted.

37. Embedding the Config Server

The Config Server runs best as a standalone application. However, if need be, you can embed it in another application. To do so, use the @EnableConfigServer annotation. An optional property named spring.cloud.config.server.bootstrap can be useful in this case. It is a flag to indicate whether the server should configure itself from its own remote repository. By default, the flag is off, because it can delay startup. However, when embedded in another application, it makes sense to initialize the same way as any other application. When setting spring.cloud.config.server.bootstrap to true you must also use a composite environment repository configuration. For example

spring:
  application:
    name: configserver
  profiles:
    active: composite
  cloud:
    config:
      server:
        composite:
          - type: native
            search-locations: ${HOME}/Desktop/config
        bootstrap: true
If you use the bootstrap flag, the config server needs to have its name and repository URI configured in bootstrap.yml.

To change the location of the server endpoints, you can (optionally) set spring.cloud.config.server.prefix (for example, /config), to serve the resources under a prefix. The prefix should start but not end with a /. It is applied to the @RequestMappings in the Config Server (that is, underneath the Spring Boot server.servletPath and server.contextPath prefixes).

If you want to read the configuration for an application directly from the backend repository (instead of from the config server), you basically want an embedded config server with no endpoints. You can switch off the endpoints entirely by not using the @EnableConfigServer annotation (set spring.cloud.config.server.bootstrap=true).

38. Push Notifications and Spring Cloud Bus

Many source code repository providers (such as Github, Gitlab, Gitea, Gitee, Gogs, or Bitbucket) notify you of changes in a repository through a webhook. You can configure the webhook through the provider’s user interface as a URL and a set of events in which you are interested. For instance, Github uses a POST to the webhook with a JSON body containing a list of commits and a header (X-Github-Event) set to push. If you add a dependency on the spring-cloud-config-monitor library and activate the Spring Cloud Bus in your Config Server, then a /monitor endpoint is enabled.

When the webhook is activated, the Config Server sends a RefreshRemoteApplicationEvent targeted at the applications it thinks might have changed. The change detection can be strategized. However, by default, it looks for changes in files that match the application name (for example, foo.properties is targeted at the foo application, while application.properties is targeted at all applications). The strategy to use when you want to override the behavior is PropertyPathNotificationExtractor, which accepts the request headers and body as parameters and returns a list of file paths that changed.

The default configuration works out of the box with Github, Gitlab, Gitea, Gitee, Gogs or Bitbucket. In addition to the JSON notifications from Github, Gitlab, Gitee, or Bitbucket, you can trigger a change notification by POSTing to /monitor with form-encoded body parameters in the pattern of path={application}. Doing so broadcasts to applications matching the {application} pattern (which can contain wildcards).

The RefreshRemoteApplicationEvent is transmitted only if the spring-cloud-bus is activated in both the Config Server and in the client application.
The default configuration also detects filesystem changes in local git repositories. In that case, the webhook is not used. However, as soon as you edit a config file, a refresh is broadcast.

39. Spring Cloud Config Client

A Spring Boot application can take immediate advantage of the Spring Config Server (or other external property sources provided by the application developer). It also picks up some additional useful features related to Environment change events.

39.1. Spring Boot Config Data Import

Spring Boot 2.4 introduced a new way to import configuration data via the spring.config.import property. This is now the default way to bind to Config Server.

To optionally connect to config server set the following in application.properties:

application.properties
spring.config.import=optional:configserver:

This will connect to the Config Server at the default location of "http://localhost:8888". Removing the optional: prefix will cause the Config Client to fail if it is unable to connect to Config Server. To change the location of Config Server either set spring.cloud.config.uri or add the url to the spring.config.import statement such as, spring.config.import=optional:configserver:http://myhost:8888. The location in the import property has precedence over the uri property.

A bootstrap file (properties or yaml) is not needed for the Spring Boot Config Data method of import via spring.config.import.

39.2. Config First Bootstrap

To use the legacy bootstrap way of connecting to Config Server, bootstrap must be enabled via a property or the spring-cloud-starter-bootstrap starter. The property is spring.cloud.bootstrap.enabled=true. It must be set as a System Property or environment variable. Once bootstrap has been enabled any application with Spring Cloud Config Client on the classpath will connect to Config Server as follows: When a config client starts, it binds to the Config Server (through the spring.cloud.config.uri bootstrap configuration property) and initializes Spring Environment with remote property sources.

The net result of this behavior is that all client applications that want to consume the Config Server need a bootstrap.yml (or an environment variable) with the server address set in spring.cloud.config.uri (it defaults to "http://localhost:8888").

39.3. Discovery First Bootstrap

If you use a DiscoveryClient implementation, such as Spring Cloud Netflix and Eureka Service Discovery or Spring Cloud Consul, you can have the Config Server register with the Discovery Service. However, in the default “Config First” mode, clients cannot take advantage of the registration.

If you prefer to use DiscoveryClient to locate the Config Server, you can do so by setting spring.cloud.config.discovery.enabled=true (the default is false). The net result of doing so is that client applications all need a bootstrap.yml (or an environment variable) with the appropriate discovery configuration. For example, with Spring Cloud Netflix, you need to define the Eureka server address (for example, in eureka.client.serviceUrl.defaultZone). The price for using this option is an extra network round trip on startup, to locate the service registration. The benefit is that, as long as the Discovery Service is a fixed point, the Config Server can change its coordinates. The default service ID is configserver, but you can change that on the client by setting spring.cloud.config.discovery.serviceId (and on the server, in the usual way for a service, such as by setting spring.application.name).

The discovery client implementations all support some kind of metadata map (for example, we have eureka.instance.metadataMap for Eureka). Some additional properties of the Config Server may need to be configured in its service registration metadata so that clients can connect correctly. If the Config Server is secured with HTTP Basic, you can configure the credentials as user and password. Also, if the Config Server has a context path, you can set configPath. For example, the following YAML file is for a Config Server that is a Eureka client:

eureka:
  instance:
    ...
    metadataMap:
      user: osufhalskjrtl
      password: lviuhlszvaorhvlo5847
      configPath: /config

39.4. Config Client Fail Fast

In some cases, you may want to fail startup of a service if it cannot connect to the Config Server. If this is the desired behavior, set the bootstrap configuration property spring.cloud.config.fail-fast=true to make the client halt with an Exception.

To get similar functionality using spring.config.import, simply omit the optional: prefix.

39.5. Config Client Retry

If you expect that the config server may occasionally be unavailable when your application starts, you can make it keep trying after a failure. First, you need to set spring.cloud.config.fail-fast=true. Then you need to add spring-retry and spring-boot-starter-aop to your classpath. The default behavior is to retry six times with an initial backoff interval of 1000ms and an exponential multiplier of 1.1 for subsequent backoffs. You can configure these properties (and others) by setting the spring.cloud.config.retry.* configuration properties.

To take full control of the retry behavior, add a @Bean of type RetryOperationsInterceptor with an ID of configServerRetryInterceptor. Spring Retry has a RetryInterceptorBuilder that supports creating one.

39.6. Locating Remote Configuration Resources

The Config Service serves property sources from /{application}/{profile}/{label}, where the default bindings in the client app are as follows:

  • "name" = ${spring.application.name}

  • "profile" = ${spring.profiles.active} (actually Environment.getActiveProfiles())

  • "label" = "master"

When setting the property ${spring.application.name} do not prefix your app name with the reserved word application- to prevent issues resolving the correct property source.

You can override all of them by setting spring.cloud.config.* (where * is name, profile or label). The label is useful for rolling back to previous versions of configuration. With the default Config Server implementation, it can be a git label, branch name, or commit ID. Label can also be provided as a comma-separated list. In that case, the items in the list are tried one by one until one succeeds. This behavior can be useful when working on a feature branch. For instance, you might want to align the config label with your branch but make it optional (in that case, use spring.cloud.config.label=myfeature,develop).

39.7. Specifying Multiple Urls for the Config Server

To ensure high availability when you have multiple instances of Config Server deployed and expect one or more instances to be unavailable from time to time, you can either specify multiple URLs (as a comma-separated list under the spring.cloud.config.uri property) or have all your instances register in a Service Registry like Eureka ( if using Discovery-First Bootstrap mode ). Note that doing so ensures high availability only when the Config Server is not running (that is, when the application has exited) or when a connection timeout has occurred. For example, if the Config Server returns a 500 (Internal Server Error) response or the Config Client receives a 401 from the Config Server (due to bad credentials or other causes), the Config Client does not try to fetch properties from other URLs. An error of that kind indicates a user issue rather than an availability problem.

If you use HTTP basic security on your Config Server, it is currently possible to support per-Config Server auth credentials only if you embed the credentials in each URL you specify under the spring.cloud.config.uri property. If you use any other kind of security mechanism, you cannot (currently) support per-Config Server authentication and authorization.

39.8. Configuring Timeouts

If you want to configure timeout thresholds:

  • Read timeouts can be configured by using the property spring.cloud.config.request-read-timeout.

  • Connection timeouts can be configured by using the property spring.cloud.config.request-connect-timeout.

39.9. Security

If you use HTTP Basic security on the server, clients need to know the password (and username if it is not the default). You can specify the username and password through the config server URI or via separate username and password properties, as shown in the following example:

spring:
  cloud:
    config:
     uri: https://user:[email protected]

The following example shows an alternate way to pass the same information:

spring:
  cloud:
    config:
     uri: https://myconfig.mycompany.com
     username: user
     password: secret

The spring.cloud.config.password and spring.cloud.config.username values override anything that is provided in the URI.

If you deploy your apps on Cloud Foundry, the best way to provide the password is through service credentials (such as in the URI, since it does not need to be in a config file). The following example works locally and for a user-provided service on Cloud Foundry named configserver:

spring:
  cloud:
    config:
     uri: ${vcap.services.configserver.credentials.uri:http://user:[email protected]:8888}

If config server requires client side TLS certificate, you can configure client side TLS certificate and trust store via properties, as shown in following example:

spring:
  cloud:
    config:
      uri: https://myconfig.myconfig.com
      tls:
        enabled: true
        key-store: <path-of-key-store>
        key-store-type: PKCS12
        key-store-password: <key-store-password>
        key-password: <key-password>
        trust-store: <path-of-trust-store>
        trust-store-type: PKCS12
        trust-store-password: <trust-store-password>

The spring.cloud.config.tls.enabled needs to be true to enable config client side TLS. When spring.cloud.config.tls.trust-store is omitted, a JVM default trust store is used. The default value for spring.cloud.config.tls.key-store-type and spring.cloud.config.tls.trust-store-type is PKCS12. When password properties are omitted, empty password is assumed.

If you use another form of security, you might need to provide a RestTemplate to the ConfigServicePropertySourceLocator (for example, by grabbing it in the bootstrap context and injecting it).

39.9.1. Health Indicator

The Config Client supplies a Spring Boot Health Indicator that attempts to load configuration from the Config Server. The health indicator can be disabled by setting health.config.enabled=false. The response is also cached for performance reasons. The default cache time to live is 5 minutes. To change that value, set the health.config.time-to-live property (in milliseconds).

39.9.2. Providing A Custom RestTemplate

In some cases, you might need to customize the requests made to the config server from the client. Typically, doing so involves passing special Authorization headers to authenticate requests to the server. To provide a custom RestTemplate:

  1. Create a new configuration bean with an implementation of PropertySourceLocator, as shown in the following example:

CustomConfigServiceBootstrapConfiguration.java
@Configuration
public class CustomConfigServiceBootstrapConfiguration {
    @Bean
    public ConfigServicePropertySourceLocator configServicePropertySourceLocator() {
        ConfigClientProperties clientProperties = configClientProperties();
       ConfigServicePropertySourceLocator configServicePropertySourceLocator =  new ConfigServicePropertySourceLocator(clientProperties);
        configServicePropertySourceLocator.setRestTemplate(customRestTemplate(clientProperties));
        return configServicePropertySourceLocator;
    }
}
For a simplified approach to adding Authorization headers, the spring.cloud.config.headers.* property can be used instead.
  1. In resources/META-INF, create a file called spring.factories and specify your custom configuration, as shown in the following example:

spring.factories
org.springframework.cloud.bootstrap.BootstrapConfiguration = com.my.config.client.CustomConfigServiceBootstrapConfiguration

39.9.3. Vault

When using Vault as a backend to your config server, the client needs to supply a token for the server to retrieve values from Vault. This token can be provided within the client by setting spring.cloud.config.token in bootstrap.yml, as shown in the following example:

spring:
  cloud:
    config:
      token: YourVaultToken

39.10. Nested Keys In Vault

Vault supports the ability to nest keys in a value stored in Vault, as shown in the following example:

echo -n '{"appA": {"secret": "appAsecret"}, "bar": "baz"}' | vault write secret/myapp -

This command writes a JSON object to your Vault. To access these values in Spring, you would use the traditional dot(.) annotation, as shown in the following example

@Value("${appA.secret}")
String name = "World";

The preceding code would sets the value of the name variable to appAsecret.

Spring Cloud Consul

2020.0.0-SNAPSHOT

This project provides Consul integrations for Spring Boot apps through autoconfiguration and binding to the Spring Environment and other Spring programming model idioms. With a few simple annotations you can quickly enable and configure the common patterns inside your application and build large distributed systems with Consul based components. The patterns provided include Service Discovery, Control Bus and Configuration. Intelligent Routing (Zuul) and Client Side Load Balancing (Ribbon), Circuit Breaker (Hystrix) are provided by integration with Spring Cloud Netflix.

40. Install Consul

Please see the installation documentation for instructions on how to install Consul.

41. Consul Agent

A Consul Agent client must be available to all Spring Cloud Consul applications. By default, the Agent client is expected to be at localhost:8500. See the Agent documentation for specifics on how to start an Agent client and how to connect to a cluster of Consul Agent Servers. For development, after you have installed consul, you may start a Consul Agent using the following command:

./src/main/bash/local_run_consul.sh

This will start an agent in server mode on port 8500, with the ui available at localhost:8500

42. Service Discovery with Consul

Service Discovery is one of the key tenets of a microservice based architecture. Trying to hand configure each client or some form of convention can be very difficult to do and can be very brittle. Consul provides Service Discovery services via an HTTP API and DNS. Spring Cloud Consul leverages the HTTP API for service registration and discovery. This does not prevent non-Spring Cloud applications from leveraging the DNS interface. Consul Agents servers are run in a cluster that communicates via a gossip protocol and uses the Raft consensus protocol.

42.1. How to activate

To activate Consul Service Discovery use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-consul-discovery. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

42.2. Registering with Consul

When a client registers with Consul, it provides meta-data about itself such as host and port, id, name and tags. An HTTP Check is created by default that Consul hits the /actuator/health endpoint every 10 seconds. If the health check fails, the service instance is marked as critical.

Example Consul client:

@SpringBootApplication
@RestController
public class Application {

    @RequestMapping("/")
    public String home() {
        return "Hello world";
    }

    public static void main(String[] args) {
        new SpringApplicationBuilder(Application.class).web(true).run(args);
    }

}

(i.e. utterly normal Spring Boot app). If the Consul client is located somewhere other than localhost:8500, the configuration is required to locate the client. Example:

application.yml
spring:
  cloud:
    consul:
      host: localhost
      port: 8500
If you use Spring Cloud Consul Config, and you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true or use spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.

The default service name, instance id and port, taken from the Environment, are ${spring.application.name}, the Spring Context ID and ${server.port} respectively.

To disable the Consul Discovery Client you can set spring.cloud.consul.discovery.enabled to false. Consul Discovery Client will also be disabled when spring.cloud.discovery.enabled is set to false.

To disable the service registration you can set spring.cloud.consul.discovery.register to false.

42.2.1. Registering Management as a Separate Service

When management server port is set to something different than the application port, by setting management.server.port property, management service will be registered as a separate service than the application service. For example:

application.yml
spring:
  application:
    name: myApp
management:
  server:
    port: 4452

Above configuration will register following 2 services:

  • Application Service:

ID: myApp
Name: myApp
  • Management Service:

ID: myApp-management
Name: myApp-management

Management service will inherit its instanceId and serviceName from the application service. For example:

application.yml
spring:
  application:
    name: myApp
management:
  server:
    port: 4452
spring:
  cloud:
    consul:
      discovery:
        instance-id: custom-service-id
        serviceName: myprefix-${spring.application.name}

Above configuration will register following 2 services:

  • Application Service:

ID: custom-service-id
Name: myprefix-myApp
  • Management Service:

ID: custom-service-id-management
Name: myprefix-myApp-management

Further customization is possible via following properties:

/** Port to register the management service under (defaults to management port) */
spring.cloud.consul.discovery.management-port

/** Suffix to use when registering management service (defaults to "management" */
spring.cloud.consul.discovery.management-suffix

/** Tags to use when registering management service (defaults to "management" */
spring.cloud.consul.discovery.management-tags

42.2.2. HTTP Health Check

The health check for a Consul instance defaults to "/actuator/health", which is the default location of the health endpoint in a Spring Boot Actuator application. You need to change this, even for an Actuator application, if you use a non-default context path or servlet path (e.g. server.servletPath=/foo) or management endpoint path (e.g. management.server.servlet.context-path=/admin).

The interval that Consul uses to check the health endpoint may also be configured. "10s" and "1m" represent 10 seconds and 1 minute respectively.

This example illustrates the above (see the spring.cloud.consul.discovery.health-check-* properties in the appendix page for more options).

application.yml
spring:
  cloud:
    consul:
      discovery:
        healthCheckPath: ${management.server.servlet.context-path}/actuator/health
        healthCheckInterval: 15s

You can disable the HTTP health check entirely by setting spring.cloud.consul.discovery.register-health-check=false.

Applying Headers

Headers can be applied to health check requests. For example, if you’re trying to register a Spring Cloud Config server that uses Vault Backend:

application.yml
spring:
  cloud:
    consul:
      discovery:
        health-check-headers:
          X-Config-Token: 6442e58b-d1ea-182e-cfa5-cf9cddef0722

According to the HTTP standard, each header can have more than one values, in which case, an array can be supplied:

application.yml
spring:
  cloud:
    consul:
      discovery:
        health-check-headers:
          X-Config-Token:
            - "6442e58b-d1ea-182e-cfa5-cf9cddef0722"
            - "Some other value"

42.2.3. Actuator Health Indicator(s)

If the service instance is a Spring Boot Actuator application, it may be provided the following Actuator health indicators.

DiscoveryClientHealthIndicator

When Consul Service Discovery is active, a DiscoverClientHealthIndicator is configured and made available to the Actuator health endpoint. See here for configuration options.

ConsulHealthIndicator

An indicator is configured that verifies the health of the ConsulClient.

By default, it retrieves the Consul leader node status and all registered services. In deployments that have many registered services it may be costly to retrieve all services on every health check. To skip the service retrieval and only check the leader node status set spring.cloud.consul.health-indicator.include-services-query=false.

To disable the indicator set management.health.consul.enabled=false.

When the application runs in bootstrap context mode (the default), this indicator is loaded into the bootstrap context and is not made available to the Actuator health endpoint.

42.2.4. Metadata

Consul supports metadata on services. Spring Cloud’s ServiceInstance has a Map<String, String> metadata field which is populated from a services meta field. To populate the meta field set values on spring.cloud.consul.discovery.metadata or spring.cloud.consul.discovery.management-metadata properties.

application.yml
spring:
  cloud:
    consul:
      discovery:
        metadata:
          myfield: myvalue
          anotherfield: anothervalue

The above configuration will result in a service who’s meta field contains myfield→myvalue and anotherfield→anothervalue.

Generated Metadata

The Consul Auto Registration will generate a few entries automatically.

Table 4. Auto Generated Metadata
Key Value

'group'

Property spring.cloud.consul.discovery.instance-group. This values is only generated if instance-group is not empty.'

'secure'

True if property spring.cloud.consul.discovery.scheme equals 'https', otherwise false.

Property spring.cloud.consul.discovery.default-zone-metadata-name, defaults to 'zone'

Property spring.cloud.consul.discovery.instance-zone. This values is only generated if instance-zone is not empty.'

Older versions of Spring Cloud Consul populated the ServiceInstance.getMetadata() method from Spring Cloud Commons by parsing the spring.cloud.consul.discovery.tags property. This is no longer supported, please migrate to using the spring.cloud.consul.discovery.metadata map.

42.2.5. Making the Consul Instance ID Unique

By default a consul instance is registered with an ID that is equal to its Spring Application Context ID. By default, the Spring Application Context ID is ${spring.application.name}:comma,separated,profiles:${server.port}. For most cases, this will allow multiple instances of one service to run on one machine. If further uniqueness is required, Using Spring Cloud you can override this by providing a unique identifier in spring.cloud.consul.discovery.instanceId. For example:

application.yml
spring:
  cloud:
    consul:
      discovery:
        instanceId: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}

With this metadata, and multiple service instances deployed on localhost, the random value will kick in there to make the instance unique. In Cloudfoundry the vcap.application.instance_id will be populated automatically in a Spring Boot application, so the random value will not be needed.

42.3. Looking up services

42.3.1. Using Load-balancer

Spring Cloud has support for Feign (a REST client builder) and also Spring RestTemplate for looking up services using the logical service names/ids instead of physical URLs. Both Feign and the discovery-aware RestTemplate utilize Spring Cloud LoadBalancer for client-side load balancing.

If you want to access service STORES using the RestTemplate simply declare:

@LoadBalanced
@Bean
public RestTemplate loadbalancedRestTemplate() {
     return new RestTemplate();
}

and use it like this (notice how we use the STORES service name/id from Consul instead of a fully qualified domainname):

@Autowired
RestTemplate restTemplate;

public String getFirstProduct() {
   return this.restTemplate.getForObject("https://STORES/products/1", String.class);
}

If you have Consul clusters in multiple datacenters and you want to access a service in another datacenter a service name/id alone is not enough. In that case you use property spring.cloud.consul.discovery.datacenters.STORES=dc-west where STORES is the service name/id and dc-west is the datacenter where the STORES service lives.

Spring Cloud now also offers support for Spring Cloud LoadBalancer.

42.3.2. Using the DiscoveryClient

You can also use the org.springframework.cloud.client.discovery.DiscoveryClient which provides a simple API for discovery clients that is not specific to Netflix, e.g.

@Autowired
private DiscoveryClient discoveryClient;

public String serviceUrl() {
    List<ServiceInstance> list = discoveryClient.getInstances("STORES");
    if (list != null && list.size() > 0 ) {
        return list.get(0).getUri();
    }
    return null;
}

42.4. Consul Catalog Watch

The Consul Catalog Watch takes advantage of the ability of consul to watch services. The Catalog Watch makes a blocking Consul HTTP API call to determine if any services have changed. If there is new service data a Heartbeat Event is published.

To change the frequency of when the Config Watch is called change spring.cloud.consul.config.discovery.catalog-services-watch-delay. The default value is 1000, which is in milliseconds. The delay is the amount of time after the end of the previous invocation and the start of the next.

To disable the Catalog Watch set spring.cloud.consul.discovery.catalogServicesWatch.enabled=false.

The watch uses a Spring TaskScheduler to schedule the call to consul. By default it is a ThreadPoolTaskScheduler with a poolSize of 1. To change the TaskScheduler, create a bean of type TaskScheduler named with the ConsulDiscoveryClientConfiguration.CATALOG_WATCH_TASK_SCHEDULER_NAME constant.

43. Distributed Configuration with Consul

Consul provides a Key/Value Store for storing configuration and other metadata. Spring Cloud Consul Config is an alternative to the Config Server and Client. Configuration is loaded into the Spring Environment during the special "bootstrap" phase. Configuration is stored in the /config folder by default. Multiple PropertySource instances are created based on the application’s name and the active profiles that mimicks the Spring Cloud Config order of resolving properties. For example, an application with the name "testApp" and with the "dev" profile will have the following property sources created:

config/testApp,dev/
config/testApp/
config/application,dev/
config/application/

The most specific property source is at the top, with the least specific at the bottom. Properties in the config/application folder are applicable to all applications using consul for configuration. Properties in the config/testApp folder are only available to the instances of the service named "testApp".

Configuration is currently read on startup of the application. Sending a HTTP POST to /refresh will cause the configuration to be reloaded. Config Watch will also automatically detect changes and reload the application context.

43.1. How to activate

To get started with Consul Configuration use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-consul-config. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

43.2. Spring Boot Config Data Import

Spring Boot 2.4 introduced a new way to import configuration data via the spring.config.import property. This is now the default way to get configuration from Consul.

To optionally connect to Consul set the following in application.properties:

application.properties
spring.config.import=optional:consul:

This will connect to the Consul Agent at the default location of "http://localhost:8500". Removing the optional: prefix will cause Consul Config to fail if it is unable to connect to Consul. To change the connection properties of Consul Config either set spring.cloud.consul.host and spring.cloud.consul.port or add the host/port pair to the spring.config.import statement such as, spring.config.import=optional:consul:myhost:8500. The location in the import property has precedence over the host and port propertie.

Consul Config will try to load values from four automatic contexts based on spring.cloud.consul.config.name (which defaults to the value of the spring.application.name property) and spring.cloud.consul.config.default-context (which defaults to application). If you want to specify the contexts rather than using the computed ones, you can add that information to the spring.config.import statement.

application.properties
spring.config.import=optional:consul:myhost:8500/contextone;/context/two

This will optionally load configuration only from /contextone and /context/two.

A bootstrap file (properties or yaml) is not needed for the Spring Boot Config Data method of import via spring.config.import.

43.3. Customizing

Consul Config may be customized using the following properties:

spring:
  cloud:
    consul:
      config:
        enabled: true
        prefix: configuration
        defaultContext: apps
        profileSeparator: '::'
If you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true, or included spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.
  • enabled setting this value to "false" disables Consul Config

  • prefix sets the base folder for configuration values

  • defaultContext sets the folder name used by all applications

  • profileSeparator sets the value of the separator used to separate the profile name in property sources with profiles

43.4. Config Watch

The Consul Config Watch takes advantage of the ability of consul to watch a key prefix. The Config Watch makes a blocking Consul HTTP API call to determine if any relevant configuration data has changed for the current application. If there is new configuration data a Refresh Event is published. This is equivalent to calling the /refresh actuator endpoint.

To change the frequency of when the Config Watch is called change spring.cloud.consul.config.watch.delay. The default value is 1000, which is in milliseconds. The delay is the amount of time after the end of the previous invocation and the start of the next.

To disable the Config Watch set spring.cloud.consul.config.watch.enabled=false.

The watch uses a Spring TaskScheduler to schedule the call to consul. By default it is a ThreadPoolTaskScheduler with a poolSize of 1. To change the TaskScheduler, create a bean of type TaskScheduler named with the ConsulConfigAutoConfiguration.CONFIG_WATCH_TASK_SCHEDULER_NAME constant.

43.5. YAML or Properties with Config

It may be more convenient to store a blob of properties in YAML or Properties format as opposed to individual key/value pairs. Set the spring.cloud.consul.config.format property to YAML or PROPERTIES. For example to use YAML:

spring:
  cloud:
    consul:
      config:
        format: YAML
If you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true, or included spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.

YAML must be set in the appropriate data key in consul. Using the defaults above the keys would look like:

config/testApp,dev/data
config/testApp/data
config/application,dev/data
config/application/data

You could store a YAML document in any of the keys listed above.

You can change the data key using spring.cloud.consul.config.data-key.

43.6. git2consul with Config

git2consul is a Consul community project that loads files from a git repository to individual keys into Consul. By default the names of the keys are names of the files. YAML and Properties files are supported with file extensions of .yml and .properties respectively. Set the spring.cloud.consul.config.format property to FILES. For example:

bootstrap.yml
spring:
  cloud:
    consul:
      config:
        format: FILES

Given the following keys in /config, the development profile and an application name of foo:

.gitignore
application.yml
bar.properties
foo-development.properties
foo-production.yml
foo.properties
master.ref

the following property sources would be created:

config/foo-development.properties
config/foo.properties
config/application.yml

The value of each key needs to be a properly formatted YAML or Properties file.

43.7. Fail Fast

It may be convenient in certain circumstances (like local development or certain test scenarios) to not fail if consul isn’t available for configuration. Setting spring.cloud.consul.config.fail-fast=false will cause the configuration module to log a warning rather than throw an exception. This will allow the application to continue startup normally.

If you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true, or included spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.

44. Consul Retry

If you expect that the consul agent may occasionally be unavailable when your app starts, you can ask it to keep trying after a failure. You need to add spring-retry and spring-boot-starter-aop to your classpath. The default behaviour is to retry 6 times with an initial backoff interval of 1000ms and an exponential multiplier of 1.1 for subsequent backoffs. You can configure these properties (and others) using spring.cloud.consul.retry.* configuration properties. This works with both Spring Cloud Consul Config and Discovery registration.

To take full control of the retry add a @Bean of type RetryOperationsInterceptor with id "consulRetryInterceptor". Spring Retry has a RetryInterceptorBuilder that makes it easy to create one.

45. Spring Cloud Bus with Consul

45.1. How to activate

To get started with the Consul Bus use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-consul-bus. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

See the Spring Cloud Bus documentation for the available actuator endpoints and howto send custom messages.

46. Circuit Breaker with Hystrix

Applications can use the Hystrix Circuit Breaker provided by the Spring Cloud Netflix project by including this starter in the projects pom.xml: spring-cloud-starter-hystrix. Hystrix doesn’t depend on the Netflix Discovery Client. The @EnableHystrix annotation should be placed on a configuration class (usually the main class). Then methods can be annotated with @HystrixCommand to be protected by a circuit breaker. See the documentation for more details.

47. Hystrix metrics aggregation with Turbine and Consul

Turbine (provided by the Spring Cloud Netflix project), aggregates multiple instances Hystrix metrics streams, so the dashboard can display an aggregate view. Turbine uses the DiscoveryClient interface to lookup relevant instances. To use Turbine with Spring Cloud Consul, configure the Turbine application in a manner similar to the following examples:

pom.xml
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-netflix-turbine</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-consul-discovery</artifactId>
</dependency>

Notice that the Turbine dependency is not a starter. The turbine starter includes support for Netflix Eureka.

application.yml
spring.application.name: turbine
applications: consulhystrixclient
turbine:
  aggregator:
    clusterConfig: ${applications}
  appConfig: ${applications}

The clusterConfig and appConfig sections must match, so it’s useful to put the comma-separated list of service ID’s into a separate configuration property.

Turbine.java
@EnableTurbine
@SpringBootApplication
public class Turbine {
    public static void main(String[] args) {
        SpringApplication.run(DemoturbinecommonsApplication.class, args);
    }
}

48. Configuration Properties

To see the list of all Consul related configuration properties please check the Appendix page.

index.htmladoc

Spring Cloud Function

Mark Fisher, Dave Syer, Oleg Zhurakousky, Anshul Mehra

3.1.0-SNAPSHOT


49. Introduction

Spring Cloud Function is a project with the following high-level goals:

  • Promote the implementation of business logic via functions.

  • Decouple the development lifecycle of business logic from any specific runtime target so that the same code can run as a web endpoint, a stream processor, or a task.

  • Support a uniform programming model across serverless providers, as well as the ability to run standalone (locally or in a PaaS).

  • Enable Spring Boot features (auto-configuration, dependency injection, metrics) on serverless providers.

It abstracts away all of the transport details and infrastructure, allowing the developer to keep all the familiar tools and processes, and focus firmly on business logic.

Here’s a complete, executable, testable Spring Boot application (implementing a simple string manipulation):

@SpringBootApplication
public class Application {

  @Bean
  public Function<Flux<String>, Flux<String>> uppercase() {
    return flux -> flux.map(value -> value.toUpperCase());
  }

  public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
  }
}

It’s just a Spring Boot application, so it can be built, run and tested, locally and in a CI build, the same way as any other Spring Boot application. The Function is from java.util and Flux is a Reactive Streams Publisher from Project Reactor. The function can be accessed over HTTP or messaging.

Spring Cloud Function has 4 main features:

In the nutshell Spring Cloud Function provides the following features: 1. Wrappers for @Beans of type Function, Consumer and Supplier, exposing them to the outside world as either HTTP endpoints and/or message stream listeners/publishers with RabbitMQ, Kafka etc.

  • Choice of programming styles - reactive, imperative or hybrid.

  • Function composition and adaptation (e.g., composing imperative functions with reactive).

  • Support for reactive function with multiple inputs and outputs allowing merging, joining and other complex streaming operation to be handled by functions.

  • Transparent type conversion of inputs and outputs.

  • Packaging functions for deployments, specific to the target platform (e.g., Project Riff, AWS Lambda and more)

  • Adapters to expose function to the outside world as HTTP endpoints etc.

  • Deploying a JAR file containing such an application context with an isolated classloader, so that you can pack them together in a single JVM.

  • Compiling strings which are Java function bodies into bytecode, and then turning them into @Beans that can be wrapped as above.

  • Adapters for AWS Lambda, Azure, Google Cloud Functions, Apache OpenWhisk and possibly other "serverless" service providers.

Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, please find the source code and issue trackers in the project at github.

50. Getting Started

Build from the command line (and "install" the samples):

$ ./mvnw clean install

(If you like to YOLO add -DskipTests.)

Run one of the samples, e.g.

$ java -jar spring-cloud-function-samples/function-sample/target/*.jar

This runs the app and exposes its functions over HTTP, so you can convert a string to uppercase, like this:

$ curl -H "Content-Type: text/plain" localhost:8080/uppercase -d Hello
HELLO

You can convert multiple strings (a Flux<String>) by separating them with new lines

$ curl -H "Content-Type: text/plain" localhost:8080/uppercase -d 'Hello
> World'
HELLOWORLD

(You can use QJ in a terminal to insert a new line in a literal string like that.)

51. Programming model

51.1. Function Catalog and Flexible Function Signatures

One of the main features of Spring Cloud Function is to adapt and support a range of type signatures for user-defined functions, while providing a consistent execution model. That’s why all user defined functions are transformed into a canonical representation by FunctionCatalog.

While users don’t normally have to care about the FunctionCatalog at all, it is useful to know what kind of functions are supported in user code.

It is also important to understand that Spring Cloud Function provides first class support for reactive API provided by Project Reactor allowing reactive primitives such as Mono and Flux to be used as types in user defined functions providing greater flexibility when choosing programming model for your function implementation. Reactive programming model also enables functional support for features that would be otherwise difficult to impossible to implement using imperative programming style. For more on this please read Function Arity section.

51.2. Java 8 function support

Spring Cloud Function embraces and builds on top of the 3 core functional interfaces defined by Java and available to us since Java 8.

  • Supplier<O>

  • Function<I, O>

  • Consumer<I>

51.2.1. Supplier

Supplier can be reactive - Supplier<Flux<T>> or imperative - Supplier<T>. From the invocation standpoint this should make no difference to the implementor of such Supplier. However, when used within frameworks (e.g., Spring Cloud Stream), Suppliers, especially reactive, often used to represent the source of the stream, therefore they are invoked once to get the stream (e.g., Flux) to which consumers can subscribe to. In other words such suppliers represent an equivalent of an infinite stream. However, the same reactive suppliers can also represent finite stream(s) (e.g., result set on the polled JDBC data). In those cases such reactive suppliers must be hooked up to some polling mechanism of the underlying framework.

To assist with that Spring Cloud Function provides a marker annotation org.springframework.cloud.function.context.PollableSupplier to signal that such supplier produces a finite stream and may need to be polled again. That said, it is important to understand that Spring Cloud Function itself provides no behavior for this annotation.

In addition PollableSupplier annotation exposes a splittable attribute to signal that produced stream needs to be split (see Splitter EIP)

Here is the example:

@PollableSupplier(splittable = true)
public Supplier<Flux<String>> someSupplier() {
    return () -> {
        String v1 = String.valueOf(System.nanoTime());
        String v2 = String.valueOf(System.nanoTime());
        String v3 = String.valueOf(System.nanoTime());
        return Flux.just(v1, v2, v3);
    };
}

51.2.2. Function

Function can also be written in imperative or reactive way, yet unlike Supplier and Consumer there are no special considerations for the implementor other then understanding that when used within frameworks such as Spring Cloud Stream and others, reactive function is invoked only once to pass a reference to the stream (Flux or Mono) and imperative is invoked once per event.

51.2.3. Consumer

Consumer is a little bit special because it has a void return type, which implies blocking, at least potentially. Most likely you will not need to write Consumer<Flux<?>>, but if you do need to do that, remember to subscribe to the input flux.

51.3. Function Composition

Function Composition is a feature that allows one to compose several functions into one. The core support is based on function composition feature available with Function.andThen(..) support available since Java 8. However on top of it, we provide few additional features.

51.3.1. Declarative Function Composition

This feature allows you to provide composition instruction in a declarative way using | (pipe) or , (comma) delimiter when providing spring.cloud.function.definition property.

For example

--spring.cloud.function.definition=uppercase|reverse

Here we effectively provided a definition of a single function which itself is a composition of function uppercase and function reverse. In fact that is one of the reasons why the property name is definition and not name, since the definition of a function can be a composition of several named functions. And as mentioned you can use , instead of pipe (such as …​definition=uppercase,reverse).

51.3.2. Composing non-Functions

Spring Cloud Function also supports composing Supplier with Consumer or Function as well as Function with Consumer. What’s important here is to understand the end product of such definitions. Composing Supplier with Function still results in Supplier while composing Supplier with Consumer will effectively render Runnable. Following the same logic composing Function with Consumer will result in Consumer.

And of course you can’t compose uncomposable such as Consumer and Function, Consumer and Supplier etc.

51.4. Function Routing

Since version 2.2 Spring Cloud Function provides routing feature allowing you to invoke a single function which acts as a router to an actual function you wish to invoke This feature is very useful in certain FAAS environments where maintaining configurations for several functions could be cumbersome or exposing more then one function is not possible.

The RoutingFunction is registered in FunctionCatalog under the name functionRouter. For simplicity and consistency you can also refer to RoutingFunction.FUNCTION_NAME constant.

This function has the following signature:

public class RoutingFunction implements Function<Object, Object> {
. . .
}

The routing instructions could be communicated in several ways;

Message Headers

If the input argument is of type Message<?>, you can communicate routing instruction by setting one of spring.cloud.function.definition or spring.cloud.function.routing-expression Message headers. For more static cases you can use spring.cloud.function.definition header which allows you to provide the name of a single function (e.g., …​definition=foo) or a composition instruction (e.g., …​definition=foo|bar|baz). For more dynamic cases you can use spring.cloud.function.routing-expression header which allows you to use Spring Expression Language (SpEL) and provide SpEL expression that should resolve into definition of a function (as described above).

SpEL evaluation context’s root object is the actual input argument, so in he case of Message<?> you can construct expression that has access to both payload and headers (e.g., spring.cloud.function.routing-expression=headers.function_name).

In specific execution environments/models the adapters are responsible to translate and communicate spring.cloud.function.definition and/or spring.cloud.function.routing-expression via Message header. For example, when using spring-cloud-function-web you can provide spring.cloud.function.definition as an HTTP header and the framework will propagate it as well as other HTTP headers as Message headers.

Application Properties

Routing instruction can also be communicated via spring.cloud.function.definition or spring.cloud.function.routing-expression as application properties. The rules described in the previous section apply here as well. The only difference is you provide these instructions as application properties (e.g., --spring.cloud.function.definition=foo).

When dealing with reactive inputs (e.g., Publisher), routing instructions must only be provided via Function properties. This is due to the nature of the reactive functions which are invoked only once to pass a Publisher and the rest is handled by the reactor, hence we can not access and/or rely on the routing instructions communicated via individual values (e.g., Message).

51.5. Function Arity

There are times when a stream of data needs to be categorized and organized. For example, consider a classic big-data use case of dealing with unorganized data containing, let’s say, ‘orders’ and ‘invoices’, and you want each to go into a separate data store. This is where function arity (functions with multiple inputs and outputs) support comes to play.

Let’s look at an example of such a function (full implementation details are available here),

@Bean
public Function<Flux<Integer>, Tuple2<Flux<String>, Flux<String>>> organise() {
    return flux -> ...;
}

Given that Project Reactor is a core dependency of SCF, we are using its Tuple library. Tuples give us a unique advantage by communicating to us both cardinality and type information. Both are extremely important in the context of SCSt. Cardinality lets us know how many input and output bindings need to be created and bound to the corresponding inputs and outputs of a function. Awareness of the type information ensures proper type conversion.

Also, this is where the ‘index’ part of the naming convention for binding names comes into play, since, in this function, the two output binding names are organise-out-0 and organise-out-1.

IMPORTANT: At the moment, function arity is only supported for reactive functions (Function<TupleN<Flux<?>…​>, TupleN<Flux<?>…​>>) centered on Complex event processing where evaluation and computation on confluence of events typically requires view into a stream of events rather than single event.

51.6. Type conversion (Content-Type negotiation)

Content-Type negotiation is one of the core features of Spring Cloud Function as it allows to not only transform the incoming data to the types declared by the function signature, but to do the same transformation during function composition making otherwise un-composable (by type) functions composable.

To better understand the mechanics and the necessity behind content-type negotiation, we take a look at a very simple use case by using the following function as an example:

@Bean
public Function<Person, String> personFunction {..}

The function shown in the preceding example expects a Person object as an argument and produces a String type as an output. If such function is invoked with the type Person, than all works fine. But typically function plays a role of a handler for the incoming data which most often comes in the raw format such as byte[], JSON String etc. In order for the framework to succeed in passing the incoming data as an argument to this function, it has to somehow transform the incoming data to a Person type.

Spring Cloud Function relies on two native to Spring mechanisms to accomplish that.

  1. MessageConverter - to convert from incoming Message data to a type declared by the function.

  2. ConversionService - to convert from incoming non-Message data to a type declared by the function.

This means that depending on the type of the raw data (Message or non-Message) Spring Cloud Function will apply one or the other mechanisms.

For most cases when dealing with functions that are invoked as part of some other request (e.g., HTTP, Messaging etc) the framework relies on MessageConverters, since such requests already converted to Spring Message. In other words, the framework locates and applies the appropriate MessageConverter. To accomplish that, the framework needs some instructions from the user. One of these instructions is already provided by the signature of the function itself (Person type). Consequently, in theory, that should be (and, in some cases, is) enough. However, for the majority of use cases, in order to select the appropriate MessageConverter, the framework needs an additional piece of information. That missing piece is contentType header.

Such header usually comes as part of the Message where it is injected by the corresponding adapter that created such Message in the first place. For example, HTTP POST request will have its content-type HTTP header copied to contentType header of the Message.

For cases when such header does not exist framework relies on the default content type as application/json.

51.6.1. Content Type versus Argument Type

As mentioned earlier, for the framework to select the appropriate MessageConverter, it requires argument type and, optionally, content type information. The logic for selecting the appropriate MessageConverter resides with the argument resolvers which trigger right before the invocation of the user-defined function (which is when the actual argument type is known to the framework). If the argument type does not match the type of the current payload, the framework delegates to the stack of the pre-configured MessageConverters to see if any one of them can convert the payload.

The combination of contentType and argument type is the mechanism by which framework determines if message can be converted to a target type by locating the appropriate MessageConverter. If no appropriate MessageConverter is found, an exception is thrown, which you can handle by adding a custom MessageConverter (see User-defined Message Converters).

Do not expect Message to be converted into some other type based only on the contentType. Remember that the contentType is complementary to the target type. It is a hint, which MessageConverter may or may not take into consideration.

51.6.2. Message Converters

MessageConverters define two methods:

Object fromMessage(Message<?> message, Class<?> targetClass);

Message<?> toMessage(Object payload, @Nullable MessageHeaders headers);

It is important to understand the contract of these methods and their usage, specifically in the context of Spring Cloud Stream.

The fromMessage method converts an incoming Message to an argument type. The payload of the Message could be any type, and it is up to the actual implementation of the MessageConverter to support multiple types.

51.6.3. Provided MessageConverters

As mentioned earlier, the framework already provides a stack of MessageConverters to handle most common use cases. The following list describes the provided MessageConverters, in order of precedence (the first MessageConverter that works is used):

  1. JsonMessageConverter: Supports conversion of the payload of the Message to/from POJO for cases when contentType is application/json using Jackson or Gson libraries (DEFAULT).

  2. ByteArrayMessageConverter: Supports conversion of the payload of the Message from byte[] to byte[] for cases when contentType is application/octet-stream. It is essentially a pass through and exists primarily for backward compatibility.

  3. StringMessageConverter: Supports conversion of any type to a String when contentType is text/plain.

When no appropriate converter is found, the framework throws an exception. When that happens, you should check your code and configuration and ensure you did not miss anything (that is, ensure that you provided a contentType by using a binding or a header). However, most likely, you found some uncommon case (such as a custom contentType perhaps) and the current stack of provided MessageConverters does not know how to convert. If that is the case, you can add custom MessageConverter. See User-defined Message Converters.

51.6.4. User-defined Message Converters

Spring Cloud Function exposes a mechanism to define and register additional MessageConverters. To use it, implement org.springframework.messaging.converter.MessageConverter, configure it as a @Bean. It is then appended to the existing stack of `MessageConverter`s.

It is important to understand that custom MessageConverter implementations are added to the head of the existing stack. Consequently, custom MessageConverter implementations take precedence over the existing ones, which lets you override as well as add to the existing converters.

The following example shows how to create a message converter bean to support a new content type called application/bar:

@SpringBootApplication
public static class SinkApplication {

    ...

    @Bean
    public MessageConverter customMessageConverter() {
        return new MyCustomMessageConverter();
    }
}

public class MyCustomMessageConverter extends AbstractMessageConverter {

    public MyCustomMessageConverter() {
        super(new MimeType("application", "bar"));
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return (Bar.class.equals(clazz));
    }

    @Override
    protected Object convertFromInternal(Message<?> message, Class<?> targetClass, Object conversionHint) {
        Object payload = message.getPayload();
        return (payload instanceof Bar ? payload : new Bar((byte[]) payload));
    }
}

51.7. Kotlin Lambda support

We also provide support for Kotlin lambdas (since v2.0). Consider the following:

@Bean
open fun kotlinSupplier(): () -> String {
    return  { "Hello from Kotlin" }
}

@Bean
open fun kotlinFunction(): (String) -> String {
    return  { it.toUpperCase() }
}

@Bean
open fun kotlinConsumer(): (String) -> Unit {
    return  { println(it) }
}

The above represents Kotlin lambdas configured as Spring beans. The signature of each maps to a Java equivalent of Supplier, Function and Consumer, and thus supported/recognized signatures by the framework. While mechanics of Kotlin-to-Java mapping are outside of the scope of this documentation, it is important to understand that the same rules for signature transformation outlined in "Java 8 function support" section are applied here as well.

To enable Kotlin support all you need is to add spring-cloud-function-kotlin module to your classpath which contains the appropriate autoconfiguration and supporting classes.

51.8. Function Component Scan

Spring Cloud Function will scan for implementations of Function, Consumer and Supplier in a package called functions if it exists. Using this feature you can write functions that have no dependencies on Spring - not even the @Component annotation is needed. If you want to use a different package, you can set spring.cloud.function.scan.packages. You can also use spring.cloud.function.scan.enabled=false to switch off the scan completely.

52. Standalone Web Applications

Functions could be automatically exported as HTTP endpoints.

The spring-cloud-function-web module has autoconfiguration that activates when it is included in a Spring Boot web application (with MVC support). There is also a spring-cloud-starter-function-web to collect all the optional dependencies in case you just want a simple getting started experience.

With the web configurations activated your app will have an MVC endpoint (on "/" by default, but configurable with spring.cloud.function.web.path) that can be used to access the functions in the application context where function name becomes part of the URL path. The supported content types are plain text and JSON.

Method Path Request Response Status

GET

/{supplier}

-

Items from the named supplier

200 OK

POST

/{consumer}

JSON object or text

Mirrors input and pushes request body into consumer

202 Accepted

POST

/{consumer}

JSON array or text with new lines

Mirrors input and pushes body into consumer one by one

202 Accepted

POST

/{function}

JSON object or text

The result of applying the named function

200 OK

POST

/{function}

JSON array or text with new lines

The result of applying the named function

200 OK

GET

/{function}/{item}

-

Convert the item into an object and return the result of applying the function

200 OK

As the table above shows the behaviour of the endpoint depends on the method and also the type of incoming request data. When the incoming data is single valued, and the target function is declared as obviously single valued (i.e. not returning a collection or Flux), then the response will also contain a single value. For multi-valued responses the client can ask for a server-sent event stream by sending `Accept: text/event-stream".

Functions and consumers that are declared with input and output in Message<?> will see the request headers on the input messages, and the output message headers will be converted to HTTP headers.

When POSTing text the response format might be different with Spring Boot 2.0 and older versions, depending on the content negotiation (provide content type and accept headers for the best results).

See Testing Functional Applications to see the details and example on how to test such application.

52.1. Function Mapping rules

If there is only a single function (consumer etc.) in the catalog, the name in the path is optional. In other words, providing you only have uppercase function in catalog curl -H "Content-Type: text/plain" localhost:8080/uppercase -d hello and curl -H "Content-Type: text/plain" localhost:8080/ -d hello calls are identical.

Composite functions can be addressed using pipes or commas to separate function names (pipes are legal in URL paths, but a bit awkward to type on the command line). For example, curl -H "Content-Type: text/plain" localhost:8080/uppercase,reverse -d hello.

For cases where there is more then a single function in catalog, each function will be exported and mapped with function name being part of the path (e.g., localhost:8080/uppercase). In this scenario you can still map specific function or function composition to the root path by providing spring.cloud.function.definition property

For example,

--spring.cloud.function.definition=foo|bar

The above property will compose 'foo' and 'bar' function and map the composed function to the "/" path.

52.2. Function Filtering rules

In situations where there are more then one function in catalog there may be a need to only export certain functions or function compositions. In that case you can use the same spring.cloud.function.definition property listing functions you intend to export delimited by ;. Note that in this case nothing will be mapped to the root path and functions that are not listed (including compositions) are not going to be exported

For example,

--spring.cloud.function.definition=foo;bar

This will only export function foo and function bar regardless how many functions are available in catalog (e.g., localhost:8080/foo).

--spring.cloud.function.definition=foo|bar;baz

This will only export function composition foo|bar and function baz regardless how many functions are available in catalog (e.g., localhost:8080/foo,bar).

53. Standalone Streaming Applications

To send or receive messages from a broker (such as RabbitMQ or Kafka) you can leverage spring-cloud-stream project and it’s integration with Spring Cloud Function. Please refer to Spring Cloud Function section of the Spring Cloud Stream reference manual for more details and examples.

54. Deploying a Packaged Function

Spring Cloud Function provides a "deployer" library that allows you to launch a jar file (or exploded archive, or set of jar files) with an isolated class loader and expose the functions defined in it. This is quite a powerful tool that would allow you to, for instance, adapt a function to a range of different input-output adapters without changing the target jar file. Serverless platforms often have this kind of feature built in, so you could see it as a building block for a function invoker in such a platform (indeed the Riff Java function invoker uses this library).

The standard entry point is to add spring-cloud-function-deployer to the classpath, the deployer kicks in and looks for some configuration to tell it where to find the function jar.

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-function-deployer</artifactId>
    <version>${spring.cloud.function.version}</version>
</dependency>

At a minimum the user has to provide a spring.cloud.function.location which is a URL or resource location for the archive containing the functions. It can optionally use a maven: prefix to locate the artifact via a dependency lookup (see FunctionProperties for complete details). A Spring Boot application is bootstrapped from the jar file, using the MANIFEST.MF to locate a start class, so that a standard Spring Boot fat jar works well, for example. If the target jar can be launched successfully then the result is a function registered in the main application’s FunctionCatalog. The registered function can be applied by code in the main application, even though it was created in an isolated class loader (by deault).

Here is the example of deploying a JAR which contains an 'uppercase' function and invoking it .

@SpringBootApplication
public class DeployFunctionDemo {

    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(DeployFunctionDemo.class,
                "--spring.cloud.function.location=..../target/uppercase-0.0.1-SNAPSHOT.jar",
                "--spring.cloud.function.definition=uppercase");

        FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
        Function<String, String> function = catalog.lookup("uppercase");
        System.out.println(function.apply("hello"));
    }
}

And here is the example using Maven URI (taken from one of the tests in FunctionDeployerTests):

@SpringBootApplication
public class DeployFunctionDemo {

    public static void main(String[] args) {
        String[] args = new String[] {
                "--spring.cloud.function.location=maven://oz.demo:demo-uppercase:0.0.1-SNAPSHOT",
                "--spring.cloud.function.function-class=oz.demo.uppercase.MyFunction" };

        ApplicationContext context = SpringApplication.run(DeployerApplication.class, args);
        FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
        Function<String, String> function = catalog.lookup("myFunction");

        assertThat(function.apply("bob")).isEqualTo("BOB");
    }
}

Keep in mind that Maven resource such as local and remote repositories, user, password and more are resolved using default MavenProperties which effectively use local defaults and will work for majority of cases. However if you need to customize you can simply provide a bean of type MavenProperties where you can set additional properties (see example below).

@Bean
public MavenProperties mavenProperties() {
    MavenProperties properties = new MavenProperties();
    properties.setLocalRepository("target/it/");
    return properties;
}

54.1. Supported Packaging Scenarios

Currently Spring Cloud Function supports several packaging scenarios to give you the most flexibility when it comes to deploying functions.

54.1.1. Simple JAR

This packaging option implies no dependency on anything related to Spring. For example; Consider that such JAR contains the following class:

package function.example;
. . .
public class UpperCaseFunction implements Function<String, String> {
    @Override
    public String apply(String value) {
        return value.toUpperCase();
    }
}

All you need to do is specify location and function-class properties when deploying such package:

--spring.cloud.function.location=target/it/simplestjar/target/simplestjar-1.0.0.RELEASE.jar
--spring.cloud.function.function-class=function.example.UpperCaseFunction

It’s conceivable in some cases that you might want to package multiple functions together. For such scenarios you can use spring.cloud.function.function-class property to list several classes delimiting them by ;.

For example,

--spring.cloud.function.function-class=function.example.UpperCaseFunction;function.example.ReverseFunction

Here we are identifying two functions to deploy, which we can now access in function catalog by name (e.g., catalog.lookup("reverseFunction");).

For more details please reference the complete sample available here. You can also find a corresponding test in FunctionDeployerTests.

54.1.2. Spring Boot JAR

This packaging option implies there is a dependency on Spring Boot and that the JAR was generated as Spring Boot JAR. That said, given that the deployed JAR runs in the isolated class loader, there will not be any version conflict with the Spring Boot version used by the actual deployer. For example; Consider that such JAR contains the following class (which could have some additional Spring dependencies providing Spring/Spring Boot is on the classpath):

package function.example;
. . .
public class UpperCaseFunction implements Function<String, String> {
    @Override
    public String apply(String value) {
        return value.toUpperCase();
    }
}

As before all you need to do is specify location and function-class properties when deploying such package:

--spring.cloud.function.location=target/it/simplestjar/target/simplestjar-1.0.0.RELEASE.jar
--spring.cloud.function.function-class=function.example.UpperCaseFunction

For more details please reference the complete sample available here. You can also find a corresponding test in FunctionDeployerTests.

54.1.3. Spring Boot Application

This packaging option implies your JAR is complete stand alone Spring Boot application with functions as managed Spring beans. As before there is an obvious assumption that there is a dependency on Spring Boot and that the JAR was generated as Spring Boot JAR. That said, given that the deployed JAR runs in the isolated class loader, there will not be any version conflict with the Spring Boot version used by the actual deployer. For example; Consider that such JAR contains the following class:

package function.example;
. . .
@SpringBootApplication
public class SimpleFunctionAppApplication {

    public static void main(String[] args) {
        SpringApplication.run(SimpleFunctionAppApplication.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
}

Given that we’re effectively dealing with another Spring Application context and that functions are spring managed beans, in addition to the location property we also specify definition property instead of function-class.

--spring.cloud.function.location=target/it/bootapp/target/bootapp-1.0.0.RELEASE-exec.jar
--spring.cloud.function.definition=uppercase

For more details please reference the complete sample available here. You can also find a corresponding test in FunctionDeployerTests.

This particular deployment option may or may not have Spring Cloud Function on it’s classpath. From the deployer perspective this doesn’t matter.

55. Functional Bean Definitions

Spring Cloud Function supports a "functional" style of bean declarations for small apps where you need fast startup. The functional style of bean declaration was a feature of Spring Framework 5.0 with significant enhancements in 5.1.

55.1. Comparing Functional with Traditional Bean Definitions

Here’s a vanilla Spring Cloud Function application from with the familiar @Configuration and @Bean declaration style:

@SpringBootApplication
public class DemoApplication {

  @Bean
  public Function<String, String> uppercase() {
    return value -> value.toUpperCase();
  }

  public static void main(String[] args) {
    SpringApplication.run(DemoApplication.class, args);
  }

}

Now for the functional beans: the user application code can be recast into "functional" form, like this:

@SpringBootConfiguration
public class DemoApplication implements ApplicationContextInitializer<GenericApplicationContext> {

  public static void main(String[] args) {
    FunctionalSpringApplication.run(DemoApplication.class, args);
  }

  public Function<String, String> uppercase() {
    return value -> value.toUpperCase();
  }

  @Override
  public void initialize(GenericApplicationContext context) {
    context.registerBean("demo", FunctionRegistration.class,
        () -> new FunctionRegistration<>(uppercase())
            .type(FunctionType.from(String.class).to(String.class)));
  }

}

The main differences are:

  • The main class is an ApplicationContextInitializer.

  • The @Bean methods have been converted to calls to context.registerBean()

  • The @SpringBootApplication has been replaced with @SpringBootConfiguration to signify that we are not enabling Spring Boot autoconfiguration, and yet still marking the class as an "entry point".

  • The SpringApplication from Spring Boot has been replaced with a FunctionalSpringApplication from Spring Cloud Function (it’s a subclass).

The business logic beans that you register in a Spring Cloud Function app are of type FunctionRegistration. This is a wrapper that contains both the function and information about the input and output types. In the @Bean form of the application that information can be derived reflectively, but in a functional bean registration some of it is lost unless we use a FunctionRegistration.

An alternative to using an ApplicationContextInitializer and FunctionRegistration is to make the application itself implement Function (or Consumer or Supplier). Example (equivalent to the above):

@SpringBootConfiguration
public class DemoApplication implements Function<String, String> {

  public static void main(String[] args) {
    FunctionalSpringApplication.run(DemoApplication.class, args);
  }

  @Override
  public String apply(String value) {
    return value.toUpperCase();
  }

}

It would also work if you add a separate, standalone class of type Function and register it with the SpringApplication using an alternative form of the run() method. The main thing is that the generic type information is available at runtime through the class declaration.

Suppose you have

@Component
public class CustomFunction implements Function<Flux<Foo>, Flux<Bar>> {
    @Override
    public Flux<Bar> apply(Flux<Foo> flux) {
        return flux.map(foo -> new Bar("This is a Bar object from Foo value: " + foo.getValue()));
    }

}

You register it as such:

@Override
public void initialize(GenericApplicationContext context) {
        context.registerBean("function", FunctionRegistration.class,
                () -> new FunctionRegistration<>(new CustomFunction()).type(CustomFunction.class));
}

55.2. Limitations of Functional Bean Declaration

Most Spring Cloud Function apps have a relatively small scope compared to the whole of Spring Boot, so we are able to adapt it to these functional bean definitions easily. If you step outside that limited scope, you can extend your Spring Cloud Function app by switching back to @Bean style configuration, or by using a hybrid approach. If you want to take advantage of Spring Boot autoconfiguration for integrations with external datastores, for example, you will need to use @EnableAutoConfiguration. Your functions can still be defined using the functional declarations if you want (i.e. the "hybrid" style), but in that case you will need to explicitly switch off the "full functional mode" using spring.functional.enabled=false so that Spring Boot can take back control.

56. Testing Functional Applications

Spring Cloud Function also has some utilities for integration testing that will be very familiar to Spring Boot users.

Suppose this is your application:

@SpringBootApplication
public class SampleFunctionApplication {

    public static void main(String[] args) {
        SpringApplication.run(SampleFunctionApplication.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return v -> v.toUpperCase();
    }
}

Here is an integration test for the HTTP server wrapping this application:

@SpringBootTest(classes = SampleFunctionApplication.class,
            webEnvironment = WebEnvironment.RANDOM_PORT)
public class WebFunctionTests {

    @Autowired
    private TestRestTemplate rest;

    @Test
    public void test() throws Exception {
        ResponseEntity<String> result = this.rest.exchange(
            RequestEntity.post(new URI("/uppercase")).body("hello"), String.class);
        System.out.println(result.getBody());
    }
}

or when function bean definition style is used:

@FunctionalSpringBootTest
public class WebFunctionTests {

    @Autowired
    private TestRestTemplate rest;

    @Test
    public void test() throws Exception {
        ResponseEntity<String> result = this.rest.exchange(
            RequestEntity.post(new URI("/uppercase")).body("hello"), String.class);
        System.out.println(result.getBody());
    }
}

This test is almost identical to the one you would write for the @Bean version of the same app - the only difference is the @FunctionalSpringBootTest annotation, instead of the regular @SpringBootTest. All the other pieces, like the @Autowired TestRestTemplate, are standard Spring Boot features.

And to help with correct dependencies here is the excerpt from POM

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.2.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    . . . .
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-function-web</artifactId>
        <version>3.0.1.BUILD-SNAPSHOT</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
        <exclusions>
            <exclusion>
                <groupId>org.junit.vintage</groupId>
                <artifactId>junit-vintage-engine</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

Or you could write a test for a non-HTTP app using just the FunctionCatalog. For example:

@RunWith(SpringRunner.class)
@FunctionalSpringBootTest
public class FunctionalTests {

    @Autowired
    private FunctionCatalog catalog;

    @Test
    public void words() throws Exception {
        Function<String, String> function = catalog.lookup(Function.class,
                "uppercase");
        assertThat(function.apply("hello")).isEqualTo("HELLO");
    }

}

57. Dynamic Compilation

There is a sample app that uses the function compiler to create a function from a configuration property. The vanilla "function-sample" also has that feature. And there are some scripts that you can run to see the compilation happening at run time. To run these examples, change into the scripts directory:

cd scripts

Also, start a RabbitMQ server locally (e.g. execute rabbitmq-server).

Start the Function Registry Service:

./function-registry.sh

Register a Function:

./registerFunction.sh -n uppercase -f "f->f.map(s->s.toString().toUpperCase())"

Run a REST Microservice using that Function:

./web.sh -f uppercase -p 9000
curl -H "Content-Type: text/plain" -H "Accept: text/plain" localhost:9000/uppercase -d foo

Register a Supplier:

./registerSupplier.sh -n words -f "()->Flux.just(\"foo\",\"bar\")"

Run a REST Microservice using that Supplier:

./web.sh -s words -p 9001
curl -H "Accept: application/json" localhost:9001/words

Register a Consumer:

./registerConsumer.sh -n print -t String -f "System.out::println"

Run a REST Microservice using that Consumer:

./web.sh -c print -p 9002
curl -X POST -H "Content-Type: text/plain" -d foo localhost:9002/print

Run Stream Processing Microservices:

First register a streaming words supplier:

./registerSupplier.sh -n wordstream -f "()->Flux.interval(Duration.ofMillis(1000)).map(i->\"message-\"+i)"

Then start the source (supplier), processor (function), and sink (consumer) apps (in reverse order):

./stream.sh -p 9103 -i uppercaseWords -c print
./stream.sh -p 9102 -i words -f uppercase -o uppercaseWords
./stream.sh -p 9101 -s wordstream -o words

The output will appear in the console of the sink app (one message per second, converted to uppercase):

MESSAGE-0
MESSAGE-1
MESSAGE-2
MESSAGE-3
MESSAGE-4
MESSAGE-5
MESSAGE-6
MESSAGE-7
MESSAGE-8
MESSAGE-9
...

58. Serverless Platform Adapters

As well as being able to run as a standalone process, a Spring Cloud Function application can be adapted to run one of the existing serverless platforms. In the project there are adapters for AWS Lambda, Azure, and Apache OpenWhisk. The Oracle Fn platform has its own Spring Cloud Function adapter. And Riff supports Java functions and its Java Function Invoker acts natively is an adapter for Spring Cloud Function jars.

58.1. AWS Lambda

The AWS adapter takes a Spring Cloud Function app and converts it to a form that can run in AWS Lambda.

The details of how to get stared with AWS Lambda is out of scope of this document, so the expectation is that user has some familiarity with AWS and AWS Lambda and wants to learn what additional value spring provides.

58.1.1. Getting Started

One of the goals of Spring Cloud Function framework is to provide necessary infrastructure elements to enable a simple function application to interact in a certain way in a particular environment. A simple function application (in context or Spring) is an application that contains beans of type Supplier, Function or Consumer. So, with AWS it means that a simple function bean should somehow be recognised and executed in AWS Lambda environment.

Let’s look at the example:

@SpringBootApplication
public class FunctionConfiguration {

    public static void main(String[] args) {
        SpringApplication.run(FunctionConfiguration.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
}

It shows a complete Spring Boot application with a function bean defined in it. What’s interesting is that on the surface this is just another boot app, but in the context of AWS Adapter it is also a perfectly valid AWS Lambda application. No other code or configuration is required. All you need to do is package it and deploy it, so let’s look how we can do that.

To make things simpler we’ve provided a sample project ready to be built and deployed and you can access it here.

You simply execute ./mvnw clean package to generate JAR file. All the necessary maven plugins have already been setup to generate appropriate AWS deployable JAR file. (You can read more details about JAR layout in Notes on JAR Layout).

Then you have to upload the JAR file (via AWS dashboard or AWS CLI) to AWS.

When ask about handler you specify org.springframework.cloud.function.adapter.aws.FunctionInvoker::handleRequest which is a generic request handler.

AWS deploy

That is all. Save and execute the function with some sample data which for this function is expected to be a String which function will uppercase and return back.

While org.springframework.cloud.function.adapter.aws.FunctionInvoker is a general purpose AWS’s RequestHandler implementation aimed at completely isolating you from the specifics of AWS Lambda API, for some cases you may want to specify which specific AWS’s RequestHandler you want to use. The next section will explain you how you can accomplish just that.

58.1.2. AWS Request Handlers

The adapter has a couple of generic request handlers that you can use. The most generic is (and the one we used in the Getting Started section) is org.springframework.cloud.function.adapter.aws.FunctionInvoker which is the implementation of AWS’s RequestStreamHandler. User doesn’t need to do anything other then specify it as 'handler' on AWS dashboard when deploying function. It will handle most of the case including Kinesis, streaming etc. .

If your app has more than one @Bean of type Function etc. then you can choose the one to use by configuring spring.cloud.function.definition property or environment variable. The functions are extracted from the Spring Cloud FunctionCatalog. In the event you don’t specify spring.cloud.function.definition the framework will attempt to find a default following the search order where it searches first for Function then Consumer and finally Supplier).

58.1.3. Notes on JAR Layout

You don’t need the Spring Cloud Function Web or Stream adapter at runtime in Lambda, so you might need to exclude those before you create the JAR you send to AWS. A Lambda application has to be shaded, but a Spring Boot standalone application does not, so you can run the same app using 2 separate jars (as per the sample). The sample app creates 2 jar files, one with an aws classifier for deploying in Lambda, and one executable (thin) jar that includes spring-cloud-function-web at runtime. Spring Cloud Function will try and locate a "main class" for you from the JAR file manifest, using the Start-Class attribute (which will be added for you by the Spring Boot tooling if you use the starter parent). If there is no Start-Class in your manifest you can use an environment variable or system property MAIN_CLASS when you deploy the function to AWS.

If you are not using the functional bean definitions but relying on Spring Boot’s auto-configuration, then additional transformers must be configured as part of the maven-shade-plugin execution.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </dependency>
    </dependencies>
    <configuration>
        <createDependencyReducedPom>false</createDependencyReducedPom>
        <shadedArtifactAttached>true</shadedArtifactAttached>
        <shadedClassifierName>aws</shadedClassifierName>
        <transformers>
            <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                <resource>META-INF/spring.handlers</resource>
            </transformer>
            <transformer implementation="org.springframework.boot.maven.PropertiesMergingResourceTransformer">
                <resource>META-INF/spring.factories</resource>
            </transformer>
            <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                <resource>META-INF/spring.schemas</resource>
            </transformer>
        </transformers>
    </configuration>
</plugin>

58.1.4. Build file setup

In order to run Spring Cloud Function applications on AWS Lambda, you can leverage Maven or Gradle plugins offered by the cloud platform provider.

Maven

In order to use the adapter plugin for Maven, add the plugin dependency to your pom.xml file:

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-function-adapter-aws</artifactId>
    </dependency>
</dependencies>

As pointed out in the Notes on JAR Layout, you will need a shaded jar in order to upload it to AWS Lambda. You can use the Maven Shade Plugin for that. The example of the setup can be found above.

You can use theSpring Boot Maven Plugin to generate the thin jar.

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot.experimental</groupId>
            <artifactId>spring-boot-thin-layout</artifactId>
            <version>${wrapper.version}</version>
        </dependency>
    </dependencies>
</plugin>

You can find the entire sample pom.xml file for deploying Spring Cloud Function applications to AWS Lambda with Maven here.

Gradle

In order to use the adapter plugin for Gradle, add the dependency to your build.gradle file:

dependencies {
    compile("org.springframework.cloud:spring-cloud-function-adapter-aws:${version}")
}

As pointed out in Notes on JAR Layout, you will need a shaded jar in order to upload it to AWS Lambda. You can use the Gradle Shadow Plugin for that:

buildscript {
    dependencies {
        classpath "com.github.jengelman.gradle.plugins:shadow:${shadowPluginVersion}"
    }
}
apply plugin: 'com.github.johnrengelman.shadow'

assemble.dependsOn = [shadowJar]

import com.github.jengelman.gradle.plugins.shadow.transformers.*

shadowJar {
    classifier = 'aws'
    dependencies {
        exclude(
            dependency("org.springframework.cloud:spring-cloud-function-web:${springCloudFunctionVersion}"))
    }
    // Required for Spring
    mergeServiceFiles()
    append 'META-INF/spring.handlers'
    append 'META-INF/spring.schemas'
    append 'META-INF/spring.tooling'
    transform(PropertiesFileTransformer) {
        paths = ['META-INF/spring.factories']
        mergeStrategy = "append"
    }
}

You can use the Spring Boot Gradle Plugin and Spring Boot Thin Gradle Plugin to generate the thin jar.

buildscript {
    dependencies {
        classpath("org.springframework.boot.experimental:spring-boot-thin-gradle-plugin:${wrapperVersion}")
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
    }
}
apply plugin: 'org.springframework.boot'
apply plugin: 'org.springframework.boot.experimental.thin-launcher'
assemble.dependsOn = [thinJar]

You can find the entire sample build.gradle file for deploying Spring Cloud Function applications to AWS Lambda with Gradle here.

58.1.5. Upload

Build the sample under spring-cloud-function-samples/function-sample-aws and upload the -aws jar file to Lambda. The handler can be example.Handler or org.springframework.cloud.function.adapter.aws.SpringBootStreamHandler (FQN of the class, not a method reference, although Lambda does accept method references).

./mvnw -U clean package

Using the AWS command line tools it looks like this:

aws lambda create-function --function-name Uppercase --role arn:aws:iam::[USERID]:role/service-role/[ROLE] --zip-file fileb://function-sample-aws/target/function-sample-aws-2.0.0.BUILD-SNAPSHOT-aws.jar --handler org.springframework.cloud.function.adapter.aws.SpringBootStreamHandler --description "Spring Cloud Function Adapter Example" --runtime java8 --region us-east-1 --timeout 30 --memory-size 1024 --publish

The input type for the function in the AWS sample is a Foo with a single property called "value". So you would need this to test it:

{
  "value": "test"
}
The AWS sample app is written in the "functional" style (as an ApplicationContextInitializer). This is much faster on startup in Lambda than the traditional @Bean style, so if you don’t need @Beans (or @EnableAutoConfiguration) it’s a good choice. Warm starts are not affected.

58.1.6. Type Conversion

Spring Cloud Function will attempt to transparently handle type conversion between the raw input stream and types declared by your function.

For example, if your function signature is as such Function<Foo, Bar> we will attempt to convert incoming stream event to an instance of Foo.

In the event type is not known or can not be determined (e.g., Function<?, ?>) we will attempt to convert an incoming stream event to a generic Map.

Raw Input

There are times when you may want to have access to a raw input. In this case all you need is to declare your function signature to accept InputStream. For example, Function<InputStream, ?>. In this case we will not attempt any conversion and will pass the raw input directly to a function.

58.2. Microsoft Azure

The Azure adapter bootstraps a Spring Cloud Function context and channels function calls from the Azure framework into the user functions, using Spring Boot configuration where necessary. Azure Functions has quite a unique, but invasive programming model, involving annotations in user code that are specific to the platform. The easiest way to use it with Spring Cloud is to extend a base class and write a method in it with the @FunctionName annotation which delegates to a base class method.

This project provides an adapter layer for a Spring Cloud Function application onto Azure. You can write an app with a single @Bean of type Function and it will be deployable in Azure if you get the JAR file laid out right.

There is an AzureSpringBootRequestHandler which you must extend, and provide the input and output types as annotated method parameters (enabling Azure to inspect the class and create JSON bindings). The base class has two useful methods (handleRequest and handleOutput) to which you can delegate the actual function call, so mostly the function will only ever have one line.

Example:

public class FooHandler extends AzureSpringBootRequestHandler<Foo, Bar> {
    @FunctionName("uppercase")
    public Bar execute(@HttpTrigger(name = "req", methods = {HttpMethod.GET,
            HttpMethod.POST}, authLevel = AuthorizationLevel.ANONYMOUS) HttpRequestMessage<Optional<Foo>> request,
        ExecutionContext context) {
        return handleRequest(request.getBody().get(), context);
    }
}

This Azure handler will delegate to a Function<Foo,Bar> bean (or a Function<Publisher<Foo>,Publisher<Bar>>). Some Azure triggers (e.g. @CosmosDBTrigger) result in a input type of List and in that case you can bind to List in the Azure handler, or String (the raw JSON). The List input delegates to a Function with input type Map<String,Object>, or Publisher or List of the same type. The output of the Function can be a List (one-for-one) or a single value (aggregation), and the output binding in the Azure declaration should match.

If your app has more than one @Bean of type Function etc. then you can choose the one to use by configuring function.name. Or if you make the @FunctionName in the Azure handler method match the function name it should work that way (also for function apps with multiple functions). The functions are extracted from the Spring Cloud FunctionCatalog so the default function names are the same as the bean names.

58.2.1. Accessing Azure ExecutionContext

Some time there is a need to access the target execution context provided by Azure runtime in the form of com.microsoft.azure.functions.ExecutionContext. For example one of such needs is logging, so it can appear in the Azure console.

For that purpose Spring Cloud Function will register ExecutionContext as bean in the Application context, so it could be injected into your function. For example

@Bean
public Function<Foo, Bar> uppercase(ExecutionContext targetContext) {
    return foo -> {
        targetContext.getLogger().info("Invoking 'uppercase' on " + foo.getValue());
        return new Bar(foo.getValue().toUpperCase());
    };
}

Normally type-based injection should suffice, however if need to you can also utilise the bean name under which it is registered which is targetExecutionContext.

58.2.2. Notes on JAR Layout

You don’t need the Spring Cloud Function Web at runtime in Azure, so you can exclude this before you create the JAR you deploy to Azure, but it won’t be used if you include it, so it doesn’t hurt to leave it in. A function application on Azure is an archive generated by the Maven plugin. The function lives in the JAR file generated by this project. The sample creates it as an executable jar, using the thin layout, so that Azure can find the handler classes. If you prefer you can just use a regular flat JAR file. The dependencies should not be included.

58.2.3. Build file setup

In order to run Spring Cloud Function applications on Microsoft Azure, you can leverage the Maven plugin offered by the cloud platform provider.

In order to use the adapter plugin for Maven, add the plugin dependency to your pom.xml file:

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-function-adapter-azure</artifactId>
    </dependency>
</dependencies>

Then, configure the plugin. You will need to provide Azure-specific configuration for your application, specifying the resourceGroup, appName and other optional properties, and add the package goal execution so that the function.json file required by Azure is generated for you. Full plugin documentation can be found in the plugin repository.

<plugin>
    <groupId>com.microsoft.azure</groupId>
    <artifactId>azure-functions-maven-plugin</artifactId>
    <configuration>
        <resourceGroup>${functionResourceGroup}</resourceGroup>
        <appName>${functionAppName}</appName>
    </configuration>
    <executions>
        <execution>
            <id>package-functions</id>
            <goals>
                <goal>package</goal>
            </goals>
        </execution>
    </executions>
</plugin>

You will also have to ensure that the files to be scanned by the plugin can be found in the Azure functions staging directory (see the plugin repository for more details on the staging directory and it’s default location).

You can find the entire sample pom.xml file for deploying Spring Cloud Function applications to Microsoft Azure with Maven here.

As of yet, only Maven plugin is available. Gradle plugin has not been created by the cloud platform provider.

58.2.4. Build

./mvnw -U clean package

58.2.5. Running the sample

You can run the sample locally, just like the other Spring Cloud Function samples:



and curl -H "Content-Type: text/plain" localhost:8080/api/uppercase -d '{"value": "hello foobar"}'.

You will need the az CLI app (see docs.microsoft.com/en-us/azure/azure-functions/functions-create-first-java-maven for more detail). To deploy the function on Azure runtime:

$ az login
$ mvn azure-functions:deploy

On another terminal try this: curl <azure-function-url-from-the-log>/api/uppercase -d '{"value": "hello foobar!"}'. Please ensure that you use the right URL for the function above. Alternatively you can test the function in the Azure Dashboard UI (click on the function name, go to the right hand side and click "Test" and to the bottom right, "Run").

The input type for the function in the Azure sample is a Foo with a single property called "value". So you need this to test it with something like below:

{
  "value": "foobar"
}
The Azure sample app is written in the "non-functional" style (using @Bean). The functional style (with just Function or ApplicationContextInitializer) is much faster on startup in Azure than the traditional @Bean style, so if you don’t need @Beans (or @EnableAutoConfiguration) it’s a good choice. Warm starts are not affected. :branch: master

58.3. Google Cloud Functions (Alpha)

The Google Cloud Functions adapter enables Spring Cloud Function apps to run on the Google Cloud Functions serverless platform. You can either run the function locally using the open source Google Functions Framework for Java or on GCP.

58.3.1. Project Dependencies

Start by adding the spring-cloud-function-adapter-gcp dependency to your project.

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-function-adapter-gcp</artifactId>
    </dependency>

    ...
</dependencies>

In addition, add the spring-boot-maven-plugin which will build the JAR of the function to deploy.

Notice that we also reference spring-cloud-function-adapter-gcp as a dependency of the spring-boot-maven-plugin. This is necessary because it modifies the plugin to package your function in the correct JAR format for deployment on Google Cloud Functions.
<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <outputDirectory>target/deploy</outputDirectory>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-function-adapter-gcp</artifactId>
        </dependency>
    </dependencies>
</plugin>

Finally, add the Maven plugin provided as part of the Google Functions Framework for Java. This allows you to test your functions locally via mvn function:run.

The function target should always be set to org.springframework.cloud.function.adapter.gcp.GcfJarLauncher; this is an adapter class which acts as the entry point to your Spring Cloud Function from the Google Cloud Functions platform.
<plugin>
    <groupId>com.google.cloud.functions</groupId>
    <artifactId>function-maven-plugin</artifactId>
    <version>0.9.1</version>
    <configuration>
        <functionTarget>org.springframework.cloud.function.adapter.gcp.GcfJarLauncher</functionTarget>
        <port>8080</port>
    </configuration>
</plugin>

A full example of a working pom.xml can be found in the Spring Cloud Functions GCP sample.

58.3.2. HTTP Functions

Google Cloud Functions supports deploying HTTP Functions, which are functions that are invoked by HTTP request. The sections below describe instructions for deploying a Spring Cloud Function as an HTTP Function.

Getting Started

Let’s start with a simple Spring Cloud Function example:

@SpringBootApplication
public class CloudFunctionMain {

    public static void main(String[] args) {
        SpringApplication.run(CloudFunctionMain.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
}

Specify your configuration main class in resources/META-INF/MANIFEST.MF.

Main-Class: com.example.CloudFunctionMain

Then run the function locally. This is provided by the Google Cloud Functions function-maven-plugin described in the project dependencies section.

mvn function:run

Invoke the HTTP function:

curl http://localhost:8080/ -d "hello"
Deploy to GCP

As of March 2020, Google Cloud Functions for Java is in Alpha. You can get on the whitelist to try it out.

Start by packaging your application.

mvn package

If you added the custom spring-boot-maven-plugin plugin defined above, you should see the resulting JAR in target/deploy directory. This JAR is correctly formatted for deployment to Google Cloud Functions.

Next, make sure that you have the Cloud SDK CLI installed.

From the project base directory run the following command to deploy.

gcloud alpha functions deploy function-sample-gcp-http \
--entry-point org.springframework.cloud.function.adapter.gcp.GcfJarLauncher \
--runtime java11 \
--trigger-http \
--source target/deploy \
--memory 512MB

Invoke the HTTP function:

curl https://REGION-PROJECT_ID.cloudfunctions.net/function-sample-gcp-http -d "hello"

58.3.3. Background Functions

Google Cloud Functions also supports deploying Background Functions which are invoked indirectly in response to an event, such as a message on a Cloud Pub/Sub topic, a change in a Cloud Storage bucket, or a Firebase event.

The spring-cloud-function-adapter-gcp allows for functions to be deployed as background functions as well.

The sections below describe the process for writing a Cloud Pub/Sub topic background function. However, there are a number of different event types that can trigger a background function to execute which are not discussed here; these are described in the Background Function triggers documentation.

Getting Started

Let’s start with a simple Spring Cloud Function which will run as a GCF background function:

@SpringBootApplication
public class BackgroundFunctionMain {

    public static void main(String[] args) {
        SpringApplication.run(BackgroundFunctionMain.class, args);
    }

    @Bean
    public Consumer<PubSubMessage> pubSubFunction() {
        return message -> System.out.println("The Pub/Sub message data: " + message.getData());
    }
}

In addition, create PubSubMessage class in the project with the below definition. This class represents the Pub/Sub event structure which gets passed to your function on a Pub/Sub topic event.

public class PubSubMessage {

    private String data;

    private Map<String, String> attributes;

    private String messageId;

    private String publishTime;

    public String getData() {
        return data;
    }

    public void setData(String data) {
        this.data = data;
    }

    public Map<String, String> getAttributes() {
        return attributes;
    }

    public void setAttributes(Map<String, String> attributes) {
        this.attributes = attributes;
    }

    public String getMessageId() {
        return messageId;
    }

    public void setMessageId(String messageId) {
        this.messageId = messageId;
    }

    public String getPublishTime() {
        return publishTime;
    }

    public void setPublishTime(String publishTime) {
        this.publishTime = publishTime;
    }

}

Specify your configuration main class in resources/META-INF/MANIFEST.MF.

Main-Class: com.example.BackgroundFunctionMain

Then run the function locally. This is provided by the Google Cloud Functions function-maven-plugin described in the project dependencies section.

mvn function:run

Invoke the HTTP function:

curl localhost:8080 -H "Content-Type: application/json" -d '{"data":"hello"}'

Verify that the function was invoked by viewing the logs.

Deploy to GCP

In order to deploy your background function to GCP, first package your application.

mvn package

If you added the custom spring-boot-maven-plugin plugin defined above, you should see the resulting JAR in target/deploy directory. This JAR is correctly formatted for deployment to Google Cloud Functions.

Next, make sure that you have the Cloud SDK CLI installed.

From the project base directory run the following command to deploy.

gcloud alpha functions deploy function-sample-gcp-background \
--entry-point org.springframework.cloud.function.adapter.gcp.GcfJarLauncher \
--runtime java11 \
--trigger-topic my-functions-topic \
--source target/deploy \
--memory 512MB

Google Cloud Function will now invoke the function every time a message is published to the topic specified by --trigger-topic.

For a walkthrough on testing and verifying your background function, see the instructions for running the GCF Background Function sample.

58.3.4. Sample Functions

The project provides the following sample functions as reference:

Spring Cloud Gateway

2020.0.0-SNAPSHOT

This project provides an API Gateway built on top of the Spring Ecosystem, including: Spring 5, Spring Boot 2 and Project Reactor. Spring Cloud Gateway aims to provide a simple, yet effective way to route to APIs and provide cross cutting concerns to them such as: security, monitoring/metrics, and resiliency.

59. How to Include Spring Cloud Gateway

To include Spring Cloud Gateway in your project, use the starter with a group ID of org.springframework.cloud and an artifact ID of spring-cloud-starter-gateway. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

If you include the starter, but you do not want the gateway to be enabled, set spring.cloud.gateway.enabled=false.

Spring Cloud Gateway is built on Spring Boot 2.x, Spring WebFlux, and Project Reactor. As a consequence, many of the familiar synchronous libraries (Spring Data and Spring Security, for example) and patterns you know may not apply when you use Spring Cloud Gateway. If you are unfamiliar with these projects, we suggest you begin by reading their documentation to familiarize yourself with some of the new concepts before working with Spring Cloud Gateway.
Spring Cloud Gateway requires the Netty runtime provided by Spring Boot and Spring Webflux. It does not work in a traditional Servlet Container or when built as a WAR.

60. Glossary

  • Route: The basic building block of the gateway. It is defined by an ID, a destination URI, a collection of predicates, and a collection of filters. A route is matched if the aggregate predicate is true.

  • Predicate: This is a Java 8 Function Predicate. The input type is a Spring Framework ServerWebExchange. This lets you match on anything from the HTTP request, such as headers or parameters.

  • Filter: These are instances of Spring Framework GatewayFilter that have been constructed with a specific factory. Here, you can modify requests and responses before or after sending the downstream request.

61. How It Works

The following diagram provides a high-level overview of how Spring Cloud Gateway works:

Spring Cloud Gateway Diagram

Clients make requests to Spring Cloud Gateway. If the Gateway Handler Mapping determines that a request matches a route, it is sent to the Gateway Web Handler. This handler runs the request through a filter chain that is specific to the request. The reason the filters are divided by the dotted line is that filters can run logic both before and after the proxy request is sent. All “pre” filter logic is executed. Then the proxy request is made. After the proxy request is made, the “post” filter logic is run.

URIs defined in routes without a port get default port values of 80 and 443 for the HTTP and HTTPS URIs, respectively.

62. Configuring Route Predicate Factories and Gateway Filter Factories

There are two ways to configure predicates and filters: shortcuts and fully expanded arguments. Most examples below use the shortcut way.

The name and argument names will be listed as code in the first sentance or two of the each section. The arguments are typically listed in the order that would be needed for the shortcut configuration.

62.1. Shortcut Configuration

Shortcut configuration is recognized by the filter name, followed by an equals sign (=), followed by argument values separated by commas (,).

application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: after_route
        uri: https://example.org
        predicates:
        - Cookie=mycookie,mycookievalue

The previous sample defines the Cookie Route Predicate Factory with two arguments, the cookie name, mycookie and the value to match mycookievalue.

62.2. Fully Expanded Arguments

Fully expanded arguments appear more like standard yaml configuration with name/value pairs. Typically, there will be a name key and an args key. The args key is a map of key value pairs to configure the predicate or filter.

application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: after_route
        uri: https://example.org
        predicates:
        - name: Cookie
          args:
            name: mycookie
            regexp: mycookievalue

This is the full configuration of the shortcut configuration of the Cookie predicate shown above.

63. Route Predicate Factories

Spring Cloud Gateway matches routes as part of the Spring WebFlux HandlerMapping infrastructure. Spring Cloud Gateway includes many built-in route predicate factories. All of these predicates match on different attributes of the HTTP request. You can combine multiple route predicate factories with logical and statements.

63.1. The After Route Predicate Factory

The After route predicate factory takes one parameter, a datetime (which is a java ZonedDateTime). This predicate matches requests that happen after the specified datetime. The following example configures an after route predicate:

Example 5. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: after_route
        uri: https://example.org
        predicates:
        - After=2017-01-20T17:42:47.789-07:00[America/Denver]

This route matches any request made after Jan 20, 2017 17:42 Mountain Time (Denver).

63.2. The Before Route Predicate Factory

The Before route predicate factory takes one parameter, a datetime (which is a java ZonedDateTime). This predicate matches requests that happen before the specified datetime. The following example configures a before route predicate:

Example 6. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: before_route
        uri: https://example.org
        predicates:
        - Before=2017-01-20T17:42:47.789-07:00[America/Denver]

This route matches any request made before Jan 20, 2017 17:42 Mountain Time (Denver).

63.3. The Between Route Predicate Factory

The Between route predicate factory takes two parameters, datetime1 and datetime2 which are java ZonedDateTime objects. This predicate matches requests that happen after datetime1 and before datetime2. The datetime2 parameter must be after datetime1. The following example configures a between route predicate:

Example 7. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: between_route
        uri: https://example.org
        predicates:
        - Between=2017-01-20T17:42:47.789-07:00[America/Denver], 2017-01-21T17:42:47.789-07:00[America/Denver]

This route matches any request made after Jan 20, 2017 17:42 Mountain Time (Denver) and before Jan 21, 2017 17:42 Mountain Time (Denver). This could be useful for maintenance windows.

The Cookie route predicate factory takes two parameters, the cookie name and a regexp (which is a Java regular expression). This predicate matches cookies that have the given name and whose values match the regular expression. The following example configures a cookie route predicate factory:

Example 8. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: cookie_route
        uri: https://example.org
        predicates:
        - Cookie=chocolate, ch.p

This route matches requests that have a cookie named chocolate whose value matches the ch.p regular expression.

63.5. The Header Route Predicate Factory

The Header route predicate factory takes two parameters, the header name and a regexp (which is a Java regular expression). This predicate matches with a header that has the given name whose value matches the regular expression. The following example configures a header route predicate:

Example 9. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: header_route
        uri: https://example.org
        predicates:
        - Header=X-Request-Id, \d+

This route matches if the request has a header named X-Request-Id whose value matches the \d+ regular expression (that is, it has a value of one or more digits).

63.6. The Host Route Predicate Factory

The Host route predicate factory takes one parameter: a list of host name patterns. The pattern is an Ant-style pattern with . as the separator. This predicates matches the Host header that matches the pattern. The following example configures a host route predicate:

Example 10. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: host_route
        uri: https://example.org
        predicates:
        - Host=**.somehost.org,**.anotherhost.org

URI template variables (such as {sub}.myhost.org) are supported as well.

This route matches if the request has a Host header with a value of www.somehost.org or beta.somehost.org or www.anotherhost.org.

This predicate extracts the URI template variables (such as sub, defined in the preceding example) as a map of names and values and places it in the ServerWebExchange.getAttributes() with a key defined in ServerWebExchangeUtils.URI_TEMPLATE_VARIABLES_ATTRIBUTE. Those values are then available for use by GatewayFilter factories

63.7. The Method Route Predicate Factory

The Method Route Predicate Factory takes a methods argument which is one or more parameters: the HTTP methods to match. The following example configures a method route predicate:

Example 11. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: method_route
        uri: https://example.org
        predicates:
        - Method=GET,POST

This route matches if the request method was a GET or a POST.

63.8. The Path Route Predicate Factory

The Path Route Predicate Factory takes two parameters: a list of Spring PathMatcher patterns and an optional flag called matchTrailingSlash (defaults to true). The following example configures a path route predicate:

Example 12. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: path_route
        uri: https://example.org
        predicates:
        - Path=/red/{segment},/blue/{segment}

This route matches if the request path was, for example: /red/1 or /red/1/ or /red/blue or /blue/green.

If matchTrailingSlash is set to false, then request path /red/1/ will not be matched.

This predicate extracts the URI template variables (such as segment, defined in the preceding example) as a map of names and values and places it in the ServerWebExchange.getAttributes() with a key defined in ServerWebExchangeUtils.URI_TEMPLATE_VARIABLES_ATTRIBUTE. Those values are then available for use by GatewayFilter factories

A utility method (called get) is available to make access to these variables easier. The following example shows how to use the get method:

Map<String, String> uriVariables = ServerWebExchangeUtils.getPathPredicateVariables(exchange);

String segment = uriVariables.get("segment");

63.9. The Query Route Predicate Factory

The Query route predicate factory takes two parameters: a required param and an optional regexp (which is a Java regular expression). The following example configures a query route predicate:

Example 13. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: query_route
        uri: https://example.org
        predicates:
        - Query=green

The preceding route matches if the request contained a green query parameter.

application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: query_route
        uri: https://example.org
        predicates:
        - Query=red, gree.

The preceding route matches if the request contained a red query parameter whose value matched the gree. regexp, so green and greet would match.

63.10. The RemoteAddr Route Predicate Factory

The RemoteAddr route predicate factory takes a list (min size 1) of sources, which are CIDR-notation (IPv4 or IPv6) strings, such as 192.168.0.1/16 (where 192.168.0.1 is an IP address and 16 is a subnet mask). The following example configures a RemoteAddr route predicate:

Example 14. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: remoteaddr_route
        uri: https://example.org
        predicates:
        - RemoteAddr=192.168.1.1/24

This route matches if the remote address of the request was, for example, 192.168.1.10.

63.11. The Weight Route Predicate Factory

The Weight route predicate factory takes two arguments: group and weight (an int). The weights are calculated per group. The following example configures a weight route predicate:

Example 15. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: weight_high
        uri: https://weighthigh.org
        predicates:
        - Weight=group1, 8
      - id: weight_low
        uri: https://weightlow.org
        predicates:
        - Weight=group1, 2

This route would forward ~80% of traffic to weighthigh.org and ~20% of traffic to weighlow.org

63.11.1. Modifying the Way Remote Addresses Are Resolved

By default, the RemoteAddr route predicate factory uses the remote address from the incoming request. This may not match the actual client IP address if Spring Cloud Gateway sits behind a proxy layer.

You can customize the way that the remote address is resolved by setting a custom RemoteAddressResolver. Spring Cloud Gateway comes with one non-default remote address resolver that is based off of the X-Forwarded-For header, XForwardedRemoteAddressResolver.

XForwardedRemoteAddressResolver has two static constructor methods, which take different approaches to security:

  • XForwardedRemoteAddressResolver::trustAll returns a RemoteAddressResolver that always takes the first IP address found in the X-Forwarded-For header. This approach is vulnerable to spoofing, as a malicious client could set an initial value for the X-Forwarded-For, which would be accepted by the resolver.

  • XForwardedRemoteAddressResolver::maxTrustedIndex takes an index that correlates to the number of trusted infrastructure running in front of Spring Cloud Gateway. If Spring Cloud Gateway is, for example only accessible through HAProxy, then a value of 1 should be used. If two hops of trusted infrastructure are required before Spring Cloud Gateway is accessible, then a value of 2 should be used.

Consider the following header value:

X-Forwarded-For: 0.0.0.1, 0.0.0.2, 0.0.0.3

The following maxTrustedIndex values yield the following remote addresses:

maxTrustedIndex result

[Integer.MIN_VALUE,0]

(invalid, IllegalArgumentException during initialization)

1

0.0.0.3

2

0.0.0.2

3

0.0.0.1

[4, Integer.MAX_VALUE]

0.0.0.1

The following example shows how to achieve the same configuration with Java:

Example 16. GatewayConfig.java
RemoteAddressResolver resolver = XForwardedRemoteAddressResolver
    .maxTrustedIndex(1);

...

.route("direct-route",
    r -> r.remoteAddr("10.1.1.1", "10.10.1.1/24")
        .uri("https://downstream1")
.route("proxied-route",
    r -> r.remoteAddr(resolver, "10.10.1.1", "10.10.1.1/24")
        .uri("https://downstream2")
)

64. GatewayFilter Factories

Route filters allow the modification of the incoming HTTP request or outgoing HTTP response in some manner. Route filters are scoped to a particular route. Spring Cloud Gateway includes many built-in GatewayFilter Factories.

For more detailed examples of how to use any of the following filters, take a look at the unit tests.

64.1. The AddRequestHeader GatewayFilter Factory

The AddRequestHeader GatewayFilter factory takes a name and value parameter. The following example configures an AddRequestHeader GatewayFilter:

Example 17. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_request_header_route
        uri: https://example.org
        filters:
        - AddRequestHeader=X-Request-red, blue

This listing adds X-Request-red:blue header to the downstream request’s headers for all matching requests.

AddRequestHeader is aware of the URI variables used to match a path or host. URI variables may be used in the value and are expanded at runtime. The following example configures an AddRequestHeader GatewayFilter that uses a variable:

Example 18. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_request_header_route
        uri: https://example.org
        predicates:
        - Path=/red/{segment}
        filters:
        - AddRequestHeader=X-Request-Red, Blue-{segment}

64.2. The AddRequestParameter GatewayFilter Factory

The AddRequestParameter GatewayFilter Factory takes a name and value parameter. The following example configures an AddRequestParameter GatewayFilter:

Example 19. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_request_parameter_route
        uri: https://example.org
        filters:
        - AddRequestParameter=red, blue

This will add red=blue to the downstream request’s query string for all matching requests.

AddRequestParameter is aware of the URI variables used to match a path or host. URI variables may be used in the value and are expanded at runtime. The following example configures an AddRequestParameter GatewayFilter that uses a variable:

Example 20. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_request_parameter_route
        uri: https://example.org
        predicates:
        - Host: {segment}.myhost.org
        filters:
        - AddRequestParameter=foo, bar-{segment}

64.3. The AddResponseHeader GatewayFilter Factory

The AddResponseHeader GatewayFilter Factory takes a name and value parameter. The following example configures an AddResponseHeader GatewayFilter:

Example 21. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_response_header_route
        uri: https://example.org
        filters:
        - AddResponseHeader=X-Response-Red, Blue

This adds X-Response-Foo:Bar header to the downstream response’s headers for all matching requests.

AddResponseHeader is aware of URI variables used to match a path or host. URI variables may be used in the value and are expanded at runtime. The following example configures an AddResponseHeader GatewayFilter that uses a variable:

Example 22. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: add_response_header_route
        uri: https://example.org
        predicates:
        - Host: {segment}.myhost.org
        filters:
        - AddResponseHeader=foo, bar-{segment}

64.4. The DedupeResponseHeader GatewayFilter Factory

The DedupeResponseHeader GatewayFilter factory takes a name parameter and an optional strategy parameter. name can contain a space-separated list of header names. The following example configures a DedupeResponseHeader GatewayFilter:

Example 23. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: dedupe_response_header_route
        uri: https://example.org
        filters:
        - DedupeResponseHeader=Access-Control-Allow-Credentials Access-Control-Allow-Origin

This removes duplicate values of Access-Control-Allow-Credentials and Access-Control-Allow-Origin response headers in cases when both the gateway CORS logic and the downstream logic add them.

The DedupeResponseHeader filter also accepts an optional strategy parameter. The accepted values are RETAIN_FIRST (default), RETAIN_LAST, and RETAIN_UNIQUE.

64.5. Spring Cloud CircuitBreaker GatewayFilter Factory

The Spring Cloud CircuitBreaker GatewayFilter factory uses the Spring Cloud CircuitBreaker APIs to wrap Gateway routes in a circuit breaker. Spring Cloud CircuitBreaker supports multiple libraries that can be used with Spring Cloud Gateway. Spring Cloud supports Resilience4J out of the box.

To enable the Spring Cloud CircuitBreaker filter, you need to place spring-cloud-starter-circuitbreaker-reactor-resilience4j on the classpath. The following example configures a Spring Cloud CircuitBreaker GatewayFilter:

Example 24. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: circuitbreaker_route
        uri: https://example.org
        filters:
        - CircuitBreaker=myCircuitBreaker

To configure the circuit breaker, see the configuration for the underlying circuit breaker implementation you are using.

The Spring Cloud CircuitBreaker filter can also accept an optional fallbackUri parameter. Currently, only forward: schemed URIs are supported. If the fallback is called, the request is forwarded to the controller matched by the URI. The following example configures such a fallback:

Example 25. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: circuitbreaker_route
        uri: lb://backing-service:8088
        predicates:
        - Path=/consumingServiceEndpoint
        filters:
        - name: CircuitBreaker
          args:
            name: myCircuitBreaker
            fallbackUri: forward:/inCaseOfFailureUseThis
        - RewritePath=/consumingServiceEndpoint, /backingServiceEndpoint

The following listing does the same thing in Java:

Example 26. Application.java
@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
    return builder.routes()
        .route("circuitbreaker_route", r -> r.path("/consumingServiceEndpoint")
            .filters(f -> f.circuitBreaker(c -> c.name("myCircuitBreaker").fallbackUri("forward:/inCaseOfFailureUseThis"))
                .rewritePath("/consumingServiceEndpoint", "/backingServiceEndpoint")).uri("lb://backing-service:8088")
        .build();
}

This example forwards to the /inCaseofFailureUseThis URI when the circuit breaker fallback is called. Note that this example also demonstrates the (optional) Spring Cloud LoadBalancer load-balancing (defined by the lb prefix on the destination URI).

The primary scenario is to use the fallbackUri to define an internal controller or handler within the gateway application. However, you can also reroute the request to a controller or handler in an external application, as follows:

Example 27. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: ingredients
        uri: lb://ingredients
        predicates:
        - Path=//ingredients/**
        filters:
        - name: CircuitBreaker
          args:
            name: fetchIngredients
            fallbackUri: forward:/fallback
      - id: ingredients-fallback
        uri: http://localhost:9994
        predicates:
        - Path=/fallback

In this example, there is no fallback endpoint or handler in the gateway application. However, there is one in another application, registered under localhost:9994.

In case of the request being forwarded to fallback, the Spring Cloud CircuitBreaker Gateway filter also provides the Throwable that has caused it. It is added to the ServerWebExchange as the ServerWebExchangeUtils.CIRCUITBREAKER_EXECUTION_EXCEPTION_ATTR attribute that can be used when handling the fallback within the gateway application.

For the external controller/handler scenario, headers can be added with exception details. You can find more information on doing so in the FallbackHeaders GatewayFilter Factory section.

64.5.1. Tripping The Circuit Breaker On Status Codes

In some cases you might want to trip a circuit breaker based on the status code returned from the route it wraps. The circuit breaker config object takes a list of status codes that if returned will cause the the circuit breaker to be tripped. When setting the status codes you want to trip the circuit breaker you can either use a integer with the status code value or the String representation of the HttpStatus enumeration.

Example 28. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: circuitbreaker_route
        uri: lb://backing-service:8088
        predicates:
        - Path=/consumingServiceEndpoint
        filters:
        - name: CircuitBreaker
          args:
            name: myCircuitBreaker
            fallbackUri: forward:/inCaseOfFailureUseThis
            statusCodes:
              - 500
              - "NOT_FOUND"
Example 29. Application.java
@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
    return builder.routes()
        .route("circuitbreaker_route", r -> r.path("/consumingServiceEndpoint")
            .filters(f -> f.circuitBreaker(c -> c.name("myCircuitBreaker").fallbackUri("forward:/inCaseOfFailureUseThis").addStatusCode("INTERNAL_SERVER_ERROR"))
                .rewritePath("/consumingServiceEndpoint", "/backingServiceEndpoint")).uri("lb://backing-service:8088")
        .build();
}

64.6. The FallbackHeaders GatewayFilter Factory

The FallbackHeaders factory lets you add Spring Cloud CircuitBreaker execution exception details in the headers of a request forwarded to a fallbackUri in an external application, as in the following scenario:

Example 30. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: ingredients
        uri: lb://ingredients
        predicates:
        - Path=//ingredients/**
        filters:
        - name: CircuitBreaker
          args:
            name: fetchIngredients
            fallbackUri: forward:/fallback
      - id: ingredients-fallback
        uri: http://localhost:9994
        predicates:
        - Path=/fallback
        filters:
        - name: FallbackHeaders
          args:
            executionExceptionTypeHeaderName: Test-Header

In this example, after an execution exception occurs while running the circuit breaker, the request is forwarded to the fallback endpoint or handler in an application running on localhost:9994. The headers with the exception type, message and (if available) root cause exception type and message are added to that request by the FallbackHeaders filter.

You can overwrite the names of the headers in the configuration by setting the values of the following arguments (shown with their default values):

  • executionExceptionTypeHeaderName ("Execution-Exception-Type")

  • executionExceptionMessageHeaderName ("Execution-Exception-Message")

  • rootCauseExceptionTypeHeaderName ("Root-Cause-Exception-Type")

  • rootCauseExceptionMessageHeaderName ("Root-Cause-Exception-Message")

For more information on circuit breakers and the gateway see the Spring Cloud CircuitBreaker Factory section.

64.7. The MapRequestHeader GatewayFilter Factory

The MapRequestHeader GatewayFilter factory takes fromHeader and toHeader parameters. It creates a new named header (toHeader), and the value is extracted out of an existing named header (fromHeader) from the incoming http request. If the input header does not exist, the filter has no impact. If the new named header already exists, its values are augmented with the new values. The following example configures a MapRequestHeader:

Example 31. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: map_request_header_route
        uri: https://example.org
        filters:
        - MapRequestHeader=Blue, X-Request-Red

This adds X-Request-Red:<values> header to the downstream request with updated values from the incoming HTTP request’s Blue header.

64.8. The PrefixPath GatewayFilter Factory

The PrefixPath GatewayFilter factory takes a single prefix parameter. The following example configures a PrefixPath GatewayFilter:

Example 32. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: prefixpath_route
        uri: https://example.org
        filters:
        - PrefixPath=/mypath

This will prefix /mypath to the path of all matching requests. So a request to /hello would be sent to /mypath/hello.

64.9. The PreserveHostHeader GatewayFilter Factory

The PreserveHostHeader GatewayFilter factory has no parameters. This filter sets a request attribute that the routing filter inspects to determine if the original host header should be sent, rather than the host header determined by the HTTP client. The following example configures a PreserveHostHeader GatewayFilter:

Example 33. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: preserve_host_route
        uri: https://example.org
        filters:
        - PreserveHostHeader

64.10. The RequestRateLimiter GatewayFilter Factory

The RequestRateLimiter GatewayFilter factory uses a RateLimiter implementation to determine if the current request is allowed to proceed. If it is not, a status of HTTP 429 - Too Many Requests (by default) is returned.

This filter takes an optional keyResolver parameter and parameters specific to the rate limiter (described later in this section).

keyResolver is a bean that implements the KeyResolver interface. In configuration, reference the bean by name using SpEL. #{@myKeyResolver} is a SpEL expression that references a bean named myKeyResolver. The following listing shows the KeyResolver interface:

Example 34. KeyResolver.java
public interface KeyResolver {
    Mono<String> resolve(ServerWebExchange exchange);
}

The KeyResolver interface lets pluggable strategies derive the key for limiting requests. In future milestone releases, there will be some KeyResolver implementations.

The default implementation of KeyResolver is the PrincipalNameKeyResolver, which retrieves the Principal from the ServerWebExchange and calls Principal.getName().

By default, if the KeyResolver does not find a key, requests are denied. You can adjust this behavior by setting the spring.cloud.gateway.filter.request-rate-limiter.deny-empty-key (true or false) and spring.cloud.gateway.filter.request-rate-limiter.empty-key-status-code properties.

The RequestRateLimiter is not configurable with the "shortcut" notation. The following example below is invalid:

Example 35. application.properties
# INVALID SHORTCUT CONFIGURATION
spring.cloud.gateway.routes[0].filters[0]=RequestRateLimiter=2, 2, #{@userkeyresolver}

64.10.1. The Redis RateLimiter

The Redis implementation is based off of work done at Stripe. It requires the use of the spring-boot-starter-data-redis-reactive Spring Boot starter.

The algorithm used is the Token Bucket Algorithm.

The redis-rate-limiter.replenishRate property is how many requests per second you want a user to be allowed to do, without any dropped requests. This is the rate at which the token bucket is filled.

The redis-rate-limiter.burstCapacity property is the maximum number of requests a user is allowed to do in a single second. This is the number of tokens the token bucket can hold. Setting this value to zero blocks all requests.

The redis-rate-limiter.requestedTokens property is how many tokens a request costs. This is the number of tokens taken from the bucket for each request and defaults to 1.

A steady rate is accomplished by setting the same value in replenishRate and burstCapacity. Temporary bursts can be allowed by setting burstCapacity higher than replenishRate. In this case, the rate limiter needs to be allowed some time between bursts (according to replenishRate), as two consecutive bursts will result in dropped requests (HTTP 429 - Too Many Requests). The following listing configures a redis-rate-limiter:

Rate limits bellow 1 request/s are accomplished by setting replenishRate to the wanted number of requests, requestedTokens to the timespan in seconds and burstCapacity to the product of replenishRate and requestedTokens, e.g. setting replenishRate=1, requestedTokens=60 and burstCapacity=60 will result in a limit of 1 request/min.

Example 36. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: requestratelimiter_route
        uri: https://example.org
        filters:
        - name: RequestRateLimiter
          args:
            redis-rate-limiter.replenishRate: 10
            redis-rate-limiter.burstCapacity: 20
            redis-rate-limiter.requestedTokens: 1

The following example configures a KeyResolver in Java:

Example 37. Config.java
@Bean
KeyResolver userKeyResolver() {
    return exchange -> Mono.just(exchange.getRequest().getQueryParams().getFirst("user"));
}

This defines a request rate limit of 10 per user. A burst of 20 is allowed, but, in the next second, only 10 requests are available. The KeyResolver is a simple one that gets the user request parameter (note that this is not recommended for production).

You can also define a rate limiter as a bean that implements the RateLimiter interface. In configuration, you can reference the bean by name using SpEL. #{@myRateLimiter} is a SpEL expression that references a bean with named myRateLimiter. The following listing defines a rate limiter that uses the KeyResolver defined in the previous listing:

Example 38. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: requestratelimiter_route
        uri: https://example.org
        filters:
        - name: RequestRateLimiter
          args:
            rate-limiter: "#{@myRateLimiter}"
            key-resolver: "#{@userKeyResolver}"

64.11. The RedirectTo GatewayFilter Factory

The RedirectTo GatewayFilter factory takes two parameters, status and url. The status parameter should be a 300 series redirect HTTP code, such as 301. The url parameter should be a valid URL. This is the value of the Location header. For relative redirects, you should use uri: no://op as the uri of your route definition. The following listing configures a RedirectTo GatewayFilter:

Example 39. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: prefixpath_route
        uri: https://example.org
        filters:
        - RedirectTo=302, https://acme.org

This will send a status 302 with a Location:https://acme.org header to perform a redirect.

64.12. The RemoveRequestHeader GatewayFilter Factory

The RemoveRequestHeader GatewayFilter factory takes a name parameter. It is the name of the header to be removed. The following listing configures a RemoveRequestHeader GatewayFilter:

Example 40. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: removerequestheader_route
        uri: https://example.org
        filters:
        - RemoveRequestHeader=X-Request-Foo

This removes the X-Request-Foo header before it is sent downstream.

64.13. RemoveResponseHeader GatewayFilter Factory

The RemoveResponseHeader GatewayFilter factory takes a name parameter. It is the name of the header to be removed. The following listing configures a RemoveResponseHeader GatewayFilter:

Example 41. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: removeresponseheader_route
        uri: https://example.org
        filters:
        - RemoveResponseHeader=X-Response-Foo

This will remove the X-Response-Foo header from the response before it is returned to the gateway client.

To remove any kind of sensitive header, you should configure this filter for any routes for which you may want to do so. In addition, you can configure this filter once by using spring.cloud.gateway.default-filters and have it applied to all routes.

64.14. The RemoveRequestParameter GatewayFilter Factory

The RemoveRequestParameter GatewayFilter factory takes a name parameter. It is the name of the query parameter to be removed. The following example configures a RemoveRequestParameter GatewayFilter:

Example 42. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: removerequestparameter_route
        uri: https://example.org
        filters:
        - RemoveRequestParameter=red

This will remove the red parameter before it is sent downstream.

64.15. The RewritePath GatewayFilter Factory

The RewritePath GatewayFilter factory takes a path regexp parameter and a replacement parameter. This uses Java regular expressions for a flexible way to rewrite the request path. The following listing configures a RewritePath GatewayFilter:

Example 43. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: rewritepath_route
        uri: https://example.org
        predicates:
        - Path=/red/**
        filters:
        - RewritePath=/red(?<segment>/?.*), $\{segment}

For a request path of /red/blue, this sets the path to /blue before making the downstream request. Note that the $ should be replaced with $\ because of the YAML specification.

64.16. RewriteLocationResponseHeader GatewayFilter Factory

The RewriteLocationResponseHeader GatewayFilter factory modifies the value of the Location response header, usually to get rid of backend-specific details. It takes stripVersionMode, locationHeaderName, hostValue, and protocolsRegex parameters. The following listing configures a RewriteLocationResponseHeader GatewayFilter:

Example 44. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: rewritelocationresponseheader_route
        uri: http://example.org
        filters:
        - RewriteLocationResponseHeader=AS_IN_REQUEST, Location, ,

For example, for a request of POST api.example.com/some/object/name, the Location response header value of object-service.prod.example.net/v2/some/object/id is rewritten as api.example.com/some/object/id.

The stripVersionMode parameter has the following possible values: NEVER_STRIP, AS_IN_REQUEST (default), and ALWAYS_STRIP.

  • NEVER_STRIP: The version is not stripped, even if the original request path contains no version.

  • AS_IN_REQUEST The version is stripped only if the original request path contains no version.

  • ALWAYS_STRIP The version is always stripped, even if the original request path contains version.

The hostValue parameter, if provided, is used to replace the host:port portion of the response Location header. If it is not provided, the value of the Host request header is used.

The protocolsRegex parameter must be a valid regex String, against which the protocol name is matched. If it is not matched, the filter does nothing. The default is http|https|ftp|ftps.

64.17. The RewriteResponseHeader GatewayFilter Factory

The RewriteResponseHeader GatewayFilter factory takes name, regexp, and replacement parameters. It uses Java regular expressions for a flexible way to rewrite the response header value. The following example configures a RewriteResponseHeader GatewayFilter:

Example 45. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: rewriteresponseheader_route
        uri: https://example.org
        filters:
        - RewriteResponseHeader=X-Response-Red, , password=[^&]+, password=***

For a header value of /42?user=ford&password=omg!what&flag=true, it is set to /42?user=ford&password=***&flag=true after making the downstream request. You must use $\ to mean $ because of the YAML specification.

64.18. The SaveSession GatewayFilter Factory

The SaveSession GatewayFilter factory forces a WebSession::save operation before forwarding the call downstream. This is of particular use when using something like Spring Session with a lazy data store and you need to ensure the session state has been saved before making the forwarded call. The following example configures a SaveSession GatewayFilter:

Example 46. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: save_session
        uri: https://example.org
        predicates:
        - Path=/foo/**
        filters:
        - SaveSession

If you integrate Spring Security with Spring Session and want to ensure security details have been forwarded to the remote process, this is critical.

64.19. The SecureHeaders GatewayFilter Factory

The SecureHeaders GatewayFilter factory adds a number of headers to the response, per the recommendation made in this blog post.

The following headers (shown with their default values) are added:

  • X-Xss-Protection:1 (mode=block)

  • Strict-Transport-Security (max-age=631138519)

  • X-Frame-Options (DENY)

  • X-Content-Type-Options (nosniff)

  • Referrer-Policy (no-referrer)

  • Content-Security-Policy (default-src 'self' https:; font-src 'self' https: data:; img-src 'self' https: data:; object-src 'none'; script-src https:; style-src 'self' https: 'unsafe-inline)'

  • X-Download-Options (noopen)

  • X-Permitted-Cross-Domain-Policies (none)

To change the default values, set the appropriate property in the spring.cloud.gateway.filter.secure-headers namespace. The following properties are available:

  • xss-protection-header

  • strict-transport-security

  • x-frame-options

  • x-content-type-options

  • referrer-policy

  • content-security-policy

  • x-download-options

  • x-permitted-cross-domain-policies

To disable the default values set the spring.cloud.gateway.filter.secure-headers.disable property with comma-separated values. The following example shows how to do so:

spring.cloud.gateway.filter.secure-headers.disable=x-frame-options,strict-transport-security
The lowercase full name of the secure header needs to be used to disable it..

64.20. The SetPath GatewayFilter Factory

The SetPath GatewayFilter factory takes a path template parameter. It offers a simple way to manipulate the request path by allowing templated segments of the path. This uses the URI templates from Spring Framework. Multiple matching segments are allowed. The following example configures a SetPath GatewayFilter:

Example 47. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setpath_route
        uri: https://example.org
        predicates:
        - Path=/red/{segment}
        filters:
        - SetPath=/{segment}

For a request path of /red/blue, this sets the path to /blue before making the downstream request.

64.21. The SetRequestHeader GatewayFilter Factory

The SetRequestHeader GatewayFilter factory takes name and value parameters. The following listing configures a SetRequestHeader GatewayFilter:

Example 48. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setrequestheader_route
        uri: https://example.org
        filters:
        - SetRequestHeader=X-Request-Red, Blue

This GatewayFilter replaces (rather than adding) all headers with the given name. So, if the downstream server responded with a X-Request-Red:1234, this would be replaced with X-Request-Red:Blue, which is what the downstream service would receive.

SetRequestHeader is aware of URI variables used to match a path or host. URI variables may be used in the value and are expanded at runtime. The following example configures an SetRequestHeader GatewayFilter that uses a variable:

Example 49. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setrequestheader_route
        uri: https://example.org
        predicates:
        - Host: {segment}.myhost.org
        filters:
        - SetRequestHeader=foo, bar-{segment}

64.22. The SetResponseHeader GatewayFilter Factory

The SetResponseHeader GatewayFilter factory takes name and value parameters. The following listing configures a SetResponseHeader GatewayFilter:

Example 50. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setresponseheader_route
        uri: https://example.org
        filters:
        - SetResponseHeader=X-Response-Red, Blue

This GatewayFilter replaces (rather than adding) all headers with the given name. So, if the downstream server responded with a X-Response-Red:1234, this is replaced with X-Response-Red:Blue, which is what the gateway client would receive.

SetResponseHeader is aware of URI variables used to match a path or host. URI variables may be used in the value and will be expanded at runtime. The following example configures an SetResponseHeader GatewayFilter that uses a variable:

Example 51. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setresponseheader_route
        uri: https://example.org
        predicates:
        - Host: {segment}.myhost.org
        filters:
        - SetResponseHeader=foo, bar-{segment}

64.23. The SetStatus GatewayFilter Factory

The SetStatus GatewayFilter factory takes a single parameter, status. It must be a valid Spring HttpStatus. It may be the integer value 404 or the string representation of the enumeration: NOT_FOUND. The following listing configures a SetStatus GatewayFilter:

Example 52. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setstatusstring_route
        uri: https://example.org
        filters:
        - SetStatus=BAD_REQUEST
      - id: setstatusint_route
        uri: https://example.org
        filters:
        - SetStatus=401

In either case, the HTTP status of the response is set to 401.

You can configure the SetStatus GatewayFilter to return the original HTTP status code from the proxied request in a header in the response. The header is added to the response if configured with the following property:

Example 53. application.yml
spring:
  cloud:
    gateway:
      set-status:
        original-status-header-name: original-http-status

64.24. The StripPrefix GatewayFilter Factory

The StripPrefix GatewayFilter factory takes one parameter, parts. The parts parameter indicates the number of parts in the path to strip from the request before sending it downstream. The following listing configures a StripPrefix GatewayFilter:

Example 54. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: nameRoot
        uri: https://nameservice
        predicates:
        - Path=/name/**
        filters:
        - StripPrefix=2

When a request is made through the gateway to /name/blue/red, the request made to nameservice looks like nameservice/red.

64.25. The Retry GatewayFilter Factory

The Retry GatewayFilter factory supports the following parameters:

  • retries: The number of retries that should be attempted.

  • statuses: The HTTP status codes that should be retried, represented by using org.springframework.http.HttpStatus.

  • methods: The HTTP methods that should be retried, represented by using org.springframework.http.HttpMethod.

  • series: The series of status codes to be retried, represented by using org.springframework.http.HttpStatus.Series.

  • exceptions: A list of thrown exceptions that should be retried.

  • backoff: The configured exponential backoff for the retries. Retries are performed after a backoff interval of firstBackoff * (factor ^ n), where n is the iteration. If maxBackoff is configured, the maximum backoff applied is limited to maxBackoff. If basedOnPreviousValue is true, the backoff is calculated byusing prevBackoff * factor.

The following defaults are configured for Retry filter, if enabled:

  • retries: Three times

  • series: 5XX series

  • methods: GET method

  • exceptions: IOException and TimeoutException

  • backoff: disabled

The following listing configures a Retry GatewayFilter:

Example 55. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: retry_test
        uri: http://localhost:8080/flakey
        predicates:
        - Host=*.retry.com
        filters:
        - name: Retry
          args:
            retries: 3
            statuses: BAD_GATEWAY
            methods: GET,POST
            backoff:
              firstBackoff: 10ms
              maxBackoff: 50ms
              factor: 2
              basedOnPreviousValue: false
When using the retry filter with a forward: prefixed URL, the target endpoint should be written carefully so that, in case of an error, it does not do anything that could result in a response being sent to the client and committed. For example, if the target endpoint is an annotated controller, the target controller method should not return ResponseEntity with an error status code. Instead, it should throw an Exception or signal an error (for example, through a Mono.error(ex) return value), which the retry filter can be configured to handle by retrying.
When using the retry filter with any HTTP method with a body, the body will be cached and the gateway will become memory constrained. The body is cached in a request attribute defined by ServerWebExchangeUtils.CACHED_REQUEST_BODY_ATTR. The type of the object is a org.springframework.core.io.buffer.DataBuffer.

64.26. The RequestSize GatewayFilter Factory

When the request size is greater than the permissible limit, the RequestSize GatewayFilter factory can restrict a request from reaching the downstream service. The filter takes a maxSize parameter. The maxSize is a `DataSize type, so values can be defined as a number followed by an optional DataUnit suffix such as 'KB' or 'MB'. The default is 'B' for bytes. It is the permissible size limit of the request defined in bytes. The following listing configures a RequestSize GatewayFilter:

Example 56. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: request_size_route
        uri: http://localhost:8080/upload
        predicates:
        - Path=/upload
        filters:
        - name: RequestSize
          args:
            maxSize: 5000000

The RequestSize GatewayFilter factory sets the response status as 413 Payload Too Large with an additional header errorMessage when the request is rejected due to size. The following example shows such an errorMessage:

errorMessage` : `Request size is larger than permissible limit. Request size is 6.0 MB where permissible limit is 5.0 MB
The default request size is set to five MB if not provided as a filter argument in the route definition.

64.27. The SetRequestHostHeader GatewayFilter Factory

There are certain situation when the host header may need to be overridden. In this situation, the SetRequestHostHeader GatewayFilter factory can replace the existing host header with a specified vaue. The filter takes a host parameter. The following listing configures a SetRequestHostHeader GatewayFilter:

Example 57. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: set_request_host_header_route
        uri: http://localhost:8080/headers
        predicates:
        - Path=/headers
        filters:
        - name: SetRequestHostHeader
          args:
            host: example.org

The SetRequestHostHeader GatewayFilter factory replaces the value of the host header with example.org.

64.28. Modify a Request Body GatewayFilter Factory

You can use the ModifyRequestBody filter filter to modify the request body before it is sent downstream by the gateway.

This filter can be configured only by using the Java DSL.

The following listing shows how to modify a request body GatewayFilter:

@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
    return builder.routes()
        .route("rewrite_request_obj", r -> r.host("*.rewriterequestobj.org")
            .filters(f -> f.prefixPath("/httpbin")
                .modifyRequestBody(String.class, Hello.class, MediaType.APPLICATION_JSON_VALUE,
                    (exchange, s) -> return Mono.just(new Hello(s.toUpperCase())))).uri(uri))
        .build();
}

static class Hello {
    String message;

    public Hello() { }

    public Hello(String message) {
        this.message = message;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}
if the request has no body, the RewriteFilter will be passed null. Mono.empty() should be returned to assign a missing body in the request.

64.29. Modify a Response Body GatewayFilter Factory

You can use the ModifyResponseBody filter to modify the response body before it is sent back to the client.

This filter can be configured only by using the Java DSL.

The following listing shows how to modify a response body GatewayFilter:

@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
    return builder.routes()
        .route("rewrite_response_upper", r -> r.host("*.rewriteresponseupper.org")
            .filters(f -> f.prefixPath("/httpbin")
                .modifyResponseBody(String.class, String.class,
                    (exchange, s) -> Mono.just(s.toUpperCase()))).uri(uri))
        .build();
}
if the response has no body, the RewriteFilter will be passed null. Mono.empty() should be returned to assign a missing body in the response.

64.30. Token Relay GatewayFilter Factory

A Token Relay is where an OAuth2 consumer acts as a Client and forwards the incoming token to outgoing resource requests. The consumer can be a pure Client (like an SSO application) or a Resource Server.

Spring Cloud Gateway can forward OAuth2 access tokens downstream to the services it is proxying. To add this functionlity to gateway you need to add the TokenRelayGatewayFilterFactory like this:

App.java
@Bean
public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
    return builder.routes()
            .route("resource", r -> r.path("/resource")
                    .filters(f -> f.tokenRelay())
                    .uri("http://localhost:9000"))
            .build();
}

or this

application.yaml
spring:
  cloud:
    gateway:
      routes:
      - id: resource
        uri: http://localhost:9000
        predicates:
        - Path=/resource
        filters:
        - TokenRelay=

and it will (in addition to logging the user in and grabbing a token) pass the authentication token downstream to the services (in this case /resource).

To enable this for Spring Cloud Gateway add the following dependencies

  • org.springframework.boot:spring-boot-starter-oauth2-client

How does it work? The filter extracts an access token from the currently authenticated user, and puts it in a request header for the downstream requests.

For a full working sample see this project.

A TokenRelayGatewayFilterFactory bean will only be created if the proper spring.security.oauth2.client.* properties are set which will trigger creation of a ReactiveClientRegistrationRepository bean.
The default implementation of ReactiveOAuth2AuthorizedClientService used by TokenRelayGatewayFilterFactory uses an in-memory data store. You will need to provide your own implementation ReactiveOAuth2AuthorizedClientService if you need a more robust solution.

64.31. Default Filters

To add a filter and apply it to all routes, you can use spring.cloud.gateway.default-filters. This property takes a list of filters. The following listing defines a set of default filters:

Example 58. application.yml
spring:
  cloud:
    gateway:
      default-filters:
      - AddResponseHeader=X-Response-Default-Red, Default-Blue
      - PrefixPath=/httpbin

65. Global Filters

The GlobalFilter interface has the same signature as GatewayFilter. These are special filters that are conditionally applied to all routes.

This interface and its usage are subject to change in future milestone releases.

65.1. Combined Global Filter and GatewayFilter Ordering

When a request matches a route, the filtering web handler adds all instances of GlobalFilter and all route-specific instances of GatewayFilter to a filter chain. This combined filter chain is sorted by the org.springframework.core.Ordered interface, which you can set by implementing the getOrder() method.

As Spring Cloud Gateway distinguishes between “pre” and “post” phases for filter logic execution (see How it Works), the filter with the highest precedence is the first in the “pre”-phase and the last in the “post”-phase.

The following listing configures a filter chain:

Example 59. ExampleConfiguration.java
@Bean
public GlobalFilter customFilter() {
    return new CustomGlobalFilter();
}

public class CustomGlobalFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        log.info("custom global filter");
        return chain.filter(exchange);
    }

    @Override
    public int getOrder() {
        return -1;
    }
}

65.2. Forward Routing Filter

The ForwardRoutingFilter looks for a URI in the exchange attribute ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a forward scheme (such as forward:///localendpoint), it uses the Spring DispatcherHandler to handle the request. The path part of the request URL is overridden with the path in the forward URL. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute.

65.3. The LoadBalancerClient Filter

The LoadBalancerClientFilter looks for a URI in the exchange attribute named ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a scheme of lb (such as lb://myservice), it uses the Spring Cloud LoadBalancerClient to resolve the name (myservice in this case) to an actual host and port and replaces the URI in the same attribute. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute. The filter also looks in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR attribute to see if it equals lb. If so, the same rules apply. The following listing configures a LoadBalancerClientFilter:

Example 60. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: myRoute
        uri: lb://service
        predicates:
        - Path=/service/**
By default, when a service instance cannot be found in the LoadBalancer, a 503 is returned. You can configure the Gateway to return a 404 by setting spring.cloud.gateway.loadbalancer.use404=true.
The isSecure value of the ServiceInstance returned from the LoadBalancer overrides the scheme specified in the request made to the Gateway. For example, if the request comes into the Gateway over HTTPS but the ServiceInstance indicates it is not secure, the downstream request is made over HTTP. The opposite situation can also apply. However, if GATEWAY_SCHEME_PREFIX_ATTR is specified for the route in the Gateway configuration, the prefix is stripped and the resulting scheme from the route URL overrides the ServiceInstance configuration.
Gateway supports all the LoadBalancer features. You can read more about them in the Spring Cloud Commons documentation.

65.4. The ReactiveLoadBalancerClientFilter

The ReactiveLoadBalancerClientFilter looks for a URI in the exchange attribute named ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a lb scheme (such as lb://myservice), it uses the Spring Cloud ReactorLoadBalancer to resolve the name (myservice in this example) to an actual host and port and replaces the URI in the same attribute. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute. The filter also looks in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR attribute to see if it equals lb. If so, the same rules apply. The following listing configures a ReactiveLoadBalancerClientFilter:

Example 61. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: myRoute
        uri: lb://service
        predicates:
        - Path=/service/**
By default, when a service instance cannot be found by the ReactorLoadBalancer, a 503 is returned. You can configure the gateway to return a 404 by setting spring.cloud.gateway.loadbalancer.use404=true.
The isSecure value of the ServiceInstance returned from the ReactiveLoadBalancerClientFilter overrides the scheme specified in the request made to the Gateway. For example, if the request comes into the Gateway over HTTPS but the ServiceInstance indicates it is not secure, the downstream request is made over HTTP. The opposite situation can also apply. However, if GATEWAY_SCHEME_PREFIX_ATTR is specified for the route in the Gateway configuration, the prefix is stripped and the resulting scheme from the route URL overrides the ServiceInstance configuration.

65.5. The Netty Routing Filter

The Netty routing filter runs if the URL located in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute has a http or https scheme. It uses the Netty HttpClient to make the downstream proxy request. The response is put in the ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR exchange attribute for use in a later filter. (There is also an experimental WebClientHttpRoutingFilter that performs the same function but does not require Netty.)

65.6. The Netty Write Response Filter

The NettyWriteResponseFilter runs if there is a Netty HttpClientResponse in the ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR exchange attribute. It runs after all other filters have completed and writes the proxy response back to the gateway client response. (There is also an experimental WebClientWriteResponseFilter that performs the same function but does not require Netty.)

65.7. The RouteToRequestUrl Filter

If there is a Route object in the ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR exchange attribute, the RouteToRequestUrlFilter runs. It creates a new URI, based off of the request URI but updated with the URI attribute of the Route object. The new URI is placed in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute`.

If the URI has a scheme prefix, such as lb:ws://serviceid, the lb scheme is stripped from the URI and placed in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR for use later in the filter chain.

65.8. The Websocket Routing Filter

If the URL located in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute has a ws or wss scheme, the websocket routing filter runs. It uses the Spring WebSocket infrastructure to forward the websocket request downstream.

You can load-balance websockets by prefixing the URI with lb, such as lb:ws://serviceid.

If you use SockJS as a fallback over normal HTTP, you should configure a normal HTTP route as well as the websocket Route.

The following listing configures a websocket routing filter:

Example 62. application.yml
spring:
  cloud:
    gateway:
      routes:
      # SockJS route
      - id: websocket_sockjs_route
        uri: http://localhost:3001
        predicates:
        - Path=/websocket/info/**
      # Normal Websocket route
      - id: websocket_route
        uri: ws://localhost:3001
        predicates:
        - Path=/websocket/**

65.9. The Gateway Metrics Filter

To enable gateway metrics, add spring-boot-starter-actuator as a project dependency. Then, by default, the gateway metrics filter runs as long as the property spring.cloud.gateway.metrics.enabled is not set to false. This filter adds a timer metric named gateway.requests with the following tags:

  • routeId: The route ID.

  • routeUri: The URI to which the API is routed.

  • outcome: The outcome, as classified by HttpStatus.Series.

  • status: The HTTP status of the request returned to the client.

  • httpStatusCode: The HTTP Status of the request returned to the client.

  • httpMethod: The HTTP method used for the request.

These metrics are then available to be scraped from /actuator/metrics/gateway.requests and can be easily integrated with Prometheus to create a Grafana dashboard.

To enable the prometheus endpoint, add micrometer-registry-prometheus as a project dependency.

65.10. Marking An Exchange As Routed

After the gateway has routed a ServerWebExchange, it marks that exchange as “routed” by adding gatewayAlreadyRouted to the exchange attributes. Once a request has been marked as routed, other routing filters will not route the request again, essentially skipping the filter. There are convenience methods that you can use to mark an exchange as routed or check if an exchange has already been routed.

  • ServerWebExchangeUtils.isAlreadyRouted takes a ServerWebExchange object and checks if it has been “routed”.

  • ServerWebExchangeUtils.setAlreadyRouted takes a ServerWebExchange object and marks it as “routed”.

66. HttpHeadersFilters

HttpHeadersFilters are applied to requests before sending them downstream, such as in the NettyRoutingFilter.

66.1. Forwarded Headers Filter

The Forwarded Headers Filter creates a Forwarded header to send to the downstream service. It adds the Host header, scheme and port of the current request to any existing Forwarded header.

66.2. RemoveHopByHop Headers Filter

The RemoveHopByHop Headers Filter removes headers from forwarded requests. The default list of headers that is removed comes from the IETF.

The default removed headers are:
  • Connection

  • Keep-Alive

  • Proxy-Authenticate

  • Proxy-Authorization

  • TE

  • Trailer

  • Transfer-Encoding

  • Upgrade

To change this, set the spring.cloud.gateway.filter.remove-hop-by-hop.headers property to the list of header names to remove.

66.3. XForwarded Headers Filter

The XForwarded Headers Filter creates various a X-Forwarded-* headers to send to the downstream service. It users the Host header, scheme, port and path of the current request to create the various headers.

Creating of individual headers can be controlled by the following boolean properties (defaults to true):

  • spring.cloud.gateway.x-forwarded.for-enabled

  • spring.cloud.gateway.x-forwarded.host-enabled

  • spring.cloud.gateway.x-forwarded.port-enabled

  • spring.cloud.gateway.x-forwarded.proto-enabled

  • spring.cloud.gateway.x-forwarded.prefix-enabled

Appending multiple headers can be controlled by the following boolean properties (defaults to true):

  • spring.cloud.gateway.x-forwarded.for-append

  • spring.cloud.gateway.x-forwarded.host-append

  • spring.cloud.gateway.x-forwarded.port-append

  • spring.cloud.gateway.x-forwarded.proto-append

  • spring.cloud.gateway.x-forwarded.prefix-append

67. TLS and SSL

The gateway can listen for requests on HTTPS by following the usual Spring server configuration. The following example shows how to do so:

Example 63. application.yml
server:
  ssl:
    enabled: true
    key-alias: scg
    key-store-password: scg1234
    key-store: classpath:scg-keystore.p12
    key-store-type: PKCS12

You can route gateway routes to both HTTP and HTTPS backends. If you are routing to an HTTPS backend, you can configure the gateway to trust all downstream certificates with the following configuration:

Example 64. application.yml
spring:
  cloud:
    gateway:
      httpclient:
        ssl:
          useInsecureTrustManager: true

Using an insecure trust manager is not suitable for production. For a production deployment, you can configure the gateway with a set of known certificates that it can trust with the following configuration:

Example 65. application.yml
spring:
  cloud:
    gateway:
      httpclient:
        ssl:
          trustedX509Certificates:
          - cert1.pem
          - cert2.pem

If the Spring Cloud Gateway is not provisioned with trusted certificates, the default trust store is used (which you can override by setting the javax.net.ssl.trustStore system property).

67.1. TLS Handshake

The gateway maintains a client pool that it uses to route to backends. When communicating over HTTPS, the client initiates a TLS handshake. A number of timeouts are associated with this handshake. You can configure these timeouts can be configured (defaults shown) as follows:

Example 66. application.yml
spring:
  cloud:
    gateway:
      httpclient:
        ssl:
          handshake-timeout-millis: 10000
          close-notify-flush-timeout-millis: 3000
          close-notify-read-timeout-millis: 0

68. Configuration

Configuration for Spring Cloud Gateway is driven by a collection of RouteDefinitionLocator instances. The following listing shows the definition of the RouteDefinitionLocator interface:

Example 67. RouteDefinitionLocator.java
public interface RouteDefinitionLocator {
    Flux<RouteDefinition> getRouteDefinitions();
}

By default, a PropertiesRouteDefinitionLocator loads properties by using Spring Boot’s @ConfigurationProperties mechanism.

The earlier configuration examples all use a shortcut notation that uses positional arguments rather than named ones. The following two examples are equivalent:

Example 68. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: setstatus_route
        uri: https://example.org
        filters:
        - name: SetStatus
          args:
            status: 401
      - id: setstatusshortcut_route
        uri: https://example.org
        filters:
        - SetStatus=401

For some usages of the gateway, properties are adequate, but some production use cases benefit from loading configuration from an external source, such as a database. Future milestone versions will have RouteDefinitionLocator implementations based off of Spring Data Repositories, such as Redis, MongoDB, and Cassandra.

69. Route Metadata Configuration

You can configure additional parameters for each route by using metadata, as follows:

Example 69. application.yml
spring:
  cloud:
    gateway:
      routes:
      - id: route_with_metadata
        uri: https://example.org
        metadata:
          optionName: "OptionValue"
          compositeObject:
            name: "value"
          iAmNumber: 1

You could acquire all metadata properties from an exchange, as follows:

Route route = exchange.getAttribute(GATEWAY_ROUTE_ATTR);
// get all metadata properties
route.getMetadata();
// get a single metadata property
route.getMetadata(someKey);

70. Http timeouts configuration

Http timeouts (response and connect) can be configured for all routes and overridden for each specific route.

70.1. Global timeouts

To configure Global http timeouts:
connect-timeout must be specified in milliseconds.
response-timeout must be specified as a java.time.Duration

global http timeouts example
spring:
  cloud:
    gateway:
      httpclient:
        connect-timeout: 1000
        response-timeout: 5s

70.2. Per-route timeouts

To configure per-route timeouts:
connect-timeout must be specified in milliseconds.
response-timeout must be specified in milliseconds.

per-route http timeouts configuration via configuration
      - id: per_route_timeouts
        uri: https://example.org
        predicates:
          - name: Path
            args:
              pattern: /delay/{timeout}
        metadata:
          response-timeout: 200
          connect-timeout: 200
per-route timeouts configuration using Java DSL
import static org.springframework.cloud.gateway.support.RouteMetadataUtils.CONNECT_TIMEOUT_ATTR;
import static org.springframework.cloud.gateway.support.RouteMetadataUtils.RESPONSE_TIMEOUT_ATTR;

      @Bean
      public RouteLocator customRouteLocator(RouteLocatorBuilder routeBuilder){
         return routeBuilder.routes()
               .route("test1", r -> {
                  return r.host("*.somehost.org").and().path("/somepath")
                        .filters(f -> f.addRequestHeader("header1", "header-value-1"))
                        .uri("http://someuri")
                        .metadata(RESPONSE_TIMEOUT_ATTR, 200)
                        .metadata(CONNECT_TIMEOUT_ATTR, 200);
               })
               .build();
      }

70.3. Fluent Java Routes API

To allow for simple configuration in Java, the RouteLocatorBuilder bean includes a fluent API. The following listing shows how it works:

Example 70. GatewaySampleApplication.java
// static imports from GatewayFilters and RoutePredicates
@Bean
public RouteLocator customRouteLocator(RouteLocatorBuilder builder, ThrottleGatewayFilterFactory throttle) {
    return builder.routes()
            .route(r -> r.host("**.abc.org").and().path("/image/png")
                .filters(f ->
                        f.addResponseHeader("X-TestHeader", "foobar"))
                .uri("http://httpbin.org:80")
            )
            .route(r -> r.path("/image/webp")
                .filters(f ->
                        f.addResponseHeader("X-AnotherHeader", "baz"))
                .uri("http://httpbin.org:80")
                .metadata("key", "value")
            )
            .route(r -> r.order(-1)
                .host("**.throttle.org").and().path("/get")
                .filters(f -> f.filter(throttle.apply(1,
                        1,
                        10,
                        TimeUnit.SECONDS)))
                .uri("http://httpbin.org:80")
                .metadata("key", "value")
            )
            .build();
}

This style also allows for more custom predicate assertions. The predicates defined by RouteDefinitionLocator beans are combined using logical and. By using the fluent Java API, you can use the and(), or(), and negate() operators on the Predicate class.

70.4. The DiscoveryClient Route Definition Locator

You can configure the gateway to create routes based on services registered with a DiscoveryClient compatible service registry.

To enable this, set spring.cloud.gateway.discovery.locator.enabled=true and make sure a DiscoveryClient implementation (such as Netflix Eureka, Consul, or Zookeeper) is on the classpath and enabled.

70.4.1. Configuring Predicates and Filters For DiscoveryClient Routes

By default, the gateway defines a single predicate and filter for routes created with a DiscoveryClient.

The default predicate is a path predicate defined with the pattern /serviceId/**, where serviceId is the ID of the service from the DiscoveryClient.

The default filter is a rewrite path filter with the regex /serviceId/(?<remaining>.*) and the replacement /${remaining}. This strips the service ID from the path before the request is sent downstream.

If you want to customize the predicates or filters used by the DiscoveryClient routes, set spring.cloud.gateway.discovery.locator.predicates[x] and spring.cloud.gateway.discovery.locator.filters[y]. When doing so, you need to make sure to include the default predicate and filter shown earlier, if you want to retain that functionality. The following example shows what this looks like:

Example 71. application.properties
spring.cloud.gateway.discovery.locator.predicates[0].name: Path
spring.cloud.gateway.discovery.locator.predicates[0].args[pattern]: "'/'+serviceId+'/**'"
spring.cloud.gateway.discovery.locator.predicates[1].name: Host
spring.cloud.gateway.discovery.locator.predicates[1].args[pattern]: "'**.foo.com'"
spring.cloud.gateway.discovery.locator.filters[0].name: CircuitBreaker
spring.cloud.gateway.discovery.locator.filters[0].args[name]: serviceId
spring.cloud.gateway.discovery.locator.filters[1].name: RewritePath
spring.cloud.gateway.discovery.locator.filters[1].args[regexp]: "'/' + serviceId + '/(?<remaining>.*)'"
spring.cloud.gateway.discovery.locator.filters[1].args[replacement]: "'/${remaining}'"

71. Reactor Netty Access Logs

To enable Reactor Netty access logs, set -Dreactor.netty.http.server.accessLogEnabled=true.

It must be a Java System Property, not a Spring Boot property.

You can configure the logging system to have a separate access log file. The following example creates a Logback configuration:

Example 72. logback.xml
    <appender name="accessLog" class="ch.qos.logback.core.FileAppender">
        <file>access_log.log</file>
        <encoder>
            <pattern>%msg%n</pattern>
        </encoder>
    </appender>
    <appender name="async" class="ch.qos.logback.classic.AsyncAppender">
        <appender-ref ref="accessLog" />
    </appender>

    <logger name="reactor.netty.http.server.AccessLog" level="INFO" additivity="false">
        <appender-ref ref="async"/>
    </logger>

72. CORS Configuration

You can configure the gateway to control CORS behavior. The “global” CORS configuration is a map of URL patterns to Spring Framework CorsConfiguration. The following example configures CORS:

Example 73. application.yml
spring:
  cloud:
    gateway:
      globalcors:
        cors-configurations:
          '[/**]':
            allowedOrigins: "https://docs.spring.io"
            allowedMethods:
            - GET

In the preceding example, CORS requests are allowed from requests that originate from docs.spring.io for all GET requested paths.

To provide the same CORS configuration to requests that are not handled by some gateway route predicate, set the spring.cloud.gateway.globalcors.add-to-simple-url-handler-mapping property to true. This is useful when you try to support CORS preflight requests and your route predicate does not evalute to true because the HTTP method is options.

73. Actuator API

The /gateway actuator endpoint lets you monitor and interact with a Spring Cloud Gateway application. To be remotely accessible, the endpoint has to be enabled and exposed over HTTP or JMX in the application properties. The following listing shows how to do so:

Example 74. application.properties
management.endpoint.gateway.enabled=true # default value
management.endpoints.web.exposure.include=gateway

73.1. Verbose Actuator Format

A new, more verbose format has been added to Spring Cloud Gateway. It adds more detail to each route, letting you view the predicates and filters associated with each route along with any configuration that is available. The following example configures /actuator/gateway/routes:

[
  {
    "predicate": "(Hosts: [**.addrequestheader.org] && Paths: [/headers], match trailing slash: true)",
    "route_id": "add_request_header_test",
    "filters": [
      "[[AddResponseHeader X-Response-Default-Foo = 'Default-Bar'], order = 1]",
      "[[AddRequestHeader X-Request-Foo = 'Bar'], order = 1]",
      "[[PrefixPath prefix = '/httpbin'], order = 2]"
    ],
    "uri": "lb://testservice",
    "order": 0
  }
]

This feature is enabled by default. To disable it, set the following property:

Example 75. application.properties
spring.cloud.gateway.actuator.verbose.enabled=false

This will default to true in a future release.

73.2. Retrieving Route Filters

This section details how to retrieve route filters, including:

73.2.1. Global Filters

To retrieve the global filters applied to all routes, make a GET request to /actuator/gateway/globalfilters. The resulting response is similar to the following:

{
  "or[email protected]77856cc5": 10100,
  "o[email protected]4f6fd101": 10000,
  "or[email protected]32d22650": -1,
  "[email protected]6459d9": 2147483647,
  "[email protected]5e0": 2147483647,
  "[email protected]d23": 0,
  "org.s[email protected]135064ea": 2147483637,
  "[email protected]23c05889": 2147483646
}

The response contains the details of the global filters that are in place. For each global filter, there is a string representation of the filter object (for example, or[email protected]77856cc5) and the corresponding order in the filter chain.}

73.2.2. Route Filters

To retrieve the GatewayFilter factories applied to routes, make a GET request to /actuator/gateway/routefilters. The resulting response is similar to the following:

{
  "[[email protected] configClass = AbstractNameValueGatewayFilterFactory.NameValueConfig]": null,
  "[[email protected] configClass = Object]": null,
  "[[email protected] configClass = Object]": null
}

The response contains the details of the GatewayFilter factories applied to any particular route. For each factory there is a string representation of the corresponding object (for example, [[email protected] configClass = Object]). Note that the null value is due to an incomplete implementation of the endpoint controller, because it tries to set the order of the object in the filter chain, which does not apply to a GatewayFilter factory object.

73.3. Refreshing the Route Cache

To clear the routes cache, make a POST request to /actuator/gateway/refresh. The request returns a 200 without a response body.

73.4. Retrieving the Routes Defined in the Gateway

To retrieve the routes defined in the gateway, make a GET request to /actuator/gateway/routes. The resulting response is similar to the following:

[{
  "route_id": "first_route",
  "route_object": {
    "predicate": "org.springframework.cloud.gateway.handler.predicate.PathRoutePredicateFactory$$Lambda$432/[email protected]",
    "filters": [
      "OrderedGatewayFilter{delegate=org.springframework.cloud.gateway.filter.factory.PreserveHostHeaderGatewayFilterFactory$$Lambda$436/[email protected], order=0}"
    ]
  },
  "order": 0
},
{
  "route_id": "second_route",
  "route_object": {
    "predicate": "org.springframework.cloud.gateway.handler.predicate.PathRoutePredicateFactory$$Lambda$432/[email protected]",
    "filters": []
  },
  "order": 0
}]

The response contains the details of all the routes defined in the gateway. The following table describes the structure of each element (each is a route) of the response:

Path Type Description

route_id

String

The route ID.

route_object.predicate

Object

The route predicate.

route_object.filters

Array

The GatewayFilter factories applied to the route.

order

Number

The route order.

73.5. Retrieving Information about a Particular Route

To retrieve information about a single route, make a GET request to /actuator/gateway/routes/{id} (for example, /actuator/gateway/routes/first_route). The resulting response is similar to the following:

{
  "id": "first_route",
  "predicates": [{
    "name": "Path",
    "args": {"_genkey_0":"/first"}
  }],
  "filters": [],
  "uri": "https://www.uri-destination.org",
  "order": 0
}]

The following table describes the structure of the response:

Path Type Description

id

String

The route ID.

predicates

Array

The collection of route predicates. Each item defines the name and the arguments of a given predicate.

filters

Array

The collection of filters applied to the route.

uri

String

The destination URI of the route.

order

Number

The route order.

73.6. Creating and Deleting a Particular Route

To create a route, make a POST request to /gateway/routes/{id_route_to_create} with a JSON body that specifies the fields of the route (see Retrieving Information about a Particular Route).

To delete a route, make a DELETE request to /gateway/routes/{id_route_to_delete}.

73.7. Recap: The List of All endpoints

The folloiwng table below summarizes the Spring Cloud Gateway actuator endpoints (note that each endpoint has /actuator/gateway as the base-path):

ID HTTP Method Description

globalfilters

GET

Displays the list of global filters applied to the routes.

routefilters

GET

Displays the list of GatewayFilter factories applied to a particular route.

refresh

POST

Clears the routes cache.

routes

GET

Displays the list of routes defined in the gateway.

routes/{id}

GET

Displays information about a particular route.

routes/{id}

POST

Adds a new route to the gateway.

routes/{id}

DELETE

Removes an existing route from the gateway.

74. Troubleshooting

This section covers common problems that may arise when you use Spring Cloud Gateway.

74.1. Log Levels

The following loggers may contain valuable troubleshooting information at the DEBUG and TRACE levels:

  • org.springframework.cloud.gateway

  • org.springframework.http.server.reactive

  • org.springframework.web.reactive

  • org.springframework.boot.autoconfigure.web

  • reactor.netty

  • redisratelimiter

74.2. Wiretap

The Reactor Netty HttpClient and HttpServer can have wiretap enabled. When combined with setting the reactor.netty log level to DEBUG or TRACE, it enables the logging of information, such as headers and bodies sent and received across the wire. To enable wiretap, set spring.cloud.gateway.httpserver.wiretap=true or spring.cloud.gateway.httpclient.wiretap=true for the HttpServer and HttpClient, respectively.

75. Developer Guide

These are basic guides to writing some custom components of the gateway.

75.1. Writing Custom Route Predicate Factories

In order to write a Route Predicate you will need to implement RoutePredicateFactory. There is an abstract class called AbstractRoutePredicateFactory which you can extend.

MyRoutePredicateFactory.java
public class MyRoutePredicateFactory extends AbstractRoutePredicateFactory<HeaderRoutePredicateFactory.Config> {

    public MyRoutePredicateFactory() {
        super(Config.class);
    }

    @Override
    public Predicate<ServerWebExchange> apply(Config config) {
        // grab configuration from Config object
        return exchange -> {
            //grab the request
            ServerHttpRequest request = exchange.getRequest();
            //take information from the request to see if it
            //matches configuration.
            return matches(config, request);
        };
    }

    public static class Config {
        //Put the configuration properties for your filter here
    }

}

75.2. Writing Custom GatewayFilter Factories

To write a GatewayFilter, you must implement GatewayFilterFactory. You can extend an abstract class called AbstractGatewayFilterFactory. The following examples show how to do so:

Example 76. PreGatewayFilterFactory.java
public class PreGatewayFilterFactory extends AbstractGatewayFilterFactory<PreGatewayFilterFactory.Config> {

    public PreGatewayFilterFactory() {
        super(Config.class);
    }

    @Override
    public GatewayFilter apply(Config config) {
        // grab configuration from Config object
        return (exchange, chain) -> {
            //If you want to build a "pre" filter you need to manipulate the
            //request before calling chain.filter
            ServerHttpRequest.Builder builder = exchange.getRequest().mutate();
            //use builder to manipulate the request
            return chain.filter(exchange.mutate().request(builder.build()).build());
        };
    }

    public static class Config {
        //Put the configuration properties for your filter here
    }

}
PostGatewayFilterFactory.java
public class PostGatewayFilterFactory extends AbstractGatewayFilterFactory<PostGatewayFilterFactory.Config> {

    public PostGatewayFilterFactory() {
        super(Config.class);
    }

    @Override
    public GatewayFilter apply(Config config) {
        // grab configuration from Config object
        return (exchange, chain) -> {
            return chain.filter(exchange).then(Mono.fromRunnable(() -> {
                ServerHttpResponse response = exchange.getResponse();
                //Manipulate the response in some way
            }));
        };
    }

    public static class Config {
        //Put the configuration properties for your filter here
    }

}

75.2.1. Naming Custom Filters And References In Configuration

Custom filters class names should end in GatewayFilterFactory.

For example, to reference a filter named Something in configuration files, the filter must be in a class named SomethingGatewayFilterFactory.

It is possible to create a gateway filter named without the GatewayFilterFactory suffix, such as class AnotherThing. This filter could be referenced as AnotherThing in configuration files. This is not a supported naming convention and this syntax may be removed in future releases. Please update the filter name to be compliant.

75.3. Writing Custom Global Filters

To write a custom global filter, you must implement GlobalFilter interface. This applies the filter to all requests.

The following examples show how to set up global pre and post filters, respectively:

@Bean
public GlobalFilter customGlobalFilter() {
    return (exchange, chain) -> exchange.getPrincipal()
        .map(Principal::getName)
        .defaultIfEmpty("Default User")
        .map(userName -> {
          //adds header to proxied request
          exchange.getRequest().mutate().header("CUSTOM-REQUEST-HEADER", userName).build();
          return exchange;
        })
        .flatMap(chain::filter);
}

@Bean
public GlobalFilter customGlobalPostFilter() {
    return (exchange, chain) -> chain.filter(exchange)
        .then(Mono.just(exchange))
        .map(serverWebExchange -> {
          //adds header to response
          serverWebExchange.getResponse().getHeaders().set("CUSTOM-RESPONSE-HEADER",
              HttpStatus.OK.equals(serverWebExchange.getResponse().getStatusCode()) ? "It worked": "It did not work");
          return serverWebExchange;
        })
        .then();
}

76. Building a Simple Gateway by Using Spring MVC or Webflux

The following describes an alternative style gateway. None of the prior documentation applies to what follows.

Spring Cloud Gateway provides a utility object called ProxyExchange. You can use it inside a regular Spring web handler as a method parameter. It supports basic downstream HTTP exchanges through methods that mirror the HTTP verbs. With MVC, it also supports forwarding to a local handler through the forward() method. To use the ProxyExchange, include the right module in your classpath (either spring-cloud-gateway-mvc or spring-cloud-gateway-webflux).

The following MVC example proxies a request to /test downstream to a remote server:

@RestController
@SpringBootApplication
public class GatewaySampleApplication {

    @Value("${remote.home}")
    private URI home;

    @GetMapping("/test")
    public ResponseEntity<?> proxy(ProxyExchange<byte[]> proxy) throws Exception {
        return proxy.uri(home.toString() + "/image/png").get();
    }

}

The following example does the same thing with Webflux:

@RestController
@SpringBootApplication
public class GatewaySampleApplication {

    @Value("${remote.home}")
    private URI home;

    @GetMapping("/test")
    public Mono<ResponseEntity<?>> proxy(ProxyExchange<byte[]> proxy) throws Exception {
        return proxy.uri(home.toString() + "/image/png").get();
    }

}

Convenience methods on the ProxyExchange enable the handler method to discover and enhance the URI path of the incoming request. For example, you might want to extract the trailing elements of a path to pass them downstream:

@GetMapping("/proxy/path/**")
public ResponseEntity<?> proxyPath(ProxyExchange<byte[]> proxy) throws Exception {
  String path = proxy.path("/proxy/path/");
  return proxy.uri(home.toString() + "/foos/" + path).get();
}

All the features of Spring MVC and Webflux are available to gateway handler methods. As a result, you can inject request headers and query parameters, for instance, and you can constrain the incoming requests with declarations in the mapping annotation. See the documentation for @RequestMapping in Spring MVC for more details of those features.

You can add headers to the downstream response by using the header() methods on ProxyExchange.

You can also manipulate response headers (and anything else you like in the response) by adding a mapper to the get() method (and other methods). The mapper is a Function that takes the incoming ResponseEntity and converts it to an outgoing one.

First-class support is provided for “sensitive” headers (by default, cookie and authorization), which are not passed downstream, and for “proxy” (x-forwarded-*) headers.

77. Configuration properties

To see the list of all Spring Cloud Gateway related configuration properties, see the appendix.

Spring Cloud Kubernetes

This reference guide covers how to use Spring Cloud Kubernetes.

78. Why do you need Spring Cloud Kubernetes?

Spring Cloud Kubernetes provides implementations of well known Spring Cloud interfaces allowing developers to build and run Spring Cloud applications on Kubernetes. While this project may be useful to you when building a cloud native application, it is also not a requirement in order to deploy a Spring Boot app on Kubernetes. If you are just getting started in your journey to running your Spring Boot app on Kubernetes you can accomplish a lot with nothing more than a basic Spring Boot app and Kubernetes itself. To learn more, you can get started by reading the Spring Boot reference documentation for deploying to Kubernetes and also working through the workshop material Spring and Kubernetes.

79. Starters

Starters are convenient dependency descriptors you can include in your application. Include a starter to get the dependencies and Spring Boot auto-configuration for a feature set. Starters that begin with spring-cloud-starter-kubernetes-fabric8 provide implementations using the Fabric8 Kubernetes Java Client. Starters that begin with spring-cloud-starter-kubernetes-client provide implementations using the Kubernetes Java Client.

Starter Features
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client</artifactId>
</dependency>

Discovery Client implementation that resolves service names to Kubernetes Services.

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8-config</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client-config</artifactId>
</dependency>

Load application properties from Kubernetes ConfigMaps and Secrets. Reload application properties when a ConfigMap or Secret changes.

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8-all</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client-all</artifactId>
</dependency>

All Spring Cloud Kubernetes features.

80. DiscoveryClient for Kubernetes

This project provides an implementation of Discovery Client for Kubernetes. This client lets you query Kubernetes endpoints (see services) by name. A service is typically exposed by the Kubernetes API server as a collection of endpoints that represent http and https addresses and that a client can access from a Spring Boot application running as a pod.

This is something that you get for free by adding the following dependency inside your project:

Fabric8 Kubernetes Client

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8</artifactId>
</dependency>

Kubernetes Java Client

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client</artifactId>
</dependency>

To enable loading of the DiscoveryClient, add @EnableDiscoveryClient to the according configuration or application class, as the following example shows:

@SpringBootApplication
@EnableDiscoveryClient
public class Application {
  public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
  }
}

Then you can inject the client in your code simply by autowiring it, as the following example shows:

@Autowired
private DiscoveryClient discoveryClient;

You can choose to enable DiscoveryClient from all namespaces by setting the following property in application.properties:

spring.cloud.kubernetes.discovery.all-namespaces=true

If, for any reason, you need to disable the DiscoveryClient, you can set the following property in application.properties:

spring.cloud.kubernetes.discovery.enabled=false

Some Spring Cloud components use the DiscoveryClient in order to obtain information about the local service instance. For this to work, you need to align the Kubernetes service name with the spring.application.name property.

spring.application.name has no effect as far as the name registered for the application within Kubernetes

Spring Cloud Kubernetes can also watch the Kubernetes service catalog for changes and update the DiscoveryClient implementation accordingly. In order to enable this functionality you need to add @EnableScheduling on a configuration class in your application.

81. Kubernetes native service discovery

Kubernetes itself is capable of (server side) service discovery (see: kubernetes.io/docs/concepts/services-networking/service/#discovering-services). Using native kubernetes service discovery ensures compatibility with additional tooling, such as Istio (istio.io), a service mesh that is capable of load balancing, circuit breaker, failover, and much more.

The caller service then need only refer to names resolvable in a particular Kubernetes cluster. A simple implementation might use a spring RestTemplate that refers to a fully qualified domain name (FQDN), such as {service-name}.{namespace}.svc.{cluster}.local:{service-port}.

Additionally, you can use Hystrix for:

  • Circuit breaker implementation on the caller side, by annotating the spring boot application class with @EnableCircuitBreaker

  • Fallback functionality, by annotating the respective method with @HystrixCommand(fallbackMethod=

82. Kubernetes PropertySource implementations

The most common approach to configuring your Spring Boot application is to create an application.properties or application.yaml or an application-profile.properties or application-profile.yaml file that contains key-value pairs that provide customization values to your application or Spring Boot starters. You can override these properties by specifying system properties or environment variables.

82.1. Using a ConfigMap PropertySource

Kubernetes provides a resource named ConfigMap to externalize the parameters to pass to your application in the form of key-value pairs or embedded application.properties or application.yaml files. The Spring Cloud Kubernetes Config project makes Kubernetes ConfigMap instances available during application bootstrapping and triggers hot reloading of beans or Spring context when changes are detected on observed ConfigMap instances.

The default behavior is to create a Fabric8ConfigMapPropertySource based on a Kubernetes ConfigMap that has a metadata.name value of either the name of your Spring application (as defined by its spring.application.name property) or a custom name defined within the bootstrap.properties file under the following key: spring.cloud.kubernetes.config.name.

However, more advanced configuration is possible where you can use multiple ConfigMap instances. The spring.cloud.kubernetes.config.sources list makes this possible. For example, you could define the following ConfigMap instances:

spring:
  application:
    name: cloud-k8s-app
  cloud:
    kubernetes:
      config:
        name: default-name
        namespace: default-namespace
        sources:
         # Spring Cloud Kubernetes looks up a ConfigMap named c1 in namespace default-namespace
         - name: c1
         # Spring Cloud Kubernetes looks up a ConfigMap named default-name in whatever namespace n2
         - namespace: n2
         # Spring Cloud Kubernetes looks up a ConfigMap named c3 in namespace n3
         - namespace: n3
           name: c3

In the preceding example, if spring.cloud.kubernetes.config.namespace had not been set, the ConfigMap named c1 would be looked up in the namespace that the application runs.

Any matching ConfigMap that is found is processed as follows:

  • Apply individual configuration properties.

  • Apply as yaml the content of any property named application.yaml.

  • Apply as a properties file the content of any property named application.properties.

The single exception to the aforementioned flow is when the ConfigMap contains a single key that indicates the file is a YAML or properties file. In that case, the name of the key does NOT have to be application.yaml or application.properties (it can be anything) and the value of the property is treated correctly. This features facilitates the use case where the ConfigMap was created by using something like the following:

kubectl create configmap game-config --from-file=/path/to/app-config.yaml

Assume that we have a Spring Boot application named demo that uses the following properties to read its thread pool configuration.

  • pool.size.core

  • pool.size.maximum

This can be externalized to config map in yaml format as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  pool.size.core: 1
  pool.size.max: 16

Individual properties work fine for most cases. However, sometimes, embedded yaml is more convenient. In this case, we use a single property named application.yaml to embed our yaml, as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yaml: |-
    pool:
      size:
        core: 1
        max:16

The following example also works:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  custom-name.yaml: |-
    pool:
      size:
        core: 1
        max:16

You can also configure Spring Boot applications differently depending on active profiles that are merged together when the ConfigMap is read. You can provide different property values for different profiles by using an application.properties or application.yaml property, specifying profile-specific values, each in their own document (indicated by the --- sequence), as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yml: |-
    greeting:
      message: Say Hello to the World
    farewell:
      message: Say Goodbye
    ---
    spring:
      profiles: development
    greeting:
      message: Say Hello to the Developers
    farewell:
      message: Say Goodbye to the Developers
    ---
    spring:
      profiles: production
    greeting:
      message: Say Hello to the Ops

In the preceding case, the configuration loaded into your Spring Application with the development profile is as follows:

  greeting:
    message: Say Hello to the Developers
  farewell:
    message: Say Goodbye to the Developers

However, if the production profile is active, the configuration becomes:

  greeting:
    message: Say Hello to the Ops
  farewell:
    message: Say Goodbye

If both profiles are active, the property that appears last within the ConfigMap overwrites any preceding values.

Another option is to create a different config map per profile and spring boot will automatically fetch it based on active profiles

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yml: |-
    greeting:
      message: Say Hello to the World
    farewell:
      message: Say Goodbye
kind: ConfigMap
apiVersion: v1
metadata:
  name: demo-development
data:
  application.yml: |-
    spring:
      profiles: development
    greeting:
      message: Say Hello to the Developers
    farewell:
      message: Say Goodbye to the Developers
kind: ConfigMap
apiVersion: v1
metadata:
  name: demo-production
data:
  application.yml: |-
    spring:
      profiles: production
    greeting:
      message: Say Hello to the Ops
    farewell:
      message: Say Goodbye

To tell Spring Boot which profile should be enabled at bootstrap, you can pass SPRING_PROFILES_ACTIVE environment variable. To do so, you can launch your Spring Boot application with an environment variable that you can define it in the PodSpec at the container specification. Deployment resource file, as follows:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment-name
  labels:
    app: deployment-name
spec:
  replicas: 1
  selector:
    matchLabels:
      app: deployment-name
  template:
    metadata:
      labels:
        app: deployment-name
    spec:
        containers:
        - name: container-name
          image: your-image
          env:
          - name: SPRING_PROFILES_ACTIVE
            value: "development"
You should check the security configuration section. To access config maps from inside a pod you need to have the correct Kubernetes service accounts, roles and role bindings.

Another option for using ConfigMap instances is to mount them into the Pod by running the Spring Cloud Kubernetes application and having Spring Cloud Kubernetes read them from the file system. This behavior is controlled by the spring.cloud.kubernetes.config.paths property. You can use it in addition to or instead of the mechanism described earlier. You can specify multiple (exact) file paths in spring.cloud.kubernetes.config.paths by using the , delimiter.

You have to provide the full exact path to each property file, because directories are not being recursively parsed.
If you use spring.cloud.kubernetes.config.paths or spring.cloud.kubernetes.secrets.path the automatic reload functionality will not work. You will need to make a POST request to the /actuator/refresh endpoint or restart/redeploy the application.
Table 5. Properties:
Name Type Default Description

spring.cloud.kubernetes.config.enabled

Boolean

true

Enable ConfigMaps PropertySource

spring.cloud.kubernetes.config.name

String

${spring.application.name}

Sets the name of ConfigMap to look up

spring.cloud.kubernetes.config.namespace

String

Client namespace

Sets the Kubernetes namespace where to lookup

spring.cloud.kubernetes.config.paths

List

null

Sets the paths where ConfigMap instances are mounted

spring.cloud.kubernetes.config.enableApi

Boolean

true

Enable or disable consuming ConfigMap instances through APIs

82.2. Secrets PropertySource

Kubernetes has the notion of Secrets for storing sensitive data such as passwords, OAuth tokens, and so on. This project provides integration with Secrets to make secrets accessible by Spring Boot applications. You can explicitly enable or disable This feature by setting the spring.cloud.kubernetes.secrets.enabled property.

When enabled, the Fabric8SecretsPropertySource looks up Kubernetes for Secrets from the following sources:

  1. Reading recursively from secrets mounts

  2. Named after the application (as defined by spring.application.name)

  3. Matching some labels

Note:

By default, consuming Secrets through the API (points 2 and 3 above) is not enabled for security reasons. The permission 'list' on secrets allows clients to inspect secrets values in the specified namespace. Further, we recommend that containers share secrets through mounted volumes.

If you enable consuming Secrets through the API, we recommend that you limit access to Secrets by using an authorization policy, such as RBAC. For more information about risks and best practices when consuming Secrets through the API refer to this doc.

If the secrets are found, their data is made available to the application.

Assume that we have a spring boot application named demo that uses properties to read its database configuration. We can create a Kubernetes secret by using the following command:

kubectl create secret generic db-secret --from-literal=username=user --from-literal=password=p455w0rd

The preceding command would create the following secret (which you can see by using kubectl get secrets db-secret -o yaml):

apiVersion: v1
data:
  password: cDQ1NXcwcmQ=
  username: dXNlcg==
kind: Secret
metadata:
  creationTimestamp: 2017-07-04T09:15:57Z
  name: db-secret
  namespace: default
  resourceVersion: "357496"
  selfLink: /api/v1/namespaces/default/secrets/db-secret
  uid: 63c89263-6099-11e7-b3da-76d6186905a8
type: Opaque

Note that the data contains Base64-encoded versions of the literal provided by the create command.

Your application can then use this secret — for example, by exporting the secret’s value as environment variables:

apiVersion: v1
kind: Deployment
metadata:
  name: ${project.artifactId}
spec:
   template:
     spec:
       containers:
         - env:
            - name: DB_USERNAME
              valueFrom:
                 secretKeyRef:
                   name: db-secret
                   key: username
            - name: DB_PASSWORD
              valueFrom:
                 secretKeyRef:
                   name: db-secret
                   key: password

You can select the Secrets to consume in a number of ways:

  1. By listing the directories where secrets are mapped:

    -Dspring.cloud.kubernetes.secrets.paths=/etc/secrets/db-secret,etc/secrets/postgresql

    If you have all the secrets mapped to a common root, you can set them like:

    -Dspring.cloud.kubernetes.secrets.paths=/etc/secrets
  2. By setting a named secret:

    -Dspring.cloud.kubernetes.secrets.name=db-secret
  3. By defining a list of labels:

    -Dspring.cloud.kubernetes.secrets.labels.broker=activemq
    -Dspring.cloud.kubernetes.secrets.labels.db=postgresql

As the case with ConfigMap, more advanced configuration is also possible where you can use multiple Secret instances. The spring.cloud.kubernetes.secrets.sources list makes this possible. For example, you could define the following Secret instances:

spring:
  application:
    name: cloud-k8s-app
  cloud:
    kubernetes:
      secrets:
        name: default-name
        namespace: default-namespace
        sources:
         # Spring Cloud Kubernetes looks up a Secret named s1 in namespace default-namespace
         - name: s1
         # Spring Cloud Kubernetes looks up a Secret named default-name in whatever namespace n2
         - namespace: n2
         # Spring Cloud Kubernetes looks up a Secret named s3 in namespace n3
         - namespace: n3
           name: s3

In the preceding example, if spring.cloud.kubernetes.secrets.namespace had not been set, the Secret named s1 would be looked up in the namespace that the application runs.

Table 6. Properties:
Name Type Default Description

spring.cloud.kubernetes.secrets.enabled

Boolean

true

Enable Secrets PropertySource

spring.cloud.kubernetes.secrets.name

String

${spring.application.name}

Sets the name of the secret to look up

spring.cloud.kubernetes.secrets.namespace

String

Client namespace

Sets the Kubernetes namespace where to look up

spring.cloud.kubernetes.secrets.labels

Map

null

Sets the labels used to lookup secrets

spring.cloud.kubernetes.secrets.paths

List

null

Sets the paths where secrets are mounted (example 1)

spring.cloud.kubernetes.secrets.enableApi

Boolean

false

Enables or disables consuming secrets through APIs (examples 2 and 3)

Notes:

  • The spring.cloud.kubernetes.secrets.labels property behaves as defined by Map-based binding.

  • The spring.cloud.kubernetes.secrets.paths property behaves as defined by Collection-based binding.

  • Access to secrets through the API may be restricted for security reasons. The preferred way is to mount secrets to the Pod.

You can find an example of an application that uses secrets (though it has not been updated to use the new spring-cloud-kubernetes project) at spring-boot-camel-config

82.3. PropertySource Reload

This functionality has been deprecated in the 2020.0 release. Please see the Spring Cloud Kubernetes Configuration Watcher controller for an alternative way to achieve the same functionality.

Some applications may need to detect changes on external property sources and update their internal status to reflect the new configuration. The reload feature of Spring Cloud Kubernetes is able to trigger an application reload when a related ConfigMap or Secret changes.

By default, this feature is disabled. You can enable it by using the spring.cloud.kubernetes.reload.enabled=true configuration property (for example, in the application.properties file).

The following levels of reload are supported (by setting the spring.cloud.kubernetes.reload.strategy property):

  • refresh (default): Only configuration beans annotated with @ConfigurationProperties or @RefreshScope are reloaded. This reload level leverages the refresh feature of Spring Cloud Context.

  • restart_context: the whole Spring ApplicationContext is gracefully restarted. Beans are recreated with the new configuration. In order for the restart context functionality to work properly you must enable and expose the restart actuator endpoint

management:
  endpoint:
    restart:
      enabled: true
  endpoints:
    web:
      exposure:
        include: restart
  • shutdown: the Spring ApplicationContext is shut down to activate a restart of the container. When you use this level, make sure that the lifecycle of all non-daemon threads is bound to the ApplicationContext and that a replication controller or replica set is configured to restart the pod.

Assuming that the reload feature is enabled with default settings (refresh mode), the following bean is refreshed when the config map changes:

@Configuration
@ConfigurationProperties(prefix = "bean")
public class MyConfig {

    private String message = "a message that can be changed live";

    // getter and setters

}

To see that changes effectively happen, you can create another bean that prints the message periodically, as follows

@Component
public class MyBean {

    @Autowired
    private MyConfig config;

    @Scheduled(fixedDelay = 5000)
    public void hello() {
        System.out.println("The message is: " + config.getMessage());
    }
}

You can change the message printed by the application by using a ConfigMap, as follows:

apiVersion: v1
kind: ConfigMap
metadata:
  name: reload-example
data:
  application.properties: |-
    bean.message=Hello World!

Any change to the property named bean.message in the ConfigMap associated with the pod is reflected in the output. More generally speaking, changes associated to properties prefixed with the value defined by the prefix field of the @ConfigurationProperties annotation are detected and reflected in the application. Associating a ConfigMap with a pod is explained earlier in this chapter.

The full example is available in spring-cloud-kubernetes-reload-example.

The reload feature supports two operating modes: * Event (default): Watches for changes in config maps or secrets by using the Kubernetes API (web socket). Any event produces a re-check on the configuration and, in case of changes, a reload. The view role on the service account is required in order to listen for config map changes. A higher level role (such as edit) is required for secrets (by default, secrets are not monitored). * Polling: Periodically re-creates the configuration from config maps and secrets to see if it has changed. You can configure the polling period by using the spring.cloud.kubernetes.reload.period property and defaults to 15 seconds. It requires the same role as the monitored property source. This means, for example, that using polling on file-mounted secret sources does not require particular privileges.

Table 7. Properties:
Name Type Default Description

spring.cloud.kubernetes.reload.enabled

Boolean

false

Enables monitoring of property sources and configuration reload

spring.cloud.kubernetes.reload.monitoring-config-maps

Boolean

true

Allow monitoring changes in config maps

spring.cloud.kubernetes.reload.monitoring-secrets

Boolean

false

Allow monitoring changes in secrets

spring.cloud.kubernetes.reload.strategy

Enum

refresh

The strategy to use when firing a reload (refresh, restart_context, or shutdown)

spring.cloud.kubernetes.reload.mode

Enum

event

Specifies how to listen for changes in property sources (event or polling)

spring.cloud.kubernetes.reload.period

Duration

15s

The period for verifying changes when using the polling strategy

Notes: * You should not use properties under spring.cloud.kubernetes.reload in config maps or secrets. Changing such properties at runtime may lead to unexpected results. * Deleting a property or the whole config map does not restore the original state of the beans when you use the refresh level.

83. Kubernetes Ecosystem Awareness

All of the features described earlier in this guide work equally well, regardless of whether your application is running inside Kubernetes. This is really helpful for development and troubleshooting. From a development point of view, this lets you start your Spring Boot application and debug one of the modules that is part of this project. You need not deploy it in Kubernetes, as the code of the project relies on the Fabric8 Kubernetes Java client, which is a fluent DSL that can communicate by using http protocol to the REST API of the Kubernetes Server.

To disable the integration with Kubernetes you can set spring.cloud.kubernetes.enabled to false. Please be aware that when spring-cloud-kubernetes-config is on the classpath, spring.cloud.kubernetes.enabled should be set in bootstrap.{properties|yml} (or the profile specific one) otherwise it should be in application.{properties|yml} (or the profile specific one). Also note that these properties: spring.cloud.kubernetes.config.enabled and spring.cloud.kubernetes.secrets.enabled only take effect when set in bootstrap.{properties|yml}

83.1. Kubernetes Profile Autoconfiguration

When the application runs as a pod inside Kubernetes, a Spring profile named kubernetes automatically gets activated. This lets you customize the configuration, to define beans that are applied when the Spring Boot application is deployed within the Kubernetes platform (for example, different development and production configuration).

83.2. Istio Awareness

When you include the spring-cloud-kubernetes-fabric8-istio module in the application classpath, a new profile is added to the application, provided the application is running inside a Kubernetes Cluster with Istio installed. You can then use spring @Profile("istio") annotations in your Beans and @Configuration classes.

The Istio awareness module uses me.snowdrop:istio-client to interact with Istio APIs, letting us discover traffic rules, circuit breakers, and so on, making it easy for our Spring Boot applications to consume this data to dynamically configure themselves according to the environment.

84. Pod Health Indicator

Spring Boot uses HealthIndicator to expose info about the health of an application. That makes it really useful for exposing health-related information to the user and makes it a good fit for use as readiness probes.

The Kubernetes health indicator (which is part of the core module) exposes the following info:

  • Pod name, IP address, namespace, service account, node name, and its IP address

  • A flag that indicates whether the Spring Boot application is internal or external to Kubernetes

85. Info Contributor

Spring Cloud Kubernetes includes an InfoContributor which adds Pod information to Spring Boot’s /info Acturator endpoint.

You can disable this InfoContributor by setting management.info.kubernetes.enabled to false in bootstrap.[properties | yaml].

86. Leader Election

<TBD>

87. LoadBalancer for Kubernetes

This project includes Spring Cloud Load Balancer for load balancing based on Kubernetes Endpoints and provides implementation of load balancer based on Kubernetes Service. To include it to your project add the following dependency.

Fabric8 Implementation

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8-loadbalancer</artifactId>
</dependency>

Kubernetes Java Client Implementation

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client-loadbalancer</artifactId>
</dependency>

To enable load balancing based on Kubernetes Service name use the following property. Then load balancer would try to call application using address, for example service-a.default.svc.cluster.local

spring.cloud.kubernetes.loadbalancer.mode=SERVICE

To enabled load balancing across all namespaces use the following property. Property from spring-cloud-kubernetes-discovery module is respected.

spring.cloud.kubernetes.discovery.all-namespaces=true

88. Security Configurations Inside Kubernetes

88.1. Namespace

Most of the components provided in this project need to know the namespace. For Kubernetes (1.3+), the namespace is made available to the pod as part of the service account secret and is automatically detected by the client. For earlier versions, it needs to be specified as an environment variable to the pod. A quick way to do this is as follows:

      env:
      - name: "KUBERNETES_NAMESPACE"
        valueFrom:
          fieldRef:
            fieldPath: "metadata.namespace"

88.2. Service Account

For distributions of Kubernetes that support more fine-grained role-based access within the cluster, you need to make sure a pod that runs with spring-cloud-kubernetes has access to the Kubernetes API. For any service accounts you assign to a deployment or pod, you need to make sure they have the correct roles.

Depending on the requirements, you’ll need get, list and watch permission on the following resources:

Table 8. Kubernetes Resource Permissions
Dependency Resources

spring-cloud-starter-kubernetes-fabric8

pods, services, endpoints

spring-cloud-starter-kubernetes-fabric8-config

configmaps, secrets

spring-cloud-starter-kubernetes-client

pods, services, endpoints

spring-cloud-starter-kubernetes-client-config

configmaps, secrets

For development purposes, you can add cluster-reader permissions to your default service account. On a production system you’ll likely want to provide more granular permissions.

The following Role and RoleBinding are an example for namespaced permissions for the default account:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: YOUR-NAME-SPACE
  name: namespace-reader
rules:
  - apiGroups: ["", "extensions", "apps"]
    resources: ["configmaps", "pods", "services", "endpoints", "secrets"]
    verbs: ["get", "list", "watch"]

---

kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: namespace-reader-binding
  namespace: YOUR-NAME-SPACE
subjects:
- kind: ServiceAccount
  name: default
  apiGroup: ""
roleRef:
  kind: Role
  name: namespace-reader
  apiGroup: ""

89. Service Registry Implementation

In Kubernetes service registration is controlled by the platform, the application itself does not control registration as it may do in other platforms. For this reason using spring.cloud.service-registry.auto-registration.enabled or setting @EnableDiscoveryClient(autoRegister=false) will have no effect in Spring Cloud Kubernetes.

90. Spring Cloud Kubernetes Configuration Watcher

Kubernetes provides the ability to mount a ConfigMap or Secret as a volume in the container of your application. When the contents of the ConfigMap or Secret changes, the mounted volume will be updated with those changes.

However, Spring Boot will not automatically update those changes unless you restart the application. Spring Cloud provides the ability refresh the application context without restarting the application by either hitting the actuator endpoint /refresh or via publishing a RefreshRemoteApplicationEvent using Spring Cloud Bus.

To achieve this configuration refresh of a Spring Cloud app running on Kubernetes, you can deploy the Spring Cloud Kubernetes Configuration Watcher controller into your Kubernetes cluster.

The application is published as a container and is available on Docker Hub.

Spring Cloud Kubernetes Configuration Watcher can send refresh notifications to applications in two ways.

  1. Over HTTP in which case the application being notified must of the /refresh actuator endpoint exposed and accessible from within the cluster

  2. Using Spring Cloud Bus, in which case you will need a message broker deployed to your custer for the application to use.

90.1. Deployment YAML

Below is a sample deployment YAML you can use to deploy the Kubernetes Configuration Watcher to Kubernetes.

---
apiVersion: v1
kind: List
items:
  - apiVersion: v1
    kind: Service
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher
    spec:
      ports:
        - name: http
          port: 8888
          targetPort: 8888
      selector:
        app: spring-cloud-kubernetes-configuration-watcher
      type: ClusterIP
  - apiVersion: v1
    kind: ServiceAccount
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher
  - apiVersion: rbac.authorization.k8s.io/v1
    kind: RoleBinding
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher:view
    roleRef:
      kind: Role
      apiGroup: rbac.authorization.k8s.io
      name: namespace-reader
    subjects:
      - kind: ServiceAccount
        name: spring-cloud-kubernetes-configuration-watcher
  - apiVersion: rbac.authorization.k8s.io/v1
    kind: Role
    metadata:
      namespace: default
      name: namespace-reader
    rules:
      - apiGroups: ["", "extensions", "apps"]
        resources: ["configmaps", "pods", "services", "endpoints", "secrets"]
        verbs: ["get", "list", "watch"]
  - apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: spring-cloud-kubernetes-configuration-watcher-deployment
    spec:
      selector:
        matchLabels:
          app: spring-cloud-kubernetes-configuration-watcher
      template:
        metadata:
          labels:
            app: spring-cloud-kubernetes-configuration-watcher
        spec:
          serviceAccount: spring-cloud-kubernetes-configuration-watcher
          containers:
          - name: spring-cloud-kubernetes-configuration-watcher
            image: springcloud/spring-cloud-kubernetes-configuration-watcher:2.0.0-SNAPSHOT
            imagePullPolicy: IfNotPresent
            readinessProbe:
              httpGet:
                port: 8888
                path: /actuator/health/readiness
            livenessProbe:
              httpGet:
                port: 8888
                path: /actuator/health/liveness
            ports:
            - containerPort: 8888

The Service Account and associated Role Binding is important for Spring Cloud Kubernetes Configuration to work properly. The controller needs access to read data about ConfigMaps, Pods, Services, Endpoints and Secrets in the Kubernetes cluster.

90.2. Monitoring ConfigMaps and Secrets

Spring Cloud Kubernetes Configuration Watcher will react to changes in ConfigMaps with a label of spring.cloud.kubernetes.config with the value true or any Secret with a label of spring.cloud.kubernetes.secret with the value true. If the ConfigMap or Secret does not have either of those labels or the values of those labels is not true then any changes will be ignored.

The labels Spring Cloud Kubernetes Configuration Watcher looks for on ConfigMaps and Secrets can be changed by setting spring.cloud.kubernetes.configuration.watcher.configLabel and spring.cloud.kubernetes.configuration.watcher.secretLabel respectively.

If a change is made to a ConfigMap or Secret with valid labels then Spring Cloud Kubernetes Configuration Watcher will take the name of the ConfigMap or Secret and send a notification to the application with that name.

90.3. HTTP Implementation

The HTTP implementation is what is used by default. When this implementation is used Spring Cloud Kubernetes Configuration Watcher and a change to a ConfigMap or Secret occurs then the HTTP implementation will use the Spring Cloud Kubernetes Discovery Client to fetch all instances of the application which match the name of the ConfigMap or Secret and send an HTTP POST request to the application’s actuator /refresh endpoint. By default it will send the post request to /actuator/refresh using the port registered in the discovery client.

90.3.1. Non-Default Management Port and Actuator Path

If the application is using a non-default actuator path and/or using a different port for the management endpoints, the Kubernetes service for the application can add an annotation called boot.spring.io/actuator and set its value to the path and port used by the application. For example

apiVersion: v1
kind: Service
metadata:
  labels:
    app: config-map-demo
  name: config-map-demo
  annotations:
    boot.spring.io/actuator: http://:9090/myactuator/home
spec:
  ports:
    - name: http
      port: 8080
      targetPort: 8080
  selector:
    app: config-map-demo

Another way you can choose to configure the actuator path and/or management port is by setting spring.cloud.kubernetes.configuration.watcher.actuatorPath and spring.cloud.kubernetes.configuration.watcher.actuatorPort.

90.4. Messaging Implementation

The messaging implementation can be enabled by setting profile to either bus-amqp (RabbitMQ) or bus-kafka (Kafka) when the Spring Cloud Kubernetes Configuration Watcher application is deployed to Kubernetes.

90.5. Configuring RabbitMQ

When the bus-amqp profile is enabled you will need to configure Spring RabbitMQ to point it to the location of the RabbitMQ instance you would like to use as well as any credentials necessary to authenticate. This can be done by setting the standard Spring RabbitMQ properties, for example

spring:
  rabbitmq:
    username: user
    password: password
    host: rabbitmq

90.6. Configuring Kafka

When the bus-kafka profile is enabled you will need to configure Spring Kafka to point it to the location of the Kafka Broker instance you would like to use. This can be done by setting the standard Spring Kafka properties, for example

spring:
  kafka:
    producer:
      bootstrap-servers: localhost:9092

91. Examples

Spring Cloud Kubernetes tries to make it transparent for your applications to consume Kubernetes Native Services by following the Spring Cloud interfaces.

In your applications, you need to add the spring-cloud-kubernetes-discovery dependency to your classpath and remove any other dependency that contains a DiscoveryClient implementation (that is, a Eureka discovery client). The same applies for PropertySourceLocator, where you need to add to the classpath the spring-cloud-kubernetes-config and remove any other dependency that contains a PropertySourceLocator implementation (that is, a configuration server client).

The following projects highlight the usage of these dependencies and demonstrate how you can use these libraries from any Spring Boot application:

92. Other Resources

This section lists other resources, such as presentations (slides) and videos about Spring Cloud Kubernetes.

Please feel free to submit other resources through pull requests to this repository.

93. Configuration properties

To see the list of all Kubernetes related configuration properties please check the Appendix page.

94. Building

94.1. Basic Compile and Test

To build the source you will need to install JDK 1.8.

Spring Cloud uses Maven for most build-related activities, and you should be able to get off the ground quite quickly by cloning the project you are interested in and typing

$ ./mvnw install
You can also install Maven (>=3.3.3) yourself and run the mvn command in place of ./mvnw in the examples below. If you do that you also might need to add -P spring if your local Maven settings do not contain repository declarations for spring pre-release artifacts.
Be aware that you might need to increase the amount of memory available to Maven by setting a MAVEN_OPTS environment variable with a value like -Xmx512m -XX:MaxPermSize=128m. We try to cover this in the .mvn configuration, so if you find you have to do it to make a build succeed, please raise a ticket to get the settings added to source control.

For hints on how to build the project look in .travis.yml if there is one. There should be a "script" and maybe "install" command. Also look at the "services" section to see if any services need to be running locally (e.g. mongo or rabbit). Ignore the git-related bits that you might find in "before_install" since they’re related to setting git credentials and you already have those.

The projects that require middleware generally include a docker-compose.yml, so consider using Docker Compose to run the middeware servers in Docker containers. See the README in the scripts demo repository for specific instructions about the common cases of mongo, rabbit and redis.

If all else fails, build with the command from .travis.yml (usually ./mvnw install).

94.2. Documentation

The spring-cloud-build module has a "docs" profile, and if you switch that on it will try to build asciidoc sources from src/main/asciidoc. As part of that process it will look for a README.adoc and process it by loading all the includes, but not parsing or rendering it, just copying it to ${main.basedir} (defaults to $/opt/jenkins/data/workspace/spring-cloud-release-train-docs-master-ci/train-docs/target/unpacked-docs, i.e. the root of the project). If there are any changes in the README it will then show up after a Maven build as a modified file in the correct place. Just commit it and push the change.

94.3. Working with the code

If you don’t have an IDE preference we would recommend that you use Spring Tools Suite or Eclipse when working with the code. We use the m2eclipse eclipse plugin for maven support. Other IDEs and tools should also work without issue as long as they use Maven 3.3.3 or better.

94.3.1. Activate the Spring Maven profile

Spring Cloud projects require the 'spring' Maven profile to be activated to resolve the spring milestone and snapshot repositories. Use your preferred IDE to set this profile to be active, or you may experience build errors.

94.3.2. Importing into eclipse with m2eclipse

We recommend the m2eclipse eclipse plugin when working with eclipse. If you don’t already have m2eclipse installed it is available from the "eclipse marketplace".

Older versions of m2e do not support Maven 3.3, so once the projects are imported into Eclipse you will also need to tell m2eclipse to use the right profile for the projects. If you see many different errors related to the POMs in the projects, check that you have an up to date installation. If you can’t upgrade m2e, add the "spring" profile to your settings.xml. Alternatively you can copy the repository settings from the "spring" profile of the parent pom into your settings.xml.

94.3.3. Importing into eclipse without m2eclipse

If you prefer not to use m2eclipse you can generate eclipse project metadata using the following command:

$ ./mvnw eclipse:eclipse

The generated eclipse projects can be imported by selecting import existing projects from the file menu.

95. Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.

95.1. Sign the Contributor License Agreement

Before we accept a non-trivial patch or pull request we will need you to sign the Contributor License Agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.

95.2. Code of Conduct

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to [email protected].

95.3. Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.

  • Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the eclipse-code-formatter.xml file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file.

  • Make sure all new .java files to have a simple Javadoc class comment with at least an @author tag identifying you, and preferably at least a paragraph on what the class is for.

  • Add the ASF license header comment to all new .java files (copy from existing files in the project)

  • Add yourself as an @author to the .java files that you modify substantially (more than cosmetic changes).

  • Add some Javadocs and, if you change the namespace, some XSD doc elements.

  • A few unit tests would help a lot as well — someone has to do it.

  • If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).

  • When writing a commit message please follow these conventions, if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).

95.4. Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in the spring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            └── checkstyle.xml (1)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules

95.4.1. Checkstyle configuration

Checkstyle rules are disabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml
<properties>
<maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError> (1)
        <maven-checkstyle-plugin.failsOnViolation>true
        </maven-checkstyle-plugin.failsOnViolation> (2)
        <maven-checkstyle-plugin.includeTestSourceDirectory>true
        </maven-checkstyle-plugin.includeTestSourceDirectory> (3)
</properties>

<build>
        <plugins>
            <plugin> (4)
                <groupId>io.spring.javaformat</groupId>
                <artifactId>spring-javaformat-maven-plugin</artifactId>
            </plugin>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>

    <reporting>
        <plugins>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>
    </reporting>
</build>
1 Fails the build upon Checkstyle errors
2 Fails the build upon Checkstyle violations
3 Checkstyle analyzes also the test sources
4 Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules
5 Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under ${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml
<?xml version="1.0"?>
<!DOCTYPE suppressions PUBLIC
        "-//Puppy Crawl//DTD Suppressions 1.1//EN"
        "https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
<suppressions>
    <suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/>
    <suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/>
</suppressions>

It’s advisable to copy the ${spring-cloud-build.rootFolder}/.editorconfig and ${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig
$ touch .springformat

95.5. IDE setup

95.5.1. Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin. The following files can be found in the Spring Cloud Build project.

spring-cloud-build-tools/
└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            ├── checkstyle.xml (1)
            └── intellij
                ├── Intellij_Project_Defaults.xml (4)
                └── Intellij_Spring_Boot_Java_Conventions.xml (5)
1 Default Checkstyle rules
2 File header setup
3 Default suppression rules
4 Project defaults for Intellij that apply most of Checkstyle rules
5 Project style conventions for Intellij that apply most of Checkstyle rules
Code style
Figure 5. Code style

Go to FileSettingsEditorCode style. There click on the icon next to the Scheme section. There, click on the Import Scheme value and pick the Intellij IDEA code style XML option. Import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Code style
Figure 6. Inspection profiles

Go to FileSettingsEditorInspections. There click on the icon next to the Profile section. There, click on the Import Profile and import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install the Checkstyle plugin. It’s advisable to also install the Assertions2Assertj to automatically convert the JUnit assertions

Checkstyle

Go to FileSettingsOther settingsCheckstyle. There click on the + icon in the Configuration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for the checkstyle.xml : raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

Remember to set the Scan Scope to All sources since we apply checkstyle rules for production and test sources.

Spring Cloud Netflix

2020.0.0-SNAPSHOT

This project provides Netflix OSS integrations for Spring Boot apps through autoconfiguration and binding to the Spring Environment and other Spring programming model idioms. With a few simple annotations you can quickly enable and configure the common patterns inside your application and build large distributed systems with battle-tested Netflix components. The patterns provided include Service Discovery (Eureka), Circuit Breaker (Hystrix), Intelligent Routing (Zuul) and Client Side Load Balancing (Ribbon).

96. Service Discovery: Eureka Clients

Service Discovery is one of the key tenets of a microservice-based architecture. Trying to hand-configure each client or some form of convention can be difficult to do and can be brittle. Eureka is the Netflix Service Discovery Server and Client. The server can be configured and deployed to be highly available, with each server replicating state about the registered services to the others.

96.1. How to Include Eureka Client

To include the Eureka Client in your project, use the starter with a group ID of org.springframework.cloud and an artifact ID of spring-cloud-starter-netflix-eureka-client. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

96.2. Registering with Eureka

When a client registers with Eureka, it provides meta-data about itself — such as host, port, health indicator URL, home page, and other details. Eureka receives heartbeat messages from each instance belonging to a service. If the heartbeat fails over a configurable timetable, the instance is normally removed from the registry.

The following example shows a minimal Eureka client application:

@SpringBootApplication
@RestController
public class Application {

    @RequestMapping("/")
    public String home() {
        return "Hello world";
    }

    public static void main(String[] args) {
        new SpringApplicationBuilder(Application.class).web(true).run(args);
    }

}

Note that the preceding example shows a normal Spring Boot application. By having spring-cloud-starter-netflix-eureka-client on the classpath, your application automatically registers with the Eureka Server. Configuration is required to locate the Eureka server, as shown in the following example:

application.yml
eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/

In the preceding example, defaultZone is a magic string fallback value that provides the service URL for any client that does not express a preference (in other words, it is a useful default).

The defaultZone property is case sensitive and requires camel case because the serviceUrl property is a Map<String, String>. Therefore, the defaultZone property does not follow the normal Spring Boot snake-case convention of default-zone.

The default application name (that is, the service ID), virtual host, and non-secure port (taken from the Environment) are ${spring.application.name}, ${spring.application.name} and ${server.port}, respectively.

Having spring-cloud-starter-netflix-eureka-client on the classpath makes the app into both a Eureka “instance” (that is, it registers itself) and a “client” (it can query the registry to locate other services). The instance behaviour is driven by eureka.instance.* configuration keys, but the defaults are fine if you ensure that your application has a value for spring.application.name (this is the default for the Eureka service ID or VIP).

See EurekaInstanceConfigBean and EurekaClientConfigBean for more details on the configurable options.

To disable the Eureka Discovery Client, you can set eureka.client.enabled to false. Eureka Discovery Client will also be disabled when spring.cloud.discovery.enabled is set to false.

96.3. Authenticating with the Eureka Server

HTTP basic authentication is automatically added to your eureka client if one of the eureka.client.serviceUrl.defaultZone URLs has credentials embedded in it (curl style, as follows: user:[email protected]:8761/eureka). For more complex needs, you can create a @Bean of type DiscoveryClientOptionalArgs and inject ClientFilter instances into it, all of which is applied to the calls from the client to the server.

When Eureka server requires client side certificate for authentication, the client side certificate and trust store can be configured via properties, as shown in following example:

application.yml
eureka:
  client:
    tls:
      enabled: true
      key-store: <path-of-key-store>
      key-store-type: PKCS12
      key-store-password: <key-store-password>
      key-password: <key-password>
      trust-store: <path-of-trust-store>
      trust-store-type: PKCS12
      trust-store-password: <trust-store-password>

The eureka.client.tls.enabled needs to be true to enable Eureka client side TLS. When eureka.client.tls.trust-store is omitted, a JVM default trust store is used. The default value for eureka.client.tls.key-store-type and eureka.client.tls.trust-store-type is PKCS12. When password properties are omitted, empty password is assumed.

Because of a limitation in Eureka, it is not possible to support per-server basic auth credentials, so only the first set that are found is used.

If you want to customize the RestTemplate used by the Eureka HTTP Client you may want to create a bean of EurekaClientHttpRequestFactorySupplier and provide your own logic for generating a ClientHttpRequestFactory instance.

96.4. Status Page and Health Indicator

The status page and health indicators for a Eureka instance default to /info and /health respectively, which are the default locations of useful endpoints in a Spring Boot Actuator application. You need to change these, even for an Actuator application if you use a non-default context path or servlet path (such as server.servletPath=/custom). The following example shows the default values for the two settings:

application.yml
eureka:
  instance:
    statusPageUrlPath: ${server.servletPath}/info
    healthCheckUrlPath: ${server.servletPath}/health

These links show up in the metadata that is consumed by clients and are used in some scenarios to decide whether to send requests to your application, so it is helpful if they are accurate.

In Dalston it was also required to set the status and health check URLs when changing that management context path. This requirement was removed beginning in Edgware.

96.5. Registering a Secure Application

If your app wants to be contacted over HTTPS, you can set two flags in the EurekaInstanceConfigBean:

  • eureka.instance.[nonSecurePortEnabled]=[false]

  • eureka.instance.[securePortEnabled]=[true]

Doing so makes Eureka publish instance information that shows an explicit preference for secure communication. The Spring Cloud DiscoveryClient always returns a URI starting with https for a service configured this way. Similarly, when a service is configured this way, the Eureka (native) instance information has a secure health check URL.

Because of the way Eureka works internally, it still publishes a non-secure URL for the status and home pages unless you also override those explicitly. You can use placeholders to configure the eureka instance URLs, as shown in the following example:

application.yml
eureka:
  instance:
    statusPageUrl: https://${eureka.hostname}/info
    healthCheckUrl: https://${eureka.hostname}/health
    homePageUrl: https://${eureka.hostname}/

(Note that ${eureka.hostname} is a native placeholder only available in later versions of Eureka. You could achieve the same thing with Spring placeholders as well — for example, by using ${eureka.instance.hostName}.)

If your application runs behind a proxy, and the SSL termination is in the proxy (for example, if you run in Cloud Foundry or other platforms as a service), then you need to ensure that the proxy “forwarded” headers are intercepted and handled by the application. If the Tomcat container embedded in a Spring Boot application has explicit configuration for the 'X-Forwarded-\*` headers, this happens automatically. The links rendered by your app to itself being wrong (the wrong host, port, or protocol) is a sign that you got this configuration wrong.

96.6. Eureka’s Health Checks

By default, Eureka uses the client heartbeat to determine if a client is up. Unless specified otherwise, the Discovery Client does not propagate the current health check status of the application, per the Spring Boot Actuator. Consequently, after successful registration, Eureka always announces that the application is in 'UP' state. This behavior can be altered by enabling Eureka health checks, which results in propagating application status to Eureka. As a consequence, every other application does not send traffic to applications in states other then 'UP'. The following example shows how to enable health checks for the client:

application.yml
eureka:
  client:
    healthcheck:
      enabled: true
eureka.client.healthcheck.enabled=true should only be set in application.yml. Setting the value in bootstrap.yml causes undesirable side effects, such as registering in Eureka with an UNKNOWN status.

If you require more control over the health checks, consider implementing your own com.netflix.appinfo.HealthCheckHandler.

96.7. Eureka Metadata for Instances and Clients

It is worth spending a bit of time understanding how the Eureka metadata works, so you can use it in a way that makes sense in your platform. There is standard metadata for information such as hostname, IP address, port numbers, the status page, and health check. These are published in the service registry and used by clients to contact the services in a straightforward way. Additional metadata can be added to the instance registration in the eureka.instance.metadataMap, and this metadata is accessible in the remote clients. In general, additional metadata does not change the behavior of the client, unless the client is made aware of the meaning of the metadata. There are a couple of special cases, described later in this document, where Spring Cloud already assigns meaning to the metadata map.

96.7.1. Using Eureka on Cloud Foundry

Cloud Foundry has a global router so that all instances of the same app have the same hostname (other PaaS solutions with a similar architecture have the same arrangement). This is not necessarily a barrier to using Eureka. However, if you use the router (recommended or even mandatory, depending on the way your platform was set up), you need to explicitly set the hostname and port numbers (secure or non-secure) so that they use the router. You might also want to use instance metadata so that you can distinguish between the instances on the client (for example, in a custom load balancer). By default, the eureka.instance.instanceId is vcap.application.instance_id, as shown in the following example:

application.yml
eureka:
  instance:
    hostname: ${vcap.application.uris[0]}
    nonSecurePort: 80

Depending on the way the security rules are set up in your Cloud Foundry instance, you might be able to register and use the IP address of the host VM for direct service-to-service calls. This feature is not yet available on Pivotal Web Services (PWS).

96.7.2. Using Eureka on AWS

If the application is planned to be deployed to an AWS cloud, the Eureka instance must be configured to be AWS-aware. You can do so by customizing the EurekaInstanceConfigBean as follows:

@Bean
@Profile("!default")
public EurekaInstanceConfigBean eurekaInstanceConfig(InetUtils inetUtils) {
  EurekaInstanceConfigBean bean = new EurekaInstanceConfigBean(inetUtils);
  AmazonInfo info = AmazonInfo.Builder.newBuilder().autoBuild("eureka");
  bean.setDataCenterInfo(info);
  return bean;
}

96.7.3. Changing the Eureka Instance ID

A vanilla Netflix Eureka instance is registered with an ID that is equal to its host name (that is, there is only one service per host). Spring Cloud Eureka provides a sensible default, which is defined as follows:

${spring.cloud.client.hostname}:${spring.application.name}:${spring.application.instance_id:${server.port}}}

An example is myhost:myappname:8080.

By using Spring Cloud, you can override this value by providing a unique identifier in eureka.instance.instanceId, as shown in the following example:

application.yml
eureka:
  instance:
    instanceId: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}

With the metadata shown in the preceding example and multiple service instances deployed on localhost, the random value is inserted there to make the instance unique. In Cloud Foundry, the vcap.application.instance_id is populated automatically in a Spring Boot application, so the random value is not needed.

96.8. Using the EurekaClient

Once you have an application that is a discovery client, you can use it to discover service instances from the Eureka Server. One way to do so is to use the native com.netflix.discovery.EurekaClient (as opposed to the Spring Cloud DiscoveryClient), as shown in the following example:

@Autowired
private EurekaClient discoveryClient;

public String serviceUrl() {
    InstanceInfo instance = discoveryClient.getNextServerFromEureka("STORES", false);
    return instance.getHomePageUrl();
}

Do not use the EurekaClient in a @PostConstruct method or in a @Scheduled method (or anywhere where the ApplicationContext might not be started yet). It is initialized in a SmartLifecycle (with phase=0), so the earliest you can rely on it being available is in another SmartLifecycle with a higher phase.

96.8.1. EurekaClient without Jersey

By default, EurekaClient uses Jersey for HTTP communication. If you wish to avoid dependencies from Jersey, you can exclude it from your dependencies. Spring Cloud auto-configures a transport client based on Spring RestTemplate. The following example shows Jersey being excluded:

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    <exclusions>
        <exclusion>
            <groupId>com.sun.jersey</groupId>
            <artifactId>jersey-client</artifactId>
        </exclusion>
        <exclusion>
            <groupId>com.sun.jersey</groupId>
            <artifactId>jersey-core</artifactId>
        </exclusion>
        <exclusion>
            <groupId>com.sun.jersey.contribs</groupId>
            <artifactId>jersey-apache-client4</artifactId>
        </exclusion>
    </exclusions>
</dependency>

96.9. Alternatives to the Native Netflix EurekaClient

You need not use the raw Netflix EurekaClient. Also, it is usually more convenient to use it behind a wrapper of some sort. Spring Cloud has support for Feign (a REST client builder) and Spring RestTemplate through the logical Eureka service identifiers (VIPs) instead of physical URLs. To configure Ribbon with a fixed list of physical servers, you can set <client>.ribbon.listOfServers to a comma-separated list of physical addresses (or hostnames), where <client> is the ID of the client.

You can also use the org.springframework.cloud.client.discovery.DiscoveryClient, which provides a simple API (not specific to Netflix) for discovery clients, as shown in the following example:

@Autowired
private DiscoveryClient discoveryClient;

public String serviceUrl() {
    List<ServiceInstance> list = discoveryClient.getInstances("STORES");
    if (list != null && list.size() > 0 ) {
        return list.get(0).getUri();
    }
    return null;
}

96.10. Why Is It so Slow to Register a Service?

Being an instance also involves a periodic heartbeat to the registry (through the client’s serviceUrl) with a default duration of 30 seconds. A service is not available for discovery by clients until the instance, the server, and the client all have the same metadata in their local cache (so it could take 3 heartbeats). You can change the period by setting eureka.instance.leaseRenewalIntervalInSeconds. Setting it to a value of less than 30 speeds up the process of getting clients connected to other services. In production, it is probably better to stick with the default, because of internal computations in the server that make assumptions about the lease renewal period.

96.11. Zones

If you have deployed Eureka clients to multiple zones, you may prefer that those clients use services within the same zone before trying services in another zone. To set that up, you need to configure your Eureka clients correctly.

First, you need to make sure you have Eureka servers deployed to each zone and that they are peers of each other. See the section on zones and regions for more information.

Next, you need to tell Eureka which zone your service is in. You can do so by using the metadataMap property. For example, if service 1 is deployed to both zone 1 and zone 2, you need to set the following Eureka properties in service 1:

Service 1 in Zone 1

eureka.instance.metadataMap.zone = zone1
eureka.client.preferSameZoneEureka = true

Service 1 in Zone 2

eureka.instance.metadataMap.zone = zone2
eureka.client.preferSameZoneEureka = true

96.12. Refreshing Eureka Clients

By default, the EurekaClient bean is refreshable, meaning the Eureka client properties can be changed and refreshed. When a refresh occurs clients will be unregistered from the Eureka server and there might be a brief moment of time where all instance of a given service are not available. One way to eliminate this from happening is to disable the ability to refresh Eureka clients. To do this set eureka.client.refresh.enable=false.

96.13. Using Eureka with Spring Cloud LoadBalancer

We offer support for the Spring Cloud LoadBalancer ZonePreferenceServiceInstanceListSupplier. The zone value from the Eureka instance metadata (eureka.instance.metadataMap.zone) is used for setting the value of spring-clod-loadbalancer-zone property that is used to filter service instances by zone.

If that is missing and if the spring.cloud.loadbalancer.eureka.approximateZoneFromHostname flag is set to true, it can use the domain name from the server hostname as a proxy for the zone.

If there is no other source of zone data, then a guess is made, based on the client configuration (as opposed to the instance configuration). We take eureka.client.availabilityZones, which is a map from region name to a list of zones, and pull out the first zone for the instance’s own region (that is, the eureka.client.region, which defaults to "us-east-1", for compatibility with native Netflix).

97. Service Discovery: Eureka Server

This section describes how to set up a Eureka server.

97.1. How to Include Eureka Server

To include Eureka Server in your project, use the starter with a group ID of org.springframework.cloud and an artifact ID of spring-cloud-starter-netflix-eureka-server. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

If your project already uses Thymeleaf as its template engine, the Freemarker templates of the Eureka server may not be loaded correctly. In this case it is necessary to configure the template loader manually:
application.yml
spring:
  freemarker:
    template-loader-path: classpath:/templates/
    prefer-file-system-access: false

97.2. How to Run a Eureka Server

The following example shows a minimal Eureka server:

@SpringBootApplication
@EnableEurekaServer
public class Application {

    public static void main(String[] args) {
        new SpringApplicationBuilder(Application.class).web(true).run(args);
    }

}

The server has a home page with a UI and HTTP API endpoints for the normal Eureka functionality under /eureka/*.

The following links have some Eureka background reading: flux capacitor and google group discussion.

Due to Gradle’s dependency resolution rules and the lack of a parent bom feature, depending on spring-cloud-starter-netflix-eureka-server can cause failures on application startup. To remedy this issue, add the Spring Boot Gradle plugin and import the Spring cloud starter parent bom as follows:

build.gradle
buildscript {
  dependencies {
    classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-docs-version}")
  }
}

apply plugin: "spring-boot"

dependencyManagement {
  imports {
    mavenBom "org.springframework.cloud:spring-cloud-dependencies:{spring-cloud-version}"
  }
}

97.3. High Availability, Zones and Regions

The Eureka server does not have a back end store, but the service instances in the registry all have to send heartbeats to keep their registrations up to date (so this can be done in memory). Clients also have an in-memory cache of Eureka registrations (so they do not have to go to the registry for every request to a service).

By default, every Eureka server is also a Eureka client and requires (at least one) service URL to locate a peer. If you do not provide it, the service runs and works, but it fills your logs with a lot of noise about not being able to register with the peer.

See also below for details of Ribbon support on the client side for Zones and Regions.

97.4. Standalone Mode

The combination of the two caches (client and server) and the heartbeats make a standalone Eureka server fairly resilient to failure, as long as there is some sort of monitor or elastic runtime (such as Cloud Foundry) keeping it alive. In standalone mode, you might prefer to switch off the client side behavior so that it does not keep trying and failing to reach its peers. The following example shows how to switch off the client-side behavior:

application.yml (Standalone Eureka Server)
server:
  port: 8761

eureka:
  instance:
    hostname: localhost
  client:
    registerWithEureka: false
    fetchRegistry: false
    serviceUrl:
      defaultZone: http://${eureka.instance.hostname}:${server.port}/eureka/

Notice that the serviceUrl is pointing to the same host as the local instance.

97.5. Peer Awareness

Eureka can be made even more resilient and available by running multiple instances and asking them to register with each other. In fact, this is the default behavior, so all you need to do to make it work is add a valid serviceUrl to a peer, as shown in the following example:

application.yml (Two Peer Aware Eureka Servers)
---
spring:
  profiles: peer1
eureka:
  instance:
    hostname: peer1
  client:
    serviceUrl:
      defaultZone: https://peer2/eureka/

---
spring:
  profiles: peer2
eureka:
  instance:
    hostname: peer2
  client:
    serviceUrl:
      defaultZone: https://peer1/eureka/

In the preceding example, we have a YAML file that can be used to run the same server on two hosts (peer1 and peer2) by running it in different Spring profiles. You could use this configuration to test the peer awareness on a single host (there is not much value in doing that in production) by manipulating /etc/hosts to resolve the host names. In fact, the eureka.instance.hostname is not needed if you are running on a machine that knows its own hostname (by default, it is looked up by using java.net.InetAddress).

You can add multiple peers to a system, and, as long as they are all connected to each other by at least one edge, they synchronize the registrations amongst themselves. If the peers are physically separated (inside a data center or between multiple data centers), then the system can, in principle, survive “split-brain” type failures. You can add multiple peers to a system, and as long as they are all directly connected to each other, they will synchronize the registrations amongst themselves.

application.yml (Three Peer Aware Eureka Servers)
eureka:
  client:
    serviceUrl:
      defaultZone: https://peer1/eureka/,http://peer2/eureka/,http://peer3/eureka/

---
spring:
  profiles: peer1
eureka:
  instance:
    hostname: peer1

---
spring:
  profiles: peer2
eureka:
  instance:
    hostname: peer2

---
spring:
  profiles: peer3
eureka:
  instance:
    hostname: peer3

97.6. When to Prefer IP Address

In some cases, it is preferable for Eureka to advertise the IP addresses of services rather than the hostname. Set eureka.instance.preferIpAddress to true and, when the application registers with eureka, it uses its IP address rather than its hostname.

If the hostname cannot be determined by Java, then the IP address is sent to Eureka. Only explict way of setting the hostname is by setting eureka.instance.hostname property. You can set your hostname at the run-time by using an environment variable — for example, eureka.instance.hostname=${HOST_NAME}.

97.7. Securing The Eureka Server

You can secure your Eureka server simply by adding Spring Security to your server’s classpath via spring-boot-starter-security. By default when Spring Security is on the classpath it will require that a valid CSRF token be sent with every request to the app. Eureka clients will not generally possess a valid cross site request forgery (CSRF) token you will need to disable this requirement for the /eureka/** endpoints. For example:

@EnableWebSecurity
class WebSecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.csrf().ignoringAntMatchers("/eureka/**");
        super.configure(http);
    }
}

For more information on CSRF see the Spring Security documentation.

A demo Eureka Server can be found in the Spring Cloud Samples repo.

97.8. JDK 11 Support

The JAXB modules which the Eureka server depends upon were removed in JDK 11. If you intend to use JDK 11 when running a Eureka server you must include these dependencies in your POM or Gradle file.

<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
</dependency>

98. Configuration properties

To see the list of all Spring Cloud Netflix related configuration properties please check the Appendix page.

Spring Cloud OpenFeign

2020.0.0-SNAPSHOT

This project provides OpenFeign integrations for Spring Boot apps through autoconfiguration and binding to the Spring Environment and other Spring programming model idioms.

99. Declarative REST Client: Feign

Feign is a declarative web service client. It makes writing web service clients easier. To use Feign create an interface and annotate it. It has pluggable annotation support including Feign annotations and JAX-RS annotations. Feign also supports pluggable encoders and decoders. Spring Cloud adds support for Spring MVC annotations and for using the same HttpMessageConverters used by default in Spring Web. Spring Cloud integrates Eureka, as well as Spring Cloud LoadBalancer to provide a load-balanced http client when using Feign.

99.1. How to Include Feign

To include Feign in your project use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-openfeign. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

Example spring boot app

@SpringBootApplication
@EnableFeignClients
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}
StoreClient.java
@FeignClient("stores")
public interface StoreClient {
    @RequestMapping(method = RequestMethod.GET, value = "/stores")
    List<Store> getStores();

    @RequestMapping(method = RequestMethod.GET, value = "/stores")
    Page<Store> getStores(Pageable pageable);

    @RequestMapping(method = RequestMethod.POST, value = "/stores/{storeId}", consumes = "application/json")
    Store update(@PathVariable("storeId") Long storeId, Store store);
}

In the @FeignClient annotation the String value ("stores" above) is an arbitrary client name, which is used to create a Spring Cloud LoadBalancer client. You can also specify a URL using the url attribute (absolute value or just a hostname). The name of the bean in the application context is the fully qualified name of the interface. To specify your own alias value you can use the qualifier value of the @FeignClient annotation.

The load-balancer client above will want to discover the physical addresses for the "stores" service. If your application is a Eureka client then it will resolve the service in the Eureka service registry. If you don’t want to use Eureka, you can simply configure a list of servers in your external configuration using SimpleDiscoveryClient.

Spring Cloud OpenFeign supports all the features available for the blocking mode of Spring Cloud LoadBalancer. You can read more about them in the project documentation.

99.2. Overriding Feign Defaults

A central concept in Spring Cloud’s Feign support is that of the named client. Each feign client is part of an ensemble of components that work together to contact a remote server on demand, and the ensemble has a name that you give it as an application developer using the @FeignClient annotation. Spring Cloud creates a new ensemble as an ApplicationContext on demand for each named client using FeignClientsConfiguration. This contains (amongst other things) an feign.Decoder, a feign.Encoder, and a feign.Contract. It is possible to override the name of that ensemble by using the contextId attribute of the @FeignClient annotation.

Spring Cloud lets you take full control of the feign client by declaring additional configuration (on top of the FeignClientsConfiguration) using @FeignClient. Example:

@FeignClient(name = "stores", configuration = FooConfiguration.class)
public interface StoreClient {
    //..
}

In this case the client is composed from the components already in FeignClientsConfiguration together with any in FooConfiguration (where the latter will override the former).

FooConfiguration does not need to be annotated with @Configuration. However, if it is, then take care to exclude it from any @ComponentScan that would otherwise include this configuration as it will become the default source for feign.Decoder, feign.Encoder, feign.Contract, etc., when specified. This can be avoided by putting it in a separate, non-overlapping package from any @ComponentScan or @SpringBootApplication, or it can be explicitly excluded in @ComponentScan.
Using contextId attribute of the @FeignClient annotation in addition to changing the name of the ApplicationContext ensemble, it will override the alias of the client name and it will be used as part of the name of the configuration bean created for that client.
Previously, using the url attribute, did not require the name attribute. Using name is now required.

Placeholders are supported in the name and url attributes.

@FeignClient(name = "${feign.name}", url = "${feign.url}")
public interface StoreClient {
    //..
}

Spring Cloud OpenFeign provides the following beans by default for feign (BeanType beanName: ClassName):

  • Decoder feignDecoder: ResponseEntityDecoder (which wraps a SpringDecoder)

  • Encoder feignEncoder: SpringEncoder

  • Logger feignLogger: Slf4jLogger

  • Contract feignContract: SpringMvcContract

  • Feign.Builder feignBuilder: FeignCircuitBreaker.Builder

  • Client feignClient: if Spring Cloud LoadBalancer is in the classpath, FeignBlockingLoadBalancerClient is used. If none of them is in the classpath, the default feign client is used.

spring-cloud-starter-openfeign supports spring-cloud-starter-loadbalancer. However, as is an optional dependency, you need to make sure it been added to your project if you want to use it.

The OkHttpClient and ApacheHttpClient feign clients can be used by setting feign.okhttp.enabled or feign.httpclient.enabled to true, respectively, and having them on the classpath. You can customize the HTTP client used by providing a bean of either org.apache.http.impl.client.CloseableHttpClient when using Apache or okhttp3.OkHttpClient when using OK HTTP.

Spring Cloud OpenFeign does not provide the following beans by default for feign, but still looks up beans of these types from the application context to create the feign client:

  • Logger.Level

  • Retryer

  • ErrorDecoder

  • Request.Options

  • Collection<RequestInterceptor>

  • SetterFactory

  • QueryMapEncoder

A bean of Retryer.NEVER_RETRY with the type Retryer is created by default, which will disable retrying. Notice this retrying behavior is different from the Feign default one, where it will automatically retry IOExceptions, treating them as transient network related exceptions, and any RetryableException thrown from an ErrorDecoder.

Creating a bean of one of those type and placing it in a @FeignClient configuration (such as FooConfiguration above) allows you to override each one of the beans described. Example:

@Configuration
public class FooConfiguration {
    @Bean
    public Contract feignContract() {
        return new feign.Contract.Default();
    }

    @Bean
    public BasicAuthRequestInterceptor basicAuthRequestInterceptor() {
        return new BasicAuthRequestInterceptor("user", "password");
    }
}

This replaces the SpringMvcContract with feign.Contract.Default and adds a RequestInterceptor to the collection of RequestInterceptor.

@FeignClient also can be configured using configuration properties.

application.yml

feign:
  client:
    config:
      feignName:
        connectTimeout: 5000
        readTimeout: 5000
        loggerLevel: full
        errorDecoder: com.example.SimpleErrorDecoder
        retryer: com.example.SimpleRetryer
       defaultQueryParameters:
          query: queryValue
        defaultRequestHeaders:
          header: headerValue
        requestInterceptors:
          - com.example.FooRequestInterceptor
          - com.example.BarRequestInterceptor
        decode404: false
        encoder: com.example.SimpleEncoder
        decoder: com.example.SimpleDecoder
        contract: com.example.SimpleContract

Default configurations can be specified in the @EnableFeignClients attribute defaultConfiguration in a similar manner as described above. The difference is that this configuration will apply to all feign clients.

If you prefer using configuration properties to configured all @FeignClient, you can create configuration properties with default feign name.

You can use feign.client.config.feignName.defaultQueryParameters and feign.client.config.feignName.defaultRequestHeaders to specify query parameters and headers that will be sent with every request of the client named feignName.

application.yml

feign:
  client:
    config:
      default:
        connectTimeout: 5000
        readTimeout: 5000
        loggerLevel: basic

If we create both @Configuration bean and configuration properties, configuration properties will win. It will override @Configuration values. But if you want to change the priority to @Configuration, you can change feign.client.default-to-properties to false.

If we want to create multiple feign clients with the same name or url so that they would point to the same server but each with a different custom configuration then we have to use contextId attribute of the @FeignClient in order to avoid name collision of these configuration beans.

@FeignClient(contextId = "fooClient", name = "stores", configuration = FooConfiguration.class)
public interface FooClient {
    //..
}
@FeignClient(contextId = "barClient", name = "stores", configuration = BarConfiguration.class)
public interface BarClient {
    //..
}

It is also possible to configure FeignClient not to inherit beans from the parent context. You can do this by overriding the inheritParentConfiguration() in a FeignClientConfigurer bean to return false:

@Configuration
public class CustomConfiguration{

@Bean
public FeignClientConfigurer feignClientConfigurer() {
            return new FeignClientConfigurer() {

                @Override
                public boolean inheritParentConfiguration() {
                    return false;
                }
            };

        }
}
By default, Feign clients do not encode slash / characters. You can change this behaviour, by setting the value of feign.client.decodeSlash to false.

99.3. Timeout Handling

We can configure timeouts on both the default and the named client. OpenFeign works with two timeout parameters:

  • connectTimeout prevents blocking the caller due to the long server processing time.

  • readTimeout is applied from the time of connection establishment and is triggered when returning the response takes too long.

In case the server is not running or available a packet results in connection refused. The communication ends either with an error message or in a fallback. This can happen before the connectTimeout if it is set very low. The time taken to perform a lookup and to receive such a packet causes a significant part of this delay. It is subject to change based on the remote host that involves a DNS lookup.

99.4. Creating Feign Clients Manually

In some cases it might be necessary to customize your Feign Clients in a way that is not possible using the methods above. In this case you can create Clients using the Feign Builder API. Below is an example which creates two Feign Clients with the same interface but configures each one with a separate request interceptor.

@Import(FeignClientsConfiguration.class)
class FooController {

    private FooClient fooClient;

    private FooClient adminClient;

        @Autowired
    public FooController(Decoder decoder, Encoder encoder, Client client, Contract contract) {
        this.fooClient = Feign.builder().client(client)
                .encoder(encoder)
                .decoder(decoder)
                .contract(contract)
                .requestInterceptor(new BasicAuthRequestInterceptor("user", "user"))
                .target(FooClient.class, "https://PROD-SVC");

        this.adminClient = Feign.builder().client(client)
                .encoder(encoder)
                .decoder(decoder)
                .contract(contract)
                .requestInterceptor(new BasicAuthRequestInterceptor("admin", "admin"))
                .target(FooClient.class, "https://PROD-SVC");
    }
}
In the above example FeignClientsConfiguration.class is the default configuration provided by Spring Cloud OpenFeign.
PROD-SVC is the name of the service the Clients will be making requests to.
The Feign Contract object defines what annotations and values are valid on interfaces. The autowired Contract bean provides supports for SpringMVC annotations, instead of the default Feign native annotations.

You can also use the Builder`to configure FeignClient not to inherit beans from the parent context. You can do this by overriding calling `inheritParentContext(false) on the Builder.

99.5. Feign Spring Cloud CircuitBreaker Support

If Spring Cloud CircuitBreaker is on the classpath and feign.circuitbreaker.enabled=true, Feign will wrap all methods with a circuit breaker.

To disable Spring Cloud CircuitBreaker support on a per-client basis create a vanilla Feign.Builder with the "prototype" scope, e.g.:

@Configuration
public class FooConfiguration {
        @Bean
    @Scope("prototype")
    public Feign.Builder feignBuilder() {
        return Feign.builder();
    }
}

The circuit breaker name follows this pattern <feignClientName>_<calledMethod>. When calling a @FeignClient with name foo and the called interface method is bar then the circuit breaker name will be foo_bar.

99.6. Feign Spring Cloud CircuitBreaker Fallbacks

Spring Cloud CircuitBreaker supports the notion of a fallback: a default code path that is executed when they circuit is open or there is an error. To enable fallbacks for a given @FeignClient set the fallback attribute to the class name that implements the fallback. You also need to declare your implementation as a Spring bean.

@FeignClient(name = "test", url = "http://localhost:${server.port}/", fallback = Fallback.class)
protected interface TestClient {

    @RequestMapping(method = RequestMethod.GET, value = "/hello")
    Hello getHello();

    @RequestMapping(method = RequestMethod.GET, value = "/hellonotfound")
    String getException();

}

@Component
static class Fallback implements TestClient {

    @Override
    public Hello getHello() {
        throw new NoFallbackAvailableException("Boom!", new RuntimeException());
    }

    @Override
    public String getException() {
        return "Fixed response";
    }

}

If one needs access to the cause that made the fallback trigger, one can use the fallbackFactory attribute inside @FeignClient.

@FeignClient(name = "testClientWithFactory", url = "http://localhost:${server.port}/",
        fallbackFactory = TestFallbackFactory.class)
protected interface TestClientWithFactory {

    @RequestMapping(method = RequestMethod.GET, value = "/hello")
    Hello getHello();

    @RequestMapping(method = RequestMethod.GET, value = "/hellonotfound")
    String getException();

}

@Component
static class TestFallbackFactory implements FallbackFactory<FallbackWithFactory> {

    @Override
    public FallbackWithFactory create(Throwable cause) {
        return new FallbackWithFactory();
    }

}

static class FallbackWithFactory implements TestClientWithFactory {

    @Override
    public Hello getHello() {
        throw new NoFallbackAvailableException("Boom!", new RuntimeException());
    }

    @Override
    public String getException() {
        return "Fixed response";
    }

}

99.7. Feign and @Primary

When using Feign with Spring Cloud CircuitBreaker fallbacks, there are multiple beans in the ApplicationContext of the same type. This will cause @Autowired to not work because there isn’t exactly one bean, or one marked as primary. To work around this, Spring Cloud OpenFeign marks all Feign instances as @Primary, so Spring Framework will know which bean to inject. In some cases, this may not be desirable. To turn off this behavior set the primary attribute of @FeignClient to false.

@FeignClient(name = "hello", primary = false)
public interface HelloClient {
    // methods here
}

99.8. Feign Inheritance Support

Feign supports boilerplate apis via single-inheritance interfaces. This allows grouping common operations into convenient base interfaces.

UserService.java
public interface UserService {

    @RequestMapping(method = RequestMethod.GET, value ="/users/{id}")
    User getUser(@PathVariable("id") long id);
}
UserResource.java
@RestController
public class UserResource implements UserService {

}
UserClient.java
package project.user;

@FeignClient("users")
public interface UserClient extends UserService {

}
It is generally not advisable to share an interface between a server and a client. It introduces tight coupling, and also actually doesn’t work with Spring MVC in its current form (method parameter mapping is not inherited).

99.9. Feign request/response compression

You may consider enabling the request or response GZIP compression for your Feign requests. You can do this by enabling one of the properties:

feign.compression.request.enabled=true
feign.compression.response.enabled=true

Feign request compression gives you settings similar to what you may set for your web server:

feign.compression.request.enabled=true
feign.compression.request.mime-types=text/xml,application/xml,application/json
feign.compression.request.min-request-size=2048

These properties allow you to be selective about the compressed media types and minimum request threshold length.

For http clients except OkHttpClient, default gzip decoder can be enabled to decode gzip response in UTF-8 encoding:

feign.compression.response.enabled=true
feign.compression.response.useGzipDecoder=true

99.10. Feign logging

A logger is created for each Feign client created. By default the name of the logger is the full class name of the interface used to create the Feign client. Feign logging only responds to the DEBUG level.

application.yml
logging.level.project.user.UserClient: DEBUG

The Logger.Level object that you may configure per client, tells Feign how much to log. Choices are:

  • NONE, No logging (DEFAULT).

  • BASIC, Log only the request method and URL and the response status code and execution time.

  • HEADERS, Log the basic information along with request and response headers.

  • FULL, Log the headers, body, and metadata for both requests and responses.

For example, the following would set the Logger.Level to FULL:

@Configuration
public class FooConfiguration {
    @Bean
    Logger.Level feignLoggerLevel() {
        return Logger.Level.FULL;
    }
}

99.11. Feign @QueryMap support

The OpenFeign @QueryMap annotation provides support for POJOs to be used as GET parameter maps. Unfortunately, the default OpenFeign QueryMap annotation is incompatible with Spring because it lacks a value property.

Spring Cloud OpenFeign provides an equivalent @SpringQueryMap annotation, which is used to annotate a POJO or Map parameter as a query parameter map.

For example, the Params class defines parameters param1 and param2:

// Params.java
public class Params {
    private String param1;
    private String param2;

    // [Getters and setters omitted for brevity]
}

The following feign client uses the Params class by using the @SpringQueryMap annotation:

@FeignClient("demo")
public interface DemoTemplate {

    @GetMapping(path = "/demo")
    String demoEndpoint(@SpringQueryMap Params params);
}

If you need more control over the generated query parameter map, you can implement a custom QueryMapEncoder bean.

99.12. HATEOAS support

Spring provides some APIs to create REST representations that follow the HATEOAS principle, Spring Hateoas and Spring Data REST.

If your project use the org.springframework.boot:spring-boot-starter-hateoas starter or the org.springframework.boot:spring-boot-starter-data-rest starter, Feign HATEOAS support is enabled by default.

When HATEOAS support is enabled, Feign clients are allowed to serialize and deserialize HATEOAS representation models: EntityModel, CollectionModel and PagedModel.

@FeignClient("demo")
public interface DemoTemplate {

    @GetMapping(path = "/stores")
    CollectionModel<Store> getStores();
}

99.13. Spring @MatrixVariable Support

Spring Cloud OpenFeign provides support for the Spring @MatrixVariable annotation.

If a map is passed as the method argument, the @MatrixVariable path segment is created by joining key-value pairs from the map with a =.

If a different object is passed, either the name provided in the @MatrixVariable annotation (if defined) or the annotated variable name is joined with the provided method argument using =.

IMPORTANT

Even though, on the server side, Spring does not require the users to name the path segment placeholder same as the matrix variable name, since it would be too ambiguous on the client side, Spring Cloud OpenFeign requires that you add a path segment placeholder with a name matching either the name provided in the @MatrixVariable annotation (if defined) or the annotated variable name.

For example:

@GetMapping("/objects/links/{matrixVars}")
Map<String, List<String>> getObjects(@MatrixVariable Map<String, List<String>> matrixVars);

Note that both variable name and the path segment placeholder are called matrixVars.

@FeignClient("demo")
public interface DemoTemplate {

    @GetMapping(path = "/stores")
    CollectionModel<Store> getStores();
}

99.14. Feign CollectionFormat support

We support feign.CollectionFormat by providing the @CollectionFormat annotation. You can annotate a Feign client method with it by passing the desired feign.CollectionFormat as annotation value.

In the following example, the CSV format is used instead of the default EXPLODED to process the method.

@FeignClient(name = "demo")
    protected interface PageableFeignClient {

        @CollectionFormat(feign.CollectionFormat.CSV)
        @GetMapping(path = "/page")
        ResponseEntity performRequest(Pageable page);

    }
Set the CSV format while sending Pageable as a query parameter in order for it to be encoded correctly.

99.15. Reactive Support

As the OpenFeign project does not currently support reactive clients, such as Spring WebClient, neither does Spring Cloud OpenFeign. We will add support for it here as soon as it becomes available in the core project.

Until that is done, we recommend using feign-reactive for Spring WebClient support.

99.15.1. Early Initialization Errors

Depending on how you are using your Feign clients you may see initialization errors when starting your application. To work around this problem you can use an ObjectProvider when autowiring your client.

@Autowired
ObjectProvider<TestFeginClient> testFeginClient;

100. Configuration properties

To see the list of all Sleuth related configuration properties please check the Appendix page.

Spring Cloud Security

Spring Cloud Security offers a set of primitives for building secure applications and services with minimum fuss. A declarative model which can be heavily configured externally (or centrally) lends itself to the implementation of large systems of co-operating, remote components, usually with a central indentity management service. It is also extremely easy to use in a service platform like Cloud Foundry. Building on Spring Boot and Spring Security OAuth2 we can quickly create systems that implement common patterns like single sign on, token relay and token exchange.

In a future major release, the functionality contained in this project will move to the respective projects.
Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, please find the source code and issue trackers in the project at github.

101. Quickstart

101.1. OAuth2 Single Sign On

Here’s a Spring Cloud "Hello World" app with HTTP Basic authentication and a single user account:

app.groovy
@Grab('spring-boot-starter-security')
@Controller
class Application {

  @RequestMapping('/')
  String home() {
    'Hello World'
  }

}

You can run it with spring run app.groovy and watch the logs for the password (username is "user"). So far this is just the default for a Spring Boot app.

Here’s a Spring Cloud app with OAuth2 SSO:

app.groovy
@Controller
@EnableOAuth2Sso
class Application {

  @RequestMapping('/')
  String home() {
    'Hello World'
  }

}

Spot the difference? This app will actually behave exactly the same as the previous one, because it doesn’t know it’s OAuth2 credentals yet.

You can register an app in github quite easily, so try that if you want a production app on your own domain. If you are happy to test on localhost:8080, then set up these properties in your application configuration:

application.yml
security:
  oauth2:
    client:
      clientId: bd1c0a783ccdd1c9b9e4
      clientSecret: 1a9030fbca47a5b2c28e92f19050bb77824b5ad1
      accessTokenUri: https://github.com/login/oauth/access_token
      userAuthorizationUri: https://github.com/login/oauth/authorize
      clientAuthenticationScheme: form
    resource:
      userInfoUri: https://api.github.com/user
      preferTokenInfo: false

run the app above and it will redirect to github for authorization. If you are already signed into github you won’t even notice that it has authenticated. These credentials will only work if your app is running on port 8080.

To limit the scope that the client asks for when it obtains an access token you can set security.oauth2.client.scope (comma separated or an array in YAML). By default the scope is empty and it is up to to Authorization Server to decide what the defaults should be, usually depending on the settings in the client registration that it holds.

The examples above are all Groovy scripts. If you want to write the same code in Java (or Groovy) you need to add Spring Security OAuth2 to the classpath (e.g. see the sample here).

101.2. OAuth2 Protected Resource

You want to protect an API resource with an OAuth2 token? Here’s a simple example (paired with the client above):

app.groovy
@Grab('spring-cloud-starter-security')
@RestController
@EnableResourceServer
class Application {

  @RequestMapping('/')
  def home() {
    [message: 'Hello World']
  }

}

and

application.yml
security:
  oauth2:
    resource:
      userInfoUri: https://api.github.com/user
      preferTokenInfo: false

102. More Detail

102.1. Single Sign On

All of the OAuth2 SSO and resource server features moved to Spring Boot in version 1.3. You can find documentation in the Spring Boot user guide.

102.1.1. Client Token Relay

If your app is a user facing OAuth2 client (i.e. has declared @EnableOAuth2Sso or @EnableOAuth2Client) then it has an OAuth2ClientContext in request scope from Spring Boot. You can create your own OAuth2RestTemplate from this context and an autowired OAuth2ProtectedResourceDetails, and then the context will always forward the access token downstream, also refreshing the access token automatically if it expires. (These are features of Spring Security and Spring Boot.)

Spring Boot (1.4.1) does not create an OAuth2ProtectedResourceDetails automatically if you are using client_credentials tokens. In that case you need to create your own ClientCredentialsResourceDetails and configure it with @ConfigurationProperties("security.oauth2.client").

102.1.2. Client Token Relay in Zuul Proxy

If your app also has a Spring Cloud Zuul embedded reverse proxy (using @EnableZuulProxy) then you can ask it to forward OAuth2 access tokens downstream to the services it is proxying. Thus the SSO app above can be enhanced simply like this:

app.groovy
@Controller
@EnableOAuth2Sso
@EnableZuulProxy
class Application {

}

and it will (in addition to logging the user in and grabbing a token) pass the authentication token downstream to the /proxy/* services. If those services are implemented with @EnableResourceServer then they will get a valid token in the correct header.

How does it work? The @EnableOAuth2Sso annotation pulls in spring-cloud-starter-security (which you could do manually in a traditional app), and that in turn triggers some autoconfiguration for a ZuulFilter, which itself is activated because Zuul is on the classpath (via @EnableZuulProxy). The filter just extracts an access token from the currently authenticated user, and puts it in a request header for the downstream requests.

Spring Boot does not create an OAuth2RestOperations automatically which is needed for refresh_token. In that case you need to create your own OAuth2RestOperations so OAuth2TokenRelayFilter can refresh the token if needed.

102.1.3. Resource Server Token Relay

If your app has @EnableResourceServer you might want to relay the incoming token downstream to other services. If you use a RestTemplate to contact the downstream services then this is just a matter of how to create the template with the right context.

If your service uses UserInfoTokenServices to authenticate incoming tokens (i.e. it is using the security.oauth2.user-info-uri configuration), then you can simply create an OAuth2RestTemplate using an autowired OAuth2ClientContext (it will be populated by the authentication process before it hits the backend code). Equivalently (with Spring Boot 1.4), you could inject a UserInfoRestTemplateFactory and grab its OAuth2RestTemplate in your configuration. For example:

MyConfiguration.java
@Bean
public OAuth2RestTemplate restTemplate(UserInfoRestTemplateFactory factory) {
    return factory.getUserInfoRestTemplate();
}

This rest template will then have the same OAuth2ClientContext (request-scoped) that is used by the authentication filter, so you can use it to send requests with the same access token.

If your app is not using UserInfoTokenServices but is still a client (i.e. it declares @EnableOAuth2Client or @EnableOAuth2Sso), then with Spring Security Cloud any OAuth2RestOperations that the user creates from an @Autowired OAuth2Context will also forward tokens. This feature is implemented by default as an MVC handler interceptor, so it only works in Spring MVC. If you are not using MVC you could use a custom filter or AOP interceptor wrapping an AccessTokenContextRelay to provide the same feature.

Here’s a basic example showing the use of an autowired rest template created elsewhere ("foo.com" is a Resource Server accepting the same tokens as the surrounding app):

MyController.java
@Autowired
private OAuth2RestOperations restTemplate;

@RequestMapping("/relay")
public String relay() {
    ResponseEntity<String> response =
      restTemplate.getForEntity("https://foo.com/bar", String.class);
    return "Success! (" + response.getBody() + ")";
}

If you don’t want to forward tokens (and that is a valid choice, since you might want to act as yourself, rather than the client that sent you the token), then you only need to create your own OAuth2Context instead of autowiring the default one.

Feign clients will also pick up an interceptor that uses the OAuth2ClientContext if it is available, so they should also do a token relay anywhere where a RestTemplate would.

index.htmladoc

3.1.0-SNAPSHOT

103. Preface

103.1. A Brief History of Spring’s Data Integration Journey

Spring’s journey on Data Integration started with Spring Integration. With its programming model, it provided a consistent developer experience to build applications that can embrace Enterprise Integration Patterns to connect with external systems such as, databases, message brokers, and among others.

Fast forward to the cloud-era, where microservices have become prominent in the enterprise setting. Spring Boot transformed the way how developers built Applications. With Spring’s programming model and the runtime responsibilities handled by Spring Boot, it became seamless to develop stand-alone, production-grade Spring-based microservices.

To extend this to Data Integration workloads, Spring Integration and Spring Boot were put together into a new project. Spring Cloud Stream was born.

With Spring Cloud Stream, developers can:

  • Build, test and deploy data-centric applications in isolation.

  • Apply modern microservices architecture patterns, including composition through messaging.

  • Decouple application responsibilities with event-centric thinking. An event can represent something that has happened in time, to which the downstream consumer applications can react without knowing where it originated or the producer’s identity.

  • Port the business logic onto message brokers (such as RabbitMQ, Apache Kafka, Amazon Kinesis).

  • Rely on the framework’s automatic content-type support for common use-cases. Extending to different data conversion types is possible.

  • and many more. . .

103.2. Quick Start

You can try Spring Cloud Stream in less then 5 min even before you jump into any details by following this three-step guide.

We show you how to create a Spring Cloud Stream application that receives messages coming from the messaging middleware of your choice (more on this later) and logs received messages to the console. We call it LoggingConsumer. While not very practical, it provides a good introduction to some of the main concepts and abstractions, making it easier to digest the rest of this user guide.

The three steps are as follows:

103.2.1. Creating a Sample Application by Using Spring Initializr

To get started, visit the Spring Initializr. From there, you can generate our LoggingConsumer application. To do so:

  1. In the Dependencies section, start typing stream. When the “Cloud Stream” option should appears, select it.

  2. Start typing either 'kafka' or 'rabbit'.

  3. Select “Kafka” or “RabbitMQ”.

    Basically, you choose the messaging middleware to which your application binds. We recommend using the one you have already installed or feel more comfortable with installing and running. Also, as you can see from the Initilaizer screen, there are a few other options you can choose. For example, you can choose Gradle as your build tool instead of Maven (the default).

  4. In the Artifact field, type 'logging-consumer'.

    The value of the Artifact field becomes the application name. If you chose RabbitMQ for the middleware, your Spring Initializr should now be as follows:

spring initializr
  1. Click the Generate Project button.

    Doing so downloads the zipped version of the generated project to your hard drive.

  2. Unzip the file into the folder you want to use as your project directory.

We encourage you to explore the many possibilities available in the Spring Initializr. It lets you create many different kinds of Spring applications.

103.2.2. Importing the Project into Your IDE

Now you can import the project into your IDE. Keep in mind that, depending on the IDE, you may need to follow a specific import procedure. For example, depending on how the project was generated (Maven or Gradle), you may need to follow specific import procedure (for example, in Eclipse or STS, you need to use File → Import → Maven → Existing Maven Project).

Once imported, the project must have no errors of any kind. Also, src/main/java should contain com.example.loggingconsumer.LoggingConsumerApplication.

Technically, at this point, you can run the application’s main class. It is already a valid Spring Boot application. However, it does not do anything, so we want to add some code.

103.2.3. Adding a Message Handler, Building, and Running

Modify the com.example.loggingconsumer.LoggingConsumerApplication class to look as follows:

@SpringBootApplication
public class LoggingConsumerApplication {

    public static void main(String[] args) {
        SpringApplication.run(LoggingConsumerApplication.class, args);
    }

    @Bean
    public Consumer<Person> log() {
        return person -> {
            System.out.println("Received: " + person);
        };
    }

    public static class Person {
        private String name;
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public String toString() {
            return this.name;
        }
    }
}

As you can see from the preceding listing:

  • We are using functional programming model (see Spring Cloud Function support) to define a single message handler as Consumer.

  • We are relying on framework conventions to bind such handler to the input destination binding exposed by the binder.

Doing so also lets you see one of the core features of the framework: It tries to automatically convert incoming message payloads to type Person.

You now have a fully functional Spring Cloud Stream application that does listens for messages. From here, for simplicity, we assume you selected RabbitMQ in step one. Assuming you have RabbitMQ installed and running, you can start the application by running its main method in your IDE.

You should see following output:

    --- [ main] c.s.b.r.p.RabbitExchangeQueueProvisioner : declaring queue for inbound: input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg, bound to: input
    --- [ main] o.s.a.r.c.CachingConnectionFactory       : Attempting to connect to: [localhost:5672]
    --- [ main] o.s.a.r.c.CachingConnectionFactory       : Created new connection: rabbitConnectionFactory#2a3a299:0/[email protected] . .
    . . .
    --- [ main] o.s.i.a.i.AmqpInboundChannelAdapter      : started inbound.input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg
    . . .
    --- [ main] c.e.l.LoggingConsumerApplication         : Started LoggingConsumerApplication in 2.531 seconds (JVM running for 2.897)

Go to the RabbitMQ management console or any other RabbitMQ client and send a message to input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg. The anonymous.CbMIwdkJSBO1ZoPDOtHtCg part represents the group name and is generated, so it is bound to be different in your environment. For something more predictable, you can use an explicit group name by setting spring.cloud.stream.bindings.input.group=hello (or whatever name you like).

The contents of the message should be a JSON representation of the Person class, as follows:

{"name":"Sam Spade"}

Then, in your console, you should see:

Received: Sam Spade

You can also build and package your application into a boot jar (by using ./mvnw clean install) and run the built JAR by using the java -jar command.

Now you have a working (albeit very basic) Spring Cloud Stream application.

104. What’s New in 3.0?

104.1. New Features and Enhancements

  • Routing Function - see [Routing with functions] for more details.

  • Multiple bindings with functions (multiple message handlers) - see Multiple functions in a single application for more details.

  • Functions with multiple inputs/outputs (single function that can subscribe or target multiple destinations) - see Functions with multiple input and output arguments for more details.

  • Native support for reactive programming - since v3.0.0 we no longer distribute spring-cloud-stream-reactive modules and instead relying on native reactive support provided by spring cloud function. For backward compatibility you can still bring spring-cloud-stream-reactive from previous versions.

104.2. Notable Deprecations

  • Reactive module (spring-cloud-stream-reactive) is discontinued and no longer distributed in favor of native support via spring-cloud-function. For backward compatibility you can still bring spring-cloud-stream-reactive from previous versions.

  • Test support binder spring-cloud-stream-test-support with MessageCollector in favor of a new test binder. See Testing for more details.

  • @StreamMessageConverter - deprecated as it is no longer required.

  • The original-content-type header references have been removed after it’s been deprecated in v2.0.

  • The BinderAwareChannelResolver is deprecated in favor if providing spring.cloud.stream.sendto.destination property. This is primarily for function-based programming model. For StreamListener it would still be required and thus will stay until we deprecate and eventually discontinue StreamListener and annotation-based programming model.

105. Introducing Spring Cloud Stream

Spring Cloud Stream is a framework for building message-driven microservice applications. Spring Cloud Stream builds upon Spring Boot to create standalone, production-grade Spring applications and uses Spring Integration to provide connectivity to message brokers. It provides opinionated configuration of middleware from several vendors, introducing the concepts of persistent publish-subscribe semantics, consumer groups, and partitions.

By adding spring-cloud-stream dependencies to the classpath of your application, you get immediate connectivity to a message broker exposed by the provided spring-cloud-stream binder (more on that later), and you can implement your functional requirement, which is run (based on the incoming message) by a java.util.function.Function.

The following listing shows a quick example:

@SpringBootApplication
public class SampleApplication {

    public static void main(String[] args) {
        SpringApplication.run(SampleApplication.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
}

The following listing shows the corresponding test:

@SpringBootTest(classes =  SampleApplication.class)
@Import({TestChannelBinderConfiguration.class})
class BootTestStreamApplicationTests {

    @Autowired
    private InputDestination input;

    @Autowired
    private OutputDestination output;

    @Test
    void contextLoads() {
        input.send(new GenericMessage<byte[]>("hello".getBytes()));
        assertThat(output.receive().getPayload()).isEqualTo("HELLO".getBytes());
    }
}

106. Main Concepts

Spring Cloud Stream provides a number of abstractions and primitives that simplify the writing of message-driven microservice applications. This section gives an overview of the following:

106.1. Application Model

A Spring Cloud Stream application consists of a middleware-neutral core. The application communicates with the outside world by establishing bindings between destinations exposed by the external brokers and input/output arguments in your code. Broker specific details necessary to establish bindings are handled by middleware-specific Binder implementations.

SCSt with binder
Figure 7. Spring Cloud Stream Application

106.1.1. Fat JAR

Spring Cloud Stream applications can be run in stand-alone mode from your IDE for testing. To run a Spring Cloud Stream application in production, you can create an executable (or “fat”) JAR by using the standard Spring Boot tooling provided for Maven or Gradle. See the Spring Boot Reference Guide for more details.

106.2. The Binder Abstraction

Spring Cloud Stream provides Binder implementations for Kafka and Rabbit MQ. The framework also includes a test binder for integration testing of your applications as spring-cloud-stream application. See Testing section for more details.

Binder abstraction is also one of the extension points of the framework, which means you can implement your own binder on top of Spring Cloud Stream. In the How to create a Spring Cloud Stream Binder from scratch post a community member documents in details, with an example, a set of steps necessary to implement a custom binder. The steps are also highlighted in the Implementing Custom Binders section.

Spring Cloud Stream uses Spring Boot for configuration, and the Binder abstraction makes it possible for a Spring Cloud Stream application to be flexible in how it connects to middleware. For example, deployers can dynamically choose, at runtime, the mapping between the external destinations (such as the Kafka topics or RabbitMQ exchanges) and inputs and outputs of the message handler (such as input parameter of the function and its return argument). Such configuration can be provided through external configuration properties and in any form supported by Spring Boot (including application arguments, environment variables, and application.yml or application.properties files). In the sink example from the Introducing Spring Cloud Stream section, setting the spring.cloud.stream.bindings.input.destination application property to raw-sensor-data causes it to read from the raw-sensor-data Kafka topic or from a queue bound to the raw-sensor-data RabbitMQ exchange.

Spring Cloud Stream automatically detects and uses a binder found on the classpath. You can use different types of middleware with the same code. To do so, include a different binder at build time. For more complex use cases, you can also package multiple binders with your application and have it choose the binder( and even whether to use different binders for different bindings) at runtime.

106.3. Persistent Publish-Subscribe Support

Communication between applications follows a publish-subscribe model, where data is broadcast through shared topics. This can be seen in the following figure, which shows a typical deployment for a set of interacting Spring Cloud Stream applications.

SCSt sensors
Figure 8. Spring Cloud Stream Publish-Subscribe

Data reported by sensors to an HTTP endpoint is sent to a common destination named raw-sensor-data. From the destination, it is independently processed by a microservice application that computes time-windowed averages and by another microservice application that ingests the raw data into HDFS (Hadoop Distributed File System). In order to process the data, both applications declare the topic as their input at runtime.

The publish-subscribe communication model reduces the complexity of both the producer and the consumer and lets new applications be added to the topology without disruption of the existing flow. For example, downstream from the average-calculating application, you can add an application that calculates the highest temperature values for display and monitoring. You can then add another application that interprets the same flow of averages for fault detection. Doing all communication through shared topics rather than point-to-point queues reduces coupling between microservices.

While the concept of publish-subscribe messaging is not new, Spring Cloud Stream takes the extra step of making it an opinionated choice for its application model. By using native middleware support, Spring Cloud Stream also simplifies use of the publish-subscribe model across different platforms.

106.4. Consumer Groups

While the publish-subscribe model makes it easy to connect applications through shared topics, the ability to scale up by creating multiple instances of a given application is equally important. When doing so, different instances of an application are placed in a competing consumer relationship, where only one of the instances is expected to handle a given message.

Spring Cloud Stream models this behavior through the concept of a consumer group. (Spring Cloud Stream consumer groups are similar to and inspired by Kafka consumer groups.) Each consumer binding can use the spring.cloud.stream.bindings.<bindingName>.group property to specify a group name. For the consumers shown in the following figure, this property would be set as spring.cloud.stream.bindings.<bindingName>.group=hdfsWrite or spring.cloud.stream.bindings.<bindingName>.group=average.

SCSt groups
Figure 9. Spring Cloud Stream Consumer Groups

All groups that subscribe to a given destination receive a copy of published data, but only one member of each group receives a given message from that destination. By default, when a group is not specified, Spring Cloud Stream assigns the application to an anonymous and independent single-member consumer group that is in a publish-subscribe relationship with all other consumer groups.

106.5. Consumer Types

Two types of consumer are supported:

  • Message-driven (sometimes referred to as Asynchronous)

  • Polled (sometimes referred to as Synchronous)

Prior to version 2.0, only asynchronous consumers were supported. A message is delivered as soon as it is available and a thread is available to process it.

When you wish to control the rate at which messages are processed, you might want to use a synchronous consumer.

106.5.1. Durability

Consistent with the opinionated application model of Spring Cloud Stream, consumer group subscriptions are durable. That is, a binder implementation ensures that group subscriptions are persistent and that, once at least one subscription for a group has been created, the group receives messages, even if they are sent while all applications in the group are stopped.

Anonymous subscriptions are non-durable by nature. For some binder implementations (such as RabbitMQ), it is possible to have non-durable group subscriptions.

In general, it is preferable to always specify a consumer group when binding an application to a given destination. When scaling up a Spring Cloud Stream application, you must specify a consumer group for each of its input bindings. Doing so prevents the application’s instances from receiving duplicate messages (unless that behavior is desired, which is unusual).

106.6. Partitioning Support

Spring Cloud Stream provides support for partitioning data between multiple instances of a given application. In a partitioned scenario, the physical communication medium (such as the broker topic) is viewed as being structured into multiple partitions. One or more producer application instances send data to multiple consumer application instances and ensure that data identified by common characteristics are processed by the same consumer instance.

Spring Cloud Stream provides a common abstraction for implementing partitioned processing use cases in a uniform fashion. Partitioning can thus be used whether the broker itself is naturally partitioned (for example, Kafka) or not (for example, RabbitMQ).

SCSt partitioning
Figure 10. Spring Cloud Stream Partitioning

Partitioning is a critical concept in stateful processing, where it is critical (for either performance or consistency reasons) to ensure that all related data is processed together. For example, in the time-windowed average calculation example, it is important that all measurements from any given sensor are processed by the same application instance.

To set up a partitioned processing scenario, you must configure both the data-producing and the data-consuming ends.

107. Programming Model

To understand the programming model, you should be familiar with the following core concepts:

  • Destination Binders: Components responsible to provide integration with the external messaging systems.

  • Bindings: Bridge between the external messaging systems and application provided Producers and Consumers of messages (created by the Destination Binders).

  • Message: The canonical data structure used by producers and consumers to communicate with Destination Binders (and thus other applications via external messaging systems).

SCSt overview

107.1. Destination Binders

Destination Binders are extension components of Spring Cloud Stream responsible for providing the necessary configuration and implementation to facilitate integration with external messaging systems. This integration is responsible for connectivity, delegation, and routing of messages to and from producers and consumers, data type conversion, invocation of the user code, and more.

Binders handle a lot of the boiler plate responsibilities that would otherwise fall on your shoulders. However, to accomplish that, the binder still needs some help in the form of minimalistic yet required set of instructions from the user, which typically come in the form of some type of binding configuration.

While it is out of scope of this section to discuss all of the available binder and binding configuration options (the rest of the manual covers them extensively), Binding as a concept, does require special attention. The next section discusses it in detail.

107.2. Bindings

As stated earlier, Bindings provide a bridge between the external messaging system (e.g., queue, topic etc.) and application-provided Producers and Consumers.

The following example shows a fully configured and functioning Spring Cloud Stream application that receives the payload of the message as a String type (see Content Type Negotiation section), logs it to the console and sends it down stream after converting it to upper case.

@SpringBootApplication
public class SampleApplication {

    public static void main(String[] args) {
        SpringApplication.run(SampleApplication.class, args);
    }

    @Bean
    public Function<String, String> uppercase() {
        return value -> {
            System.out.println("Received: " + value);
            return value.toUpperCase()
        };
    }
}

The above example looks no different then any vanilla spring-boot application. It defines a single bean of type Function and that it is. So, how does it became spring-cloud-stream application? It becomes spring-cloud-stream application simply based on the presence of spring-cloud-stream and binder dependencies and auto-configuration classes on the classpath effectively setting the context for your boot application as spring-cloud-stream application. And in this context beans of type Supplier, Function or Consumer are treated as defacto message handlers triggering binding of to destinations exposed by the provided binder following certain naming conventions and rules to avoid extra configuration.

107.2.1. Binding and Binding names

Binding is an abstraction that represents a bridge between sources and targets exposed by the binder and user code, This abstraction has a name and while we try to do our best to limit configuration required to run spring-cloud-stream applications, being aware of such name(s) is necessary for cases where additional per-binding configuration is required.

Throughout this manual you will see examples of configuration properties such as spring.cloud.stream.bindings.input.destination=myQueue. The input segment in this property name is what we refer to as binding name and it could derive via several mechanisms. The following sub-sections will describe the naming conventions and configuration elements used by spring-cloud-stream to control binding names.

Functional binding names

Unlike the explicit naming required by annotation-based support (legacy) used in the previous versions of spring-cloud-stream, the functional programming model defaults to a simple convention when it comes to binding names, thus greatly simplifying application configuration. Let’s look at the first example:

@SpringBootApplication
public class SampleApplication {

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }
}

In the preceding example we have an application with a single function which acts as message handler. As a Function it has an input and output. The naming convention used to name input and output bindings is as follows:

  • input - <functionName> + -in- + <index>

  • output - <functionName> + -out- + <index>

The in and out corresponds to the type of binding (such as input or output). The index is the index of the input or output binding. It is always 0 for typical single input/output function, so it’s only relevant for Functions with multiple input and output arguments.

So if for example you would want to map the input of this function to a remote destination (e.g., topic, queue etc) called "my-topic" you would do so with the following property:

--spring.cloud.stream.bindings.uppercase-in-0.destination=my-topic

Note how uppercase-in-0 is used as a segment in property name. The same goes for uppercase-out-0.

Descriptive Binding Names

Some times to improve readability you may want to give your binding a more descriptive name (such as 'account', 'orders` etc). Another way of looking at it is you can map an implicit binding name to an explicit binding name. And you can do it with spring.cloud.stream.function.bindings.<binding-name> property. This property also provides a migration path for existing applications that rely on custom interface-based bindings that require explicit names.

For example,

--spring.cloud.stream.function.bindings.uppercase-in-0=input

In the preceding example you mapped and effectively renamed uppercase-in-0 binding name to input. Now all configuration properties can refer to input binding name instead (e.g., --spring.cloud.stream.bindings.input.destination=my-topic).

While descriptive binding names may enhance the readability aspect of the configuration, they also create another level of misdirection by mapping an implicit binding name to an explicit binding name. And since all subsequent configuration properties will use the explicit binding name you must always refer to this 'bindings' property to correlate which function it actually corresponds to. We believe that for most cases (with the exception of Functional Composition) it may be an overkill, so, it is our recommendation to avoid using it altogether, especially since not using it provides a clear path between binder destination and binding name, such as spring.cloud.stream.bindings.uppercase-in-0.destination=sample-topic, where you are clearly correlating the input of uppercase function to sample-topic destination.

For more on properties and other configuration options please see Configuration Options section.

Pollable Destination Binding

While the previously described bindings support event-based message consumption, sometimes you need more control, such as rate of consumption.

Starting with version 2.0, you can now bind a pollable consumer:

The following example shows how to bind a pollable consumer:

public interface PolledBarista {

    @Input
    PollableMessageSource orders();
    . . .
}

In this case, an implementation of PollableMessageSource is bound to the orders “channel”. See Using Polled Consumers for more details.

107.3. Producing and Consuming Messages

You can write a Spring Cloud Stream application by simply writing functions and exposing them as `@Bean`s. You can also use Spring Integration annotations based configuration or Spring Cloud Stream annotation based configuration, although starting with spring-cloud-stream 3.x we recommend using functional implementations.

107.3.1. Spring Cloud Function support

Overview

Since Spring Cloud Stream v2.1, another alternative for defining stream handlers and sources is to use build-in support for Spring Cloud Function where they can be expressed as beans of type java.util.function.[Supplier/Function/Consumer].

To specify which functional bean to bind to the external destination(s) exposed by the bindings, you must provide spring.cloud.function.definition property.

In the event you only have single bean of type java.util.function.[Supplier/Function/Consumer], you can skip the spring.cloud.function.definition property, since such functional bean will be auto-discovered. However, it is considered best practice to use such property to avoid any confusion.

Here is the example of the application exposing message handler as java.util.function.Function effectively supporting pass-thru semantics by acting as consumer and producer of data.

@SpringBootApplication
public class MyFunctionBootApp {

    public static void main(String[] args) {
        SpringApplication.run(MyFunctionBootApp.class);
    }

    @Bean
    public Function<String, String> toUpperCase() {
        return s -> s.toUpperCase();
    }
}

In the preceding example, we define a bean of type java.util.function.Function called toUpperCase to be acting as message handler whose 'input' and 'output' must be bound to the external destinations exposed by the provided destination binder. By default the 'input' and 'output' binding names will be toUpperCase-in-0 and toUpperCase-out-0. Please see Functional binding names section for details on naming convention used to establish binding names.

Below are the examples of simple functional applications to support other semantics:

Here is the example of a source semantics exposed as java.util.function.Supplier

@SpringBootApplication
public static class SourceFromSupplier {

    @Bean
    public Supplier<Date> date() {
        return () -> new Date(12345L);
    }
}

Here is the example of a sink semantics exposed as java.util.function.Consumer

@SpringBootApplication
public static class SinkFromConsumer {

    @Bean
    public Consumer<String> sink() {
        return System.out::println;
    }
}
Suppliers (Sources)

Function and Consumer are pretty straightforward when it comes to how their invocation is triggered. They are triggered based on data (events) sent to the destination they are bound to. In other words, they are classic event-driven components.

However, Supplier is in its own category when it comes to triggering. Since it is, by definition, the source (the origin) of the data, it does not subscribe to any in-bound destination and, therefore, has to be triggered by some other mechanism(s). There is also a question of Supplier implementation, which could be imperative or reactive and which directly relates to the triggering of such suppliers.

Consider the following sample:

@SpringBootApplication
public static class SupplierConfiguration {

    @Bean
    public Supplier<String> stringSupplier() {
        return () -> "Hello from Supplier";
    }
}

The preceding Supplier bean produces a string whenever its get() method is invoked. However, who invokes this method and how often? The framework provides a default polling mechanism (answering the question of "Who?") that will trigger the invocation of the supplier and by default it will do so every second (answering the question of "How often?"). In other words, the above configuration produces a single message every second and each message is sent to an output destination that is exposed by the binder. To learn how to customize the polling mechanism, see Polling Configuration Properties section.

Consider a different example:

@SpringBootApplication
public static class SupplierConfiguration {

    @Bean
    public Supplier<Flux<String>> stringSupplier() {
        return () -> Flux.fromStream(Stream.generate(new Supplier<String>() {
            @Override
            public String get() {
                try {
                    Thread.sleep(1000);
                    return "Hello from Supplier";
                } catch (Exception e) {
                    // ignore
                }
            }
        })).subscribeOn(Schedulers.elastic()).share();
    }
}

The preceding Supplier bean adopts the reactive programming style. Typically, and unlike the imperative supplier, it should be triggered only once, given that the invocation of its get() method produces (supplies) the continuous stream of messages and not an individual message.

The framework recognizes the difference in the programming style and guarantees that such a supplier is triggered only once.

However, imagine the use case where you want to poll some data source and return a finite stream of data representing the result set. The reactive programming style is a perfect mechanism for such a Supplier. However, given the finite nature of the produced stream, such Supplier still needs to be invoked periodically.

Consider the following sample, which emulates such use case by producing a finite stream of data:

@SpringBootApplication
public static class SupplierConfiguration {

    @PollableBean
    public Supplier<Flux<String>> stringSupplier() {
        return () -> Flux.just("hello", "bye");
    }
}

The bean itself is annotated with PollableBean annotation (sub-set of @Bean), thus signaling to the framework that although the implementation of such a supplier is reactive, it still needs to be polled.

There is a splittable attribute defined in PollableBean which signals to the post processors of this annotation that the result produced by the annotated component has to be split and is set to true by default. It means that the framework will split the returning sending out each item as an individual message. If this is not he desired behavior you can set it to false at which point such supplier will simply return the produced Flux without splitting it.
Consumer (Reactive)

Reactive Consumer is a little bit special because it has a void return type, leaving framework with no reference to subscribe to. Most likely you will not need to write Consumer<Flux<?>>, and instead write it as a Function<Flux<?>, Mono<Void>> invoking then operator as the last operator on your stream.

For example:

public Function<Flux<?>, Mono<Void>>`consumer() {
    return flux -> flux.map(..).filter(..).then();
}

But if you do need to write an explicit Consumer<Flux<?>>, remember to subscribe to the incoming Flux.

Polling Configuration Properties

The following properties are exposed by org.springframework.cloud.stream.config.DefaultPollerProperties and are prefixed with spring.cloud.stream.poller:

fixedDelay

Fixed delay for default poller in milliseconds.

Default: 1000L.

maxMessagesPerPoll

Maximum messages for each polling event of the default poller.

Default: 1L.

cron

Cron expression value for the Cron Trigger.

Default: none.

initialDelay

Initial delay for periodic triggers.

Default: 0.

timeUnit

The TimeUnit to apply to delay values.

Default: MILLISECONDS.

For example --spring.cloud.stream.poller.fixed-delay=2000 sets the poller interval to poll every two seconds.

Sending arbitrary data to an output (e.g. Foreign event-driven sources)

There are cases where the actual source of data may be coming from the external (foreign) system that is not a binder. For example, the source of the data may be a classic REST endpoint. How do we bridge such source with the functional mechanism used by spring-cloud-stream?

Spring Cloud Stream provides two mechanisms, so let’s look at them in more details

Here, for both samples we’ll use a standard MVC endpoint method called delegateToSupplier bound to the root web context, delegating incoming requests to stream via two different mechanisms - imperative (via StreamBridge) and reactive (via EmitterProcessor).

Using StreamBridge
@SpringBootApplication
@Controller
public class WebSourceApplication {

    public static void main(String[] args) {
        SpringApplication.run(WebSourceApplication.class, "--spring.cloud.stream.source=toStream");
    }

    @Autowired
    private StreamBridge streamBridge;

    @RequestMapping
    @ResponseStatus(HttpStatus.ACCEPTED)
    public void delegateToSupplier(@RequestBody String body) {
        System.out.println("Sending " + body);
        streamBridge.send("toStream-out-0", body);
    }
}

Here we autowire a StreamBridge bean which allows us to send data to an output binding effectively bridging non-stream application with spring-cloud-stream. Note that preceding example does not have any source functions defined (e.g., Supplier bean) leaving the framework with no trigger to create source bindings, which would be typical for cases where configuration contains function beans. So to trigger the creation of source binding we use spring.cloud.stream.source property where you can declare the name of your sources. The provided name will be used as a trigger to create a source binding. So in the preceding example the name of the output binding will be toStream-out-0 which is consistent with the binding naming convention used by functions (see Binding and Binding names). You can use ; to signify multiple sources (e.g., --spring.cloud.stream.source=foo;bar)

Also, note that streamBridge.send(..) method takes an Object for data. This means you can send POJO or Message to it and it will go through the same routine when sending output as if it was from any Function or Supplier providing the same level of consistency as with functions. This means the output type conversion, partitioning etc are honored as if it was from the output produced by functions.

StreamBridge and Dynamic Destinations

StreamBridge can also be used for cases when output destination(s) are not known ahead of time similar to the use cases described in Routing FROM Consumer section.

Let’s look at the example

@SpringBootApplication
@Controller
public class WebSourceApplication {

    public static void main(String[] args) {
        SpringApplication.run(WebSourceApplication.class, args);
    }

    @Autowired
    private StreamBridge streamBridge;

    @RequestMapping
    @ResponseStatus(HttpStatus.ACCEPTED)
    public void delegateToSupplier(@RequestBody String body) {
        System.out.println("Sending " + body);
        streamBridge.send("myDestination", body);
    }
}

As you can see the preceding example is very similar to the previous one with the exception of explicit binding instruction provided via spring.cloud.stream.source property (which is not provided). Here we’re sending data to myDestination name which does not exist as a binding. Therefore such name will be treated as dynamic destination as described in Routing FROM Consumer section.

Using reactor API

Another approach that can be used to send arbitrary data to the output is using Reactor API.

All we need to do is declare a Supplier<Flux<whatever>> which returns EmitterProcessor from the reactor API (see Reactive Functions support for more details) to effectively provide a bridge between the actual event source (foreign source) and spring-cloud-stream. All you need to do now is feed the EmitterProcessor with data via EmitterProcessor#onNext(data) operation.

For example,

public class SampleApplication {

    public static void main(String[] args) {
               SpringApplication.run(SampleApplication.class);
    }

    EmitterProcessor<String> processor = EmitterProcessor.create();

    @Bean
    public ApplicationRunner runner() {
        Message<String> msg1 = MessageBuilder.withPayload("foo")
                .setHeader("*.events", "test1.events.billing")
                .build();
        Message<String> msg2 = MessageBuilder.withPayload("bar")
                .setHeader("*.events", "test2.events.messages")
                .build();
        return args -> {
            this.processor.onNext(msg1);
            this.processor.onNext(msg2);
        };
    }

    @Bean
    public Supplier<Flux<String>> supplier() {
               return () -> this.processor;
    }
}

In the preceding example, we are using ApplicationRunner as a foreign source to feed the stream.

A more practical example, where the foreign source is REST endpoint.

@SpringBootApplication
@Controller
public class WebSourceApplication {

    public static void main(String[] args) {
        SpringApplication.run(WebSourceApplication.class);
    }

    EmitterProcessor<String> processor = EmitterProcessor.create();

    @RequestMapping
    @ResponseStatus(HttpStatus.ACCEPTED)
    public void delegateToSupplier(@RequestBody String body) {
        processor.onNext(body);
    }

    @Bean
    public Supplier<Flux<String>> supplier() {
        return () -> this.processor;
    }
}

Same as before we declare a Supplier bean which returns Flux<String>. But given that this is a REST endpoint we send messages by simply posting to this REST endpoint.

curl -H "Content-Type: text/plain" -X POST -d "hello from the other side" http://localhost:8080/

By showing two example we want to emphasize the approach will work with any type of foreign sources.

Reactive Functions support

Since Spring Cloud Function is build on top of Project Reactor there isn’t much you need to do to benefit from reactive programming model while implementing Supplier, Function or Consumer.

For example:

@SpringBootApplication
public static class SinkFromConsumer {

    @Bean
    public Function<Flux<String>, Flux<String>> reactiveUpperCase() {
        return flux -> flux.map(val -> val.toUpperCase());
    }
}
Functional Composition

Using functional programming model you can also benefit from functional composition where you can dynamically compose complex handlers from a set of simple functions. As an example let’s add the following function bean to the application defined above

@Bean
public Function<String, String> wrapInQuotes() {
    return s -> "\"" + s + "\"";
}

and modify the spring.cloud.function.definition property to reflect your intention to compose a new function from both ‘toUpperCase’ and ‘wrapInQuotes’. To do so Spring Cloud Function relies on | (pipe) symbol. So, to finish our example our property will now look like this:

--spring.cloud.function.definition=toUpperCase|wrapInQuotes
One of the great benefits of functional composition support provided by Spring Cloud Function is the fact that you can compose reactive and imperative functions.

The result of a composition is a single function which, as you may guess, could have a very long and rather cryptic name (e.g., foo|bar|baz|xyz. . .) presenting a great deal of inconvenience when it comes to other configuration properties. This is where descriptive binding names feature described in Functional binding names section can help.

For example, if we want to give our toUpperCase|wrapInQuotes a more descriptive name we can do so with the following property spring.cloud.stream.function.bindings.toUpperCase|wrapInQuotes=quotedUpperCase allowing other configuration properties to refer to that binding name (e.g., spring.cloud.stream.bindings.quotedUpperCase.destination=myDestination).

Functions with multiple input and output arguments

Starting with version 3.0 spring-cloud-stream provides support for functions that have multiple inputs and/or multiple outputs (return values). What does this actually mean and what type of use cases it is targeting?

  • Big Data: Imagine the source of data you’re dealing with is highly un-organized and contains various types of data elements (e.g., orders, transactions etc) and you effectively need to sort it out.

  • Data aggregation: Another use case may require you to merge data elements from 2+ incoming _streams.

The above describes just a few use cases where you may need to use a single function to accept and/or produce multiple streams of data. And that is the type of use cases we are targeting here.

Also, note a slightly different emphasis on the concept of streams here. The assumption is that such functions are only valuable if they are given access to the actual streams of data (not the individual elements). So for that we are relying on abstractions provided by Project Reactor (i.e., Flux and Mono) which is already available on the classpath as part of the dependencies brought in by spring-cloud-functions.

Another important aspect is representation of multiple input and outputs. While java provides variety of different abstractions to represent multiple of something those abstractions are a) unbounded, b) lack arity and c) lack type information which are all important in this context. As an example, let’s look at Collection or an array which only allows us to describe multiple of a single type or up-cast everything to an Object, affecting the transparent type conversion feature of spring-cloud-stream and so on.

So to accommodate all these requirements the initial support is relying on the signature which utilizes another abstraction provided by Project Reactor - Tuples. However, we are working on allowing a more flexible signatures.

Please refer to Binding and Binding names section to understand the naming convention used to establish binding names used by such application.

Let’s look at the few samples:

@SpringBootApplication
public class SampleApplication {

    @Bean
    public Function<Tuple2<Flux<String>, Flux<Integer>>, Flux<String>> gather() {
        return tuple -> {
            Flux<String> stringStream = tuple.getT1();
            Flux<String> intStream = tuple.getT2().map(i -> String.valueOf(i));
            return Flux.merge(stringStream, intStream);
        };
    }
}

The above example demonstrates function which takes two inputs (first of type String and second of type Integer) and produces a single output of type String.

So, for the above example the two input bindings will be gather-in-0 and gather-in-1 and for consistency the output binding also follows the same convention and is named gather-out-0.

Knowing that will allow you to set binding specific properties. For example, the following will override content-type for gather-in-0 binding:

--spring.cloud.stream.bindings.gather-in-0.content-type=text/plain
@SpringBootApplication
public class SampleApplication {

    @Bean
    public static Function<Flux<Integer>, Tuple2<Flux<String>, Flux<String>>> scatter() {
        return flux -> {
            Flux<Integer> connectedFlux = flux.publish().autoConnect(2);
            UnicastProcessor even = UnicastProcessor.create();
            UnicastProcessor odd = UnicastProcessor.create();
            Flux<Integer> evenFlux = connectedFlux.filter(number -> number % 2 == 0).doOnNext(number -> even.onNext("EVEN: " + number));
            Flux<Integer> oddFlux = connectedFlux.filter(number -> number % 2 != 0).doOnNext(number -> odd.onNext("ODD: " + number));

            return Tuples.of(Flux.from(even).doOnSubscribe(x -> evenFlux.subscribe()), Flux.from(odd).doOnSubscribe(x -> oddFlux.subscribe()));
        };
    }
}

The above example is somewhat of a the opposite from the previous sample and demonstrates function which takes single input of type Integer and produces two outputs (both of type String).

So, for the above example the input binding is scatter-in-0 and the output bindings are scatter-out-0 and scatter-out-1.

And you test it with the following code:

@Test
public void testSingleInputMultiOutput() {
    try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
            TestChannelBinderConfiguration.getCompleteConfiguration(
                    SampleApplication.class))
                            .run("--spring.cloud.function.definition=scatter")) {

        InputDestination inputDestination = context.getBean(InputDestination.class);
        OutputDestination outputDestination = context.getBean(OutputDestination.class);

        for (int i = 0; i < 10; i++) {
            inputDestination.send(MessageBuilder.withPayload(String.valueOf(i).getBytes()).build());
        }

        int counter = 0;
        for (int i = 0; i < 5; i++) {
            Message<byte[]> even = outputDestination.receive(0, 0);
            assertThat(even.getPayload()).isEqualTo(("EVEN: " + String.valueOf(counter++)).getBytes());
            Message<byte[]> odd = outputDestination.receive(0, 1);
            assertThat(odd.getPayload()).isEqualTo(("ODD: " + String.valueOf(counter++)).getBytes());
        }
    }
}
Multiple functions in a single application

There may also be a need for grouping several message handlers in a single application. You would do so by defining several functions.

@SpringBootApplication
public class SampleApplication {

    @Bean
    public Function<String, String> uppercase() {
        return value -> value.toUpperCase();
    }

    @Bean
    public Function<String, String> reverse() {
        return value -> new StringBuilder(value).reverse().toString();
    }
}

In the above example we have configuration which defines two functions uppercase and reverse. So first, as mentioned before, we need to notice that there is a a conflict (more then one function) and therefore we need to resolve it by providing spring.cloud.function.definition property pointing to the actual function we want to bind. Except here we will use ; delimiter to point to both functions (see test case below).

As with functions with multiple inputs/outputs, please refer to Binding and Binding names section to understand the naming convention used to establish binding names used by such application.

And you test it with the following code:

@Test
public void testMultipleFunctions() {
    try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
            TestChannelBinderConfiguration.getCompleteConfiguration(
                    ReactiveFunctionConfiguration.class))
                            .run("--spring.cloud.function.definition=uppercase;reverse")) {

        InputDestination inputDestination = context.getBean(InputDestination.class);
        OutputDestination outputDestination = context.getBean(OutputDestination.class);

        Message<byte[]> inputMessage = MessageBuilder.withPayload("Hello".getBytes()).build();
        inputDestination.send(inputMessage, "uppercase-in-0");
        inputDestination.send(inputMessage, "reverse-in-0");

        Message<byte[]> outputMessage = outputDestination.receive(0, "uppercase-out-0");
        assertThat(outputMessage.getPayload()).isEqualTo("HELLO".getBytes());

        outputMessage = outputDestination.receive(0, "uppercase-out-1");
        assertThat(outputMessage.getPayload()).isEqualTo("olleH".getBytes());
    }
}
Batch Consumers

When using a MessageChannelBinder that supports batch listeners, and the feature is enabled for the consumer binding, you can set spring.cloud.stream.bindings.<binding-name>.consumer.batch-mode to true to enable the entire batch of messages to be passed to the function in a List.

@Bean
public Function<List<Person>, Person> findFirstPerson() {
    return persons -> persons.get(0);
}
Spring Integration flow as functions

When you implement a function, you may have complex requirements that fit the category of Enterprise Integration Patterns (EIP). These are best handled by using a framework such as Spring Integration (SI), which is a reference implementation of EIP.

Thankfully SI already provides support for exposing integration flows as functions via Integration flow as gateway Consider the following sample:

@SpringBootApplication
public class FunctionSampleSpringIntegrationApplication {

    public static void main(String[] args) {
        SpringApplication.run(FunctionSampleSpringIntegrationApplication.class, args);
    }

    @Bean
    public IntegrationFlow uppercaseFlow() {
        return IntegrationFlows.from(MessageFunction.class, "uppercase")
                .<String, String>transform(String::toUpperCase)
                .logAndReply(LoggingHandler.Level.WARN);
    }

    public interface MessageFunction extends Function<Message<String>, Message<String>> {

    }
}

For those who are familiar with SI you can see we define a bean of type IntegrationFlow where we declare an integration flow that we want to expose as a Function<String, String> (using SI DSL) called uppercase. The MessageFunction interface lets us explicitly declare the type of the inputs and outputs for proper type conversion. See Content Type Negotiation section for more on type conversion.

To receive raw input you can use from(Function.class, …​).

The resulting function is bound to the input and output destinations exposed by the target binder.

Please refer to Binding and Binding names section to understand the naming convention used to establish binding names used by such application.

For more details on interoperability of Spring Integration and Spring Cloud Stream specifically around functional programming model you may find this post very interesting, as it dives a bit deeper into various patterns you can apply by merging the best of Spring Integration and Spring Cloud Stream/Functions.

107.3.2. Using Polled Consumers

Overview

When using polled consumers, you poll the PollableMessageSource on demand. To define binding for polled consumer you need to provide spring.cloud.stream.pollable-source property.

Consider the following example of a polled consumer binding:

--spring.cloud.stream.pollable-source=myDestination

The pollable-source name myDestination in the preceding example will result in myDestination-in-0 binding name to stay consistent with functional programming model.

Given the polled consumer in the preceding example, you might use it as follows:

@Bean
public ApplicationRunner poller(PollableMessageSource destIn, MessageChannel destOut) {
    return args -> {
        while (someCondition()) {
            try {
                if (!destIn.poll(m -> {
                    String newPayload = ((String) m.getPayload()).toUpperCase();
                    destOut.send(new GenericMessage<>(newPayload));
                })) {
                    Thread.sleep(1000);
                }
            }
            catch (Exception e) {
                // handle failure
            }
        }
    };
}

A less manual and more Spring-like alternative would be to configure a scheduled task bean. For example,

@Scheduled(fixedDelay = 5_000)
public void poll() {
    System.out.println("Polling...");
    this.source.poll(m -> {
        System.out.println(m.getPayload());

    }, new ParameterizedTypeReference<Foo>() { });
}

The PollableMessageSource.poll() method takes a MessageHandler argument (often a lambda expression, as shown here). It returns true if the message was received and successfully processed.

As with message-driven consumers, if the MessageHandler throws an exception, messages are published to error channels, as discussed in Error Handling.

Normally, the poll() method acknowledges the message when the MessageHandler exits. If the method exits abnormally, the message is rejected (not re-queued), but see Handling Errors. You can override that behavior by taking responsibility for the acknowledgment, as shown in the following example:

@Bean
public ApplicationRunner poller(PollableMessageSource dest1In, MessageChannel dest2Out) {
    return args -> {
        while (someCondition()) {
            if (!dest1In.poll(m -> {
                StaticMessageHeaderAccessor.getAcknowledgmentCallback(m).noAutoAck();
                // e.g. hand off to another thread which can perform the ack
                // or acknowledge(Status.REQUEUE)

            })) {
                Thread.sleep(1000);
            }
        }
    };
}
You must ack (or nack) the message at some point, to avoid resource leaks.
Some messaging systems (such as Apache Kafka) maintain a simple offset in a log. If a delivery fails and is re-queued with StaticMessageHeaderAccessor.getAcknowledgmentCallback(m).acknowledge(Status.REQUEUE);, any later successfully ack’d messages are redelivered.

There is also an overloaded poll method, for which the definition is as follows:

poll(MessageHandler handler, ParameterizedTypeReference<?> type)

The type is a conversion hint that allows the incoming message payload to be converted, as shown in the following example:

boolean result = pollableSource.poll(received -> {
            Map<String, Foo> payload = (Map<String, Foo>) received.getPayload();
            ...

        }, new ParameterizedTypeReference<Map<String, Foo>>() {});
Handling Errors

By default, an error channel is configured for the pollable source; if the callback throws an exception, an ErrorMessage is sent to the error channel (<destination>.<group>.errors); this error channel is also bridged to the global Spring Integration errorChannel.

You can subscribe to either error channel with a @ServiceActivator to handle errors; without a subscription, the error will simply be logged and the message will be acknowledged as successful. If the error channel service activator throws an exception, the message will be rejected (by default) and won’t be redelivered. If the service activator throws a RequeueCurrentMessageException, the message will be requeued at the broker and will be again retrieved on a subsequent poll.

If the listener throws a RequeueCurrentMessageException directly, the message will be requeued, as discussed above, and will not be sent to the error channels.

107.4. Event Routing

Event Routing, in the context of Spring Cloud Stream, is the ability to either a) route events to a particular event subscriber or b) route events produced by an event subscriber to a particular destination. Here we’ll refer to it as route ‘TO’ and route ‘FROM’.

107.4.1. Routing TO Consumer

Routing can be achieved by relying on RoutingFunction available in Spring Cloud Function 3.0. All you need to do is enable it via --spring.cloud.stream.function.routing.enabled=true application property or provide spring.cloud.function.routing-expression property. Once enabled RoutingFunction will be bound to input destination receiving all the messages and route them to other functions based on the provided instruction.

For the purposes of binding the name of the routing destination is functionRouter-in-0 (see RoutingFunction.FUNCTION_NAME and binding naming convention Functional binding names).

Instruction could be provided with individual messages as well as application properties.

Here are couple of samples:

Using message headers
@SpringBootApplication
public class SampleApplication {

    public static void main(String[] args) {
        SpringApplication.run(SampleApplication.class,
                       "--spring.cloud.stream.function.routing.enabled=true");
    }

    @Bean
    public Consumer<String> even() {
        return value -> {
            System.out.println("EVEN: " + value);
        };
    }

    @Bean
    public Consumer<String> odd() {
        return value -> {
            System.out.println("ODD: " + value);
        };
    }
}

By sending a message to the functionRouter-in-0 destination exposed by the binder (i.e., rabbit, kafka), such message will be routed to the appropriate (‘even’ or ‘odd’) Consumer.

By default RoutingFunction will look for a spring.cloud.function.definition or spring.cloud.function.routing-expression (for more dynamic scenarios with SpEL) header and if it is found, its value will be treated as the routing instruction.

For example, setting spring.cloud.function.routing-expression header to value T(java.lang.System).currentTimeMillis() % 2 == 0 ? 'even' : 'odd' will end up semi-randomly routing request to either odd or even functions. Also, for SpEL, the root object of the evaluation context is Message so you can do evaluation on individual headers (or message) as well …​.routing-expression=headers['type']

Using application properties

The spring.cloud.function.routing-expression and/or spring.cloud.function.definition can be passed as application properties (e.g., spring.cloud.function.routing-expression=headers['type'].

@SpringBootApplication
public class RoutingStreamApplication {

  public static void main(String[] args) {
      SpringApplication.run(RoutingStreamApplication.class,
      "--spring.cloud.function.routing-expression="
      + "T(java.lang.System).nanoTime() % 2 == 0 ? 'even' : 'odd'");
  }
  @Bean
  public Consumer<Integer> even() {
    return value -> System.out.println("EVEN: " + value);
  }

  @Bean
  public Consumer<Integer> odd() {
    return value -> System.out.println("ODD: " + value);
  }
}
Passing instructions via application properties is especially important for reactive functions given that a reactive function is only invoked once to pass the Publisher, so access to the individual items is limited.

107.4.2. Routing FROM Consumer

Aside from static destinations, Spring Cloud Stream lets applications send messages to dynamically bound destinations. This is useful, for example, when the target destination needs to be determined at runtime. Applications can do so in one of two ways.

BinderAwareChannelResolver

The BinderAwareChannelResolver is a special bean registered automatically by the framework. You can autowire this bean into your application and use it to resolve output destination at runtime

The 'spring.cloud.stream.dynamicDestinations' property can be used for restricting the dynamic destination names to a known set (that is, intentionally allowed values). If this property is not set, any destination can be bound dynamically.

The following example demonstrates one of the common scenarios where REST controller uses a path variable to determine target destination:

@SpringBootApplication
@Controller
public class SourceWithDynamicDestination {

    @Autowired
    private BinderAwareChannelResolver resolver;

    @RequestMapping(value="/{target}")
    @ResponseStatus(HttpStatus.ACCEPTED)
    public void send(@RequestBody String body, @PathVariable("target") String target){
        resolver.resolveDestination(target).send(new GenericMessage<String>(body));
    }
}

Now consider what happens when we start the application on the default port (8080) and make the following requests with CURL:

curl -H "Content-Type: application/json" -X POST -d "customer-1" http://localhost:8080/customers

curl -H "Content-Type: application/json" -X POST -d "order-1" http://localhost:8080/orders

The destinations, 'customers' and 'orders', are created in the broker (in the exchange for Rabbit or in the topic for Kafka) with names of 'customers' and 'orders', and the data is published to the appropriate destinations.

spring.cloud.stream.sendto.destination

You can also delegate to the framework to dynamically resolve the output destination by specifying spring.cloud.stream.sendto.destination header set to the name of the destination to be resolved.

Consider the following example:

@SpringBootApplication
@Controller
public class SourceWithDynamicDestination {

    @Bean
    public Function<String, Message<String>> destinationAsPayload() {
        return value -> {
            return MessageBuilder.withPayload(value)
                .setHeader("spring.cloud.stream.sendto.destination", value).build();};
    }
}

Albeit trivial you can clearly see in this example, our output is a Message with spring.cloud.stream.sendto.destination header set to the value of he input argument. The framework will consult this header and will attempt to create or discover a destination with that name and send output to it.

If destination names are known in advance, you can configure the producer properties as with any other destination. Alternatively, if you register a NewDestinationBindingCallback<> bean, it is invoked just before the binding is created. The callback takes the generic type of the extended producer properties used by the binder. It has one method:

void configure(String destinationName, MessageChannel channel, ProducerProperties producerProperties,
        T extendedProducerProperties);

The following example shows how to use the RabbitMQ binder:

@Bean
public NewDestinationBindingCallback<RabbitProducerProperties> dynamicConfigurer() {
    return (name, channel, props, extended) -> {
        props.setRequiredGroups("bindThisQueue");
        extended.setQueueNameGroupOnly(true);
        extended.setAutoBindDlq(true);
        extended.setDeadLetterQueueName("myDLQ");
    };
}
If you need to support dynamic destinations with multiple binder types, use Object for the generic type and cast the extended argument as needed.

Also, please see Using StreamBridge section to see how yet another option (StreamBridge) can be utilized for similar cases.

107.5. Error Handling

In this section we’ll explain the general idea behind error handling mechanisms provided by the framework. We’ll be using Rabbit binder as an example, since individual binders define different set of properties for certain supported mechanisms specific to underlying broker capabilities (such as Kafka binder).

Errors happen, and Spring Cloud Stream provides several flexible mechanisms to deal with them. Note that the techniques are dependent on binder implementation and the capability of the underlying messaging middleware.

Whenever there is an exception during message processing, the framework will make several attempts at re-trying the same message (3 by default). For that, the framework uses Spring Retry library (for imperative functions and standard message handlers) and retryBackoff capabilities of the reactive API (for reactive functions).

Whenever a handler (function) throws an exception, it is propagated back to the binder, and the binder subsequently propagates the error back to the messaging system. Depending on the capabilities of the messaging system such system may drop the message, re-queue the message for re-processing or send the failed message to DLQ. Both Rabbit and Kafka support these concepts. However, other binders may not, so refer to your individual binder’s documentation for details on supported error-handling options.

107.5.1. Drop Failed Messages

By default, if no additional system-level configuration is provided, the messaging system drops the failed message. While acceptable in some cases, for most cases, it is not, and we need some recovery mechanism to avoid message loss.

107.5.2. DLQ - Dead Letter Queue

Perhaps the most common mechanism, DLQ allows failed messages to be sent to a special destination: the Dead Letter Queue.

When configured, failed messages are sent to this destination for subsequent re-processing or auditing and reconciliation.

Consider the following example:

@SpringBootApplication
public class SimpleStreamApplication {

    public static void main(String[] args) throws Exception {
        SpringApplication.run(SimpleStreamApplication.class,
          "--spring.cloud.function.definition=uppercase",
          "--spring.cloud.stream.bindings.uppercase-in-0.destination=uppercase",
          "--spring.cloud.stream.bindings.uppercase-in-0.group=myGroup",
          "--spring.cloud.stream.rabbit.bindings.uppercase-in-0.consumer.auto-bind-dlq=true"
        );
    }

    @Bean
    public Function<Person, Person> uppercase() {
        return personIn -> {
           throw new RuntimeException("intentional");
          });
        };
    }
}

As a reminder, in this example uppercase-in-0 segment of the property corresponds to the name of the input destination binding. The consumer segment indicates that it is a consumer property.

When using DLQ, at least the group property must be provided for proper naming of the DLQ destination. However group is often used together with destination property, as in our example.

Aside from some standard properties we also set the auto-bind-dlq to instruct the binder to create and configure DLQ destination for uppercase-in-0 binding which corresponds to uppercase destination (see corresponding property), which results in an additional Rabbit queue named uppercase.myGroup.dlq (see Kafka documentation for Kafka specific DLQ properties).

Once configured, all failed messages are routed to this destination preserving the original message for further actions.

And you can see that the error message contains more information relevant to the original error, as follows:

. . . .
x-exception-stacktrace: org.springframework.messaging.MessageHandlingException: nested exception is
      org.springframework.messaging.MessagingException: has an error, failedMessage=GenericMessage [payload=byte[15],
      headers={amqp_receivedDeliveryMode=NON_PERSISTENT, amqp_receivedRoutingKey=input.hello, amqp_deliveryTag=1,
      deliveryAttempt=3, amqp_consumerQueue=input.hello, amqp_redelivered=false, id=a15231e6-3f80-677b-5ad7-d4b1e61e486e,
      amqp_consumerTag=amq.ctag-skBFapilvtZhDsn0k3ZmQg, contentType=application/json, timestamp=1522327846136}]
      at org.spring...integ...han...MethodInvokingMessageProcessor.processMessage(MethodInvokingMessageProcessor.java:107)
      at. . . . .
Payload: blah

You can also facilitate immediate dispatch to DLQ (without re-tries) by setting max-attempts to '1'. For example,

--spring.cloud.stream.bindings.uppercase-in-0.consumer.max-attempts=1

107.5.3. Retry Template and retryBackoff

In this section we cover configuration properties relevant to configuration of retry capabilities. Given that we use two different mechanisms for imperative and reactive handlers (RetryTemplate and retryBackoff), properties that correspond to both will be identified as such.

The RetryTemplate is part of the Spring Retry library. While it is out of scope of this document to cover all of the capabilities of the RetryTemplate, we will mention the following consumer properties that are specifically related to the RetryTemplate:

maxAttempts

The number of attempts to process the message.

Default: 3. - Applies to 'retryBackoff'

backOffInitialInterval

The backoff initial interval on retry.

Default 1000 milliseconds. - Applies to 'retryBackoff'

backOffMaxInterval

The maximum backoff interval.

Default 10000 milliseconds. - Applies to 'retryBackoff'

backOffMultiplier

The backoff multiplier.

Default 2.0.

defaultRetryable

Whether exceptions thrown by the listener that are not listed in the retryableExceptions are retryable.

Default: true.

retryableExceptions

A map of Throwable class names in the key and a boolean in the value. Specify those exceptions (and subclasses) that will or won’t be retried. Also see defaultRetriable. Example: spring.cloud.stream.bindings.input.consumer.retryable-exceptions.java.lang.IllegalStateException=false.

Default: empty.

While the preceding settings are sufficient for the majority of the customization requirements, they may not satisfy certain complex requirements, at which point you may want to provide your own instance of the RetryTemplate. To do so configure it as a bean in your application configuration. The application provided instance will override the one provided by the framework. Also, to avoid conflicts you must qualify the instance of the RetryTemplate you want to be used by the binder as @StreamRetryTemplate. For example,

@StreamRetryTemplate
public RetryTemplate myRetryTemplate() {
    return new RetryTemplate();
}

As you can see from the above example you don’t need to annotate it with @Bean since @StreamRetryTemplate is a qualified @Bean.

If you need to be more precise with your RetryTemplate, you can specify the bean by name in your ConsumerProperties to associate the specific retry bean per binding.

spring.cloud.stream.bindings.<foo>.consumer.retry-template-name=<your-retry-template-bean-name>

108. Binders

Spring Cloud Stream provides a Binder abstraction for use in connecting to physical destinations at the external middleware. This section provides information about the main concepts behind the Binder SPI, its main components, and implementation-specific details.

108.1. Producers and Consumers

The following image shows the general relationship of producers and consumers:

producers consumers
Figure 11. Producers and Consumers

A producer is any component that sends messages to a binding destination. The binding destination can be bound to an external message broker with a Binder implementation for that broker. When invoking the bindProducer() method, the first parameter is the name of the destination within the broker, the second parameter is the instance if local destination to which the producer sends messages, and the third parameter contains properties (such as a partition key expression) to be used within the adapter that is created for that binding destination.

A consumer is any component that receives messages from the binding destination. As with a producer, the consumer can be bound to an external message broker. When invoking the bindConsumer() method, the first parameter is the destination name, and a second parameter provides the name of a logical group of consumers. Each group that is represented by consumer bindings for a given destination receives a copy of each message that a producer sends to that destination (that is, it follows normal publish-subscribe semantics). If there are multiple consumer instances bound with the same group name, then messages are load-balanced across those consumer instances so that each message sent by a producer is consumed by only a single consumer instance within each group (that is, it follows normal queueing semantics).

108.2. Binder SPI

The Binder SPI consists of a number of interfaces, out-of-the box utility classes, and discovery strategies that provide a pluggable mechanism for connecting to external middleware.

The key point of the SPI is the Binder interface, which is a strategy for connecting inputs and outputs to external middleware. The following listing shows the definition of the Binder interface:

public interface Binder<T, C extends ConsumerProperties, P extends ProducerProperties> {
    Binding<T> bindConsumer(String bindingName, String group, T inboundBindTarget, C consumerProperties);

    Binding<T> bindProducer(String bindingName, T outboundBindTarget, P producerProperties);
}

The interface is parameterized, offering a number of extension points:

  • Input and output bind targets.

  • Extended consumer and producer properties, allowing specific Binder implementations to add supplemental properties that can be supported in a type-safe manner.

A typical binder implementation consists of the following:

  • A class that implements the Binder interface;

  • A Spring @Configuration class that creates a bean of type Binder along with the middleware connection infrastructure.

  • A META-INF/spring.binders file found on the classpath containing one or more binder definitions, as shown in the following example:

    kafka:\
    org.springframework.cloud.stream.binder.kafka.config.KafkaBinderConfiguration
As it was mentioned earlier Binder abstraction is also one of the extension points of the framework. So if you can’t find a suitable binder in the preceding list you can implement your own binder on top of Spring Cloud Stream. In the How to create a Spring Cloud Stream Binder from scratch post a community member documents in details, with an example, a set of steps necessary to implement a custom binder. The steps are also highlighted in the Implementing Custom Binders section.

108.3. Binder Detection

Spring Cloud Stream relies on implementations of the Binder SPI to perform the task of connecting (binding) user code to message brokers. Each Binder implementation typically connects to one type of messaging system.

108.3.1. Classpath Detection

By default, Spring Cloud Stream relies on Spring Boot’s auto-configuration to configure the binding process. If a single Binder implementation is found on the classpath, Spring Cloud Stream automatically uses it. For example, a Spring Cloud Stream project that aims to bind only to RabbitMQ can add the following dependency:

<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-stream-binder-rabbit</artifactId>
</dependency>

For the specific Maven coordinates of other binder dependencies, see the documentation of that binder implementation.

108.4. Multiple Binders on the Classpath

When multiple binders are present on the classpath, the application must indicate which binder is to be used for each destination binding. Each binder configuration contains a META-INF/spring.binders file, which is a simple properties file, as shown in the following example:

rabbit:\
org.springframework.cloud.stream.binder.rabbit.config.RabbitServiceAutoConfiguration

Similar files exist for the other provided binder implementations (such as Kafka), and custom binder implementations are expected to provide them as well. The key represents an identifying name for the binder implementation, whereas the value is a comma-separated list of configuration classes that each contain one and only one bean definition of type org.springframework.cloud.stream.binder.Binder.

Binder selection can either be performed globally, using the spring.cloud.stream.defaultBinder property (for example, spring.cloud.stream.defaultBinder=rabbit) or individually, by configuring the binder on each binding. For instance, a processor application (that has bindings named input and output for read and write respectively) that reads from Kafka and writes to RabbitMQ can specify the following configuration:

spring.cloud.stream.bindings.input.binder=kafka
spring.cloud.stream.bindings.output.binder=rabbit

108.5. Connecting to Multiple Systems

By default, binders share the application’s Spring Boot auto-configuration, so that one instance of each binder found on the classpath is created. If your application should connect to more than one broker of the same type, you can specify multiple binder configurations, each with different environment settings.

Turning on explicit binder configuration disables the default binder configuration process altogether. If you do so, all binders in use must be included in the configuration. Frameworks that intend to use Spring Cloud Stream transparently may create binder configurations that can be referenced by name, but they do not affect the default binder configuration. In order to do so, a binder configuration may have its defaultCandidate flag set to false (for example, spring.cloud.stream.binders.<configurationName>.defaultCandidate=false). This denotes a configuration that exists independently of the default binder configuration process.

The following example shows a typical configuration for a processor application that connects to two RabbitMQ broker instances:

spring:
  cloud:
    stream:
      bindings:
        input:
          destination: thing1
          binder: rabbit1
        output:
          destination: thing2
          binder: rabbit2
      binders:
        rabbit1:
          type: rabbit
          environment:
            spring:
              rabbitmq:
                host: <host1>
        rabbit2:
          type: rabbit
          environment:
            spring:
              rabbitmq:
                host: <host2>
The environment property of the particular binder can also be used for any Spring Boot property, including this spr