Spring Cloud Consul

3.1.1

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 and Client Side Load Balancing, Circuit Breaker are provided by integration with other Spring Cloud projects.

1. Quick Start

This quick start walks through using Spring Cloud Consul for Service Discovery and Distributed Configuration.

First, run Consul Agent on your machine. Then you can access it and use it as a Service Registry and Configuration source with Spring Cloud Consul.

1.1. Discovery Client Usage

To use these features in an application, you can build it as a Spring Boot application that depends on spring-cloud-consul-core. The most convenient way to add the dependency is with a Spring Boot starter: org.springframework.cloud:spring-cloud-starter-consul-discovery. We recommend using dependency management and spring-boot-starter-parent. The following example shows a typical Maven configuration:

pom.xml

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

  <dependencies>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-consul-discovery</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>
  <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>
  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
    </plugins>
  </build>
</project>

The following example shows a typical Gradle setup:

build.gradle

plugins {
  id 'org.springframework.boot' version ${spring-boot-version}
  id 'io.spring.dependency-management' version ${spring-dependency-management-version}
  id 'java'
}

repositories {
  mavenCentral()
}

dependencies {
  implementation 'org.springframework.cloud:spring-cloud-starter-consul-discovery'
  testImplementation 'org.springframework.boot:spring-boot-starter-test'
}
dependencyManagement {
  imports {
    mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
  }
}

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

@SpringBootApplication
@RestController
public class Application {

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

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

}

When this HTTP server runs, it connects to Consul Agent running at the default local 8500 port. To modify the startup behavior, you can change the location of Consul Agent by using application.properties, as shown in the following example:

spring:
  cloud:
    consul:
      host: localhost
      port: 8500

You can now use DiscoveryClient, @LoadBalanced RestTemplate, or @LoadBalanced WebClient.Builder to retrieve services and instances data from Consul, 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().toString();
    }
    return null;
}

1.2. Distributed Configuration Usage

To use these features in an application, you can build it as a Spring Boot application that depends on spring-cloud-consul-core and spring-cloud-consul-config. The most convenient way to add the dependency is with a Spring Boot starter: org.springframework.cloud:spring-cloud-starter-consul-config. We recommend using dependency management and spring-boot-starter-parent. The following example shows a typical Maven configuration:

pom.xml

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

  <dependencies>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-consul-config</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>
  <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>
  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
    </plugins>
  </build>
</project>

The following example shows a typical Gradle setup:

build.gradle

plugins {
  id 'org.springframework.boot' version ${spring-boot-version}
  id 'io.spring.dependency-management' version ${spring-dependency-management-version}
  id 'java'
}

repositories {
  mavenCentral()
}

dependencies {
  implementation 'org.springframework.cloud:spring-cloud-starter-consul-config'
  testImplementation 'org.springframework.boot:spring-boot-starter-test'
}
dependencyManagement {
  imports {
    mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
  }
}

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

@SpringBootApplication
@RestController
public class Application {

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

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

}

The application retrieves configuration data from Consul.

If you use Spring Cloud Consul Config, you need to set the spring.config.import property in order to bind to Consul. You can read more about it in the Spring Boot Config Data Import section.

2. Install Consul

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

3. 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

4. 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.

4.1. 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.

4.2. 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.

4.2.1. 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

4.2.2. 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"

4.2.3. 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.

4.2.4. 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 1. 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.

4.2.5. 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.

4.3. Looking up services

4.3.1. 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.

4.3.2. 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;
}

4.4. 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.

5. 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 mimics 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.

5.1. 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.

5.2. 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.

5.3. 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

5.4. 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.

5.5. 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

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.

5.6. 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.

5.7. 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.

6. 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.

7. Spring Cloud Bus with Consul

7.1. 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.

8. 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.

9. 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);
    }
}

10. Configuration Properties

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