This project provides Zookeeper integrations for Spring Boot applications through autoconfiguration and binding to the Spring Environment and other Spring programming model idioms. With a few annotations, you can quickly enable and configure the common patterns inside your application and build large distributed systems with Zookeeper based components. The provided patterns include Service Discovery and Configuration. Integration with Spring Cloud Netflix provides Intelligent Routing (Zuul), Client Side Load Balancing (Ribbon), and Circuit Breaker (Hystrix).
1. Quick Start
This quick start walks through using Spring Cloud Zookeeper for Service Discovery and Distributed Configuration.
First, run Zookeeper on your machine. Then you can access it and use it as a Service Registry and Configuration source with Spring Cloud Zookeeper.
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-zookeeper-core
and spring-cloud-zookeeper-discovery
.
The most convenient way to add the dependency is with a Spring Boot starter: org.springframework.cloud:spring-cloud-starter-zookeeper-discovery
.
We recommend using dependency management and spring-boot-starter-parent
.
The following example shows a typical Maven configuration:
<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-zookeeper-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:
plugins {
id 'org.springframework.boot' version ${spring-boot-version}
id 'io.spring.dependency-management' version ${spring-dependency-management-version}
id 'java'
}
repositories {
mavenCentral()
}
dependencies {
implementation 'org.springframework.cloud:spring-cloud-starter-zookeeper-discovery'
testImplementation 'org.springframework.boot:spring-boot-starter-test'
}
dependencyManagement {
imports {
mavenBom "org.springframework.cloud:spring-cloud-dependencies:${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 Zookeeper, which runs on the default local port (2181).
To modify the startup behavior, you can change the location of Zookeeper by using application.properties
, as shown in the following example:
spring: cloud: zookeeper: connect-string: localhost:2181
You can now use DiscoveryClient
, @LoadBalanced RestTemplate
, or @LoadBalanced WebClient.Builder
to retrieve services and instances data from Zookeeper, 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-zookeeper-core
and spring-cloud-zookeeper-config
.
The most convenient way to add the dependency is with a Spring Boot starter: org.springframework.cloud:spring-cloud-starter-zookeeper-config
.
We recommend using dependency management and spring-boot-starter-parent
.
The following example shows a typical Maven configuration:
<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-zookeeper-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:
plugins {
id 'org.springframework.boot' version ${spring-boot-version}
id 'io.spring.dependency-management' version ${spring-dependency-management-version}
id 'java'
}
repositories {
mavenCentral()
}
dependencies {
implementation 'org.springframework.cloud:spring-cloud-starter-zookeeper-config'
testImplementation 'org.springframework.boot:spring-boot-starter-test'
}
dependencyManagement {
imports {
mavenBom "org.springframework.cloud:spring-cloud-dependencies:${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 Zookeeper.
2. Install Zookeeper
See the installation documentation for instructions on how to install Zookeeper.
Spring Cloud Zookeeper uses Apache Curator behind the scenes. While Zookeeper 3.5.x is still considered "beta" by the Zookeeper development team, the reality is that it is used in production by many users. However, Zookeeper 3.4.x is also used in production. Prior to Apache Curator 4.0, both versions of Zookeeper were supported via two versions of Apache Curator. Starting with Curator 4.0 both versions of Zookeeper are supported via the same Curator libraries.
In case you are integrating with version 3.4 you need to change the Zookeeper dependency
that comes shipped with curator
, and thus spring-cloud-zookeeper
.
To do so simply exclude that dependency and add the 3.4.x version like shown below.
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-zookeeper-all</artifactId>
<exclusions>
<exclusion>
<groupId>org.apache.zookeeper</groupId>
<artifactId>zookeeper</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.apache.zookeeper</groupId>
<artifactId>zookeeper</artifactId>
<version>3.4.12</version>
<exclusions>
<exclusion>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
</exclusion>
</exclusions>
</dependency>
compile('org.springframework.cloud:spring-cloud-starter-zookeeper-all') {
exclude group: 'org.apache.zookeeper', module: 'zookeeper'
}
compile('org.apache.zookeeper:zookeeper:3.4.12') {
exclude group: 'org.slf4j', module: 'slf4j-log4j12'
}
3. Service Discovery with Zookeeper
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. Curator(A Java library for Zookeeper) provides Service Discovery through a Service Discovery Extension. Spring Cloud Zookeeper uses this extension for service registration and discovery.
3.1. Activating
Including a dependency on
org.springframework.cloud:spring-cloud-starter-zookeeper-discovery
enables
autoconfiguration that sets up Spring Cloud Zookeeper Discovery.
For web functionality, you still need to include
org.springframework.boot:spring-boot-starter-web .
|
When working with version 3.4 of Zookeeper you need to change the way you include the dependency as described here. |
3.2. Registering with Zookeeper
When a client registers with Zookeeper, it provides metadata (such as host and port, ID, and name) about itself.
The following example shows a Zookeeper 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);
}
}
The preceding example is a normal Spring Boot application. |
If Zookeeper is located somewhere other than localhost:2181
, the configuration must
provide the location of the server, as shown in the following example:
spring:
cloud:
zookeeper:
connect-string: localhost:2181
If you use Spring Cloud Zookeeper Config, the
values shown in the preceding example need to be 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.
Having spring-cloud-starter-zookeeper-discovery
on the classpath makes the app into both
a Zookeeper “service” (that is, it registers itself) and a “client” (that is, it can
query Zookeeper to locate other services).
If you would like to disable the Zookeeper Discovery Client, you can set
spring.cloud.zookeeper.discovery.enabled
to false
.
3.3. Using the DiscoveryClient
Spring Cloud has support for
Feign
(a REST client builder),
Spring
RestTemplate
and
Spring WebFlux, using logical service names instead of physical URLs.
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, 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;
}
4. Using Spring Cloud Zookeeper with Spring Cloud Netflix Components
Spring Cloud Netflix supplies useful tools that work regardless of which DiscoveryClient
implementation you use. Feign, Turbine, Ribbon, and Zuul all work with Spring Cloud
Zookeeper.
4.1. Ribbon with Zookeeper
Spring Cloud Zookeeper provides an implementation of Ribbon’s ServerList
. When you use
the spring-cloud-starter-zookeeper-discovery
, Ribbon is autoconfigured to use the
ZookeeperServerList
by default.
5. Spring Cloud Zookeeper and Service Registry
Spring Cloud Zookeeper implements the ServiceRegistry
interface, letting developers
register arbitrary services in a programmatic way.
The ServiceInstanceRegistration
class offers a builder()
method to create a
Registration
object that can be used by the ServiceRegistry
, as shown in the following
example:
@Autowired
private ZookeeperServiceRegistry serviceRegistry;
public void registerThings() {
ZookeeperRegistration registration = ServiceInstanceRegistration.builder()
.defaultUriSpec()
.address("anyUrl")
.port(10)
.name("/a/b/c/d/anotherservice")
.build();
this.serviceRegistry.register(registration);
}
5.1. Instance Status
Netflix Eureka supports having instances that are OUT_OF_SERVICE
registered with the
server. These instances are not returned as active service instances. This is useful for
behaviors such as blue/green deployments. (Note that the Curator Service Discovery recipe
does not support this behavior.) Taking advantage of the flexible payload has let Spring
Cloud Zookeeper implement OUT_OF_SERVICE
by updating some specific metadata and then
filtering on that metadata in the Ribbon ZookeeperServerList
. The ZookeeperServerList
filters out all non-null instance statuses that do not equal UP
. If the instance status
field is empty, it is considered to be UP
for backwards compatibility. To change the
status of an instance, make a POST
with OUT_OF_SERVICE
to the ServiceRegistry
instance status actuator endpoint, as shown in the following example:
$ http POST http://localhost:8081/service-registry status=OUT_OF_SERVICE
The preceding example uses the http command from httpie.org.
|
6. Zookeeper Dependencies
The following topics cover how to work with Spring Cloud Zookeeper dependencies:
6.1. Using the Zookeeper Dependencies
Spring Cloud Zookeeper gives you a possibility to provide dependencies of your application
as properties. As dependencies, you can understand other applications that are registered
in Zookeeper and which you would like to call through
Feign
(a REST client builder),
Spring
RestTemplate
and
Spring WebFlux.
You can also use the Zookeeper Dependency Watchers functionality to control and monitor the state of your dependencies.
6.2. Activating Zookeeper Dependencies
Including a dependency on
org.springframework.cloud:spring-cloud-starter-zookeeper-discovery
enables
autoconfiguration that sets up Spring Cloud Zookeeper Dependencies. Even if you provide
the dependencies in your properties, you can turn off the dependencies. To do so, set the
spring.cloud.zookeeper.dependency.enabled
property to false (it defaults to true
).
6.3. Setting up Zookeeper Dependencies
Consider the following example of dependency representation:
spring.application.name: yourServiceName
spring.cloud.zookeeper:
dependencies:
newsletter:
path: /path/where/newsletter/has/registered/in/zookeeper
loadBalancerType: ROUND_ROBIN
contentTypeTemplate: application/vnd.newsletter.$version+json
version: v1
headers:
header1:
- value1
header2:
- value2
required: false
stubs: org.springframework:foo:stubs
mailing:
path: /path/where/mailing/has/registered/in/zookeeper
loadBalancerType: ROUND_ROBIN
contentTypeTemplate: application/vnd.mailing.$version+json
version: v1
required: true
The next few sections go through each part of the dependency one by one. The root property
name is spring.cloud.zookeeper.dependencies
.
6.3.1. Aliases
Below the root property you have to represent each dependency as an alias. This is due to
the constraints of Ribbon, which requires that the application ID be placed in the URL.
Consequently, you cannot pass any complex path, suchas /myApp/myRoute/name
). The alias
is the name you use instead of the serviceId
for DiscoveryClient
, Feign
, or
RestTemplate
.
In the previous examples, the aliases are newsletter
and mailing
. The following
example shows Feign usage with a newsletter
alias:
@FeignClient("newsletter")
public interface NewsletterService {
@RequestMapping(method = RequestMethod.GET, value = "/newsletter")
String getNewsletters();
}
6.3.2. Path
The path is represented by the path
YAML property and is the path under which the
dependency is registered under Zookeeper. As described in the
previous section, Ribbon
operates on URLs. As a result, this path is not compliant with its requirement.
That is why Spring Cloud Zookeeper maps the alias to the proper path.
6.3.3. Load Balancer Type
The load balancer type is represented by loadBalancerType
YAML property.
If you know what kind of load-balancing strategy has to be applied when calling this particular dependency, you can provide it in the YAML file, and it is automatically applied. You can choose one of the following load balancing strategies:
-
STICKY: Once chosen, the instance is always called.
-
RANDOM: Picks an instance randomly.
-
ROUND_ROBIN: Iterates over instances over and over again.
6.3.4. Content-Type
Template and Version
The Content-Type
template and version are represented by the contentTypeTemplate
and
version
YAML properties.
If you version your API in the Content-Type
header, you do not want to add this header
to each of your requests. Also, if you want to call a new version of the API, you do not
want to roam around your code to bump up the API version. That is why you can provide a
contentTypeTemplate
with a special $version
placeholder. That placeholder will be filled by the value of the
version
YAML property. Consider the following example of a contentTypeTemplate
:
application/vnd.newsletter.$version+json
Further consider the following version
:
v1
The combination of contentTypeTemplate
and version results in the creation of a
Content-Type
header for each request, as follows:
application/vnd.newsletter.v1+json
6.3.5. Default Headers
Default headers are represented by the headers
map in YAML.
Sometimes, each call to a dependency requires setting up of some default headers. To not
do that in code, you can set them up in the YAML file, as shown in the following example
headers
section:
headers:
Accept:
- text/html
- application/xhtml+xml
Cache-Control:
- no-cache
That headers
section results in adding the Accept
and Cache-Control
headers with
appropriate list of values in your HTTP request.
6.3.6. Required Dependencies
Required dependencies are represented by required
property in YAML.
If one of your dependencies is required to be up when your application boots, you can set
the required: true
property in the YAML file.
If your application cannot localize the required dependency during boot time, it throws an exception, and the Spring Context fails to set up. In other words, your application cannot start if the required dependency is not registered in Zookeeper.
You can read more about Spring Cloud Zookeeper Presence Checker later in this document.
6.3.7. Stubs
You can provide a colon-separated path to the JAR containing stubs of the dependency, as shown in the following example:
stubs: org.springframework:myApp:stubs
where:
-
org.springframework
is thegroupId
. -
myApp
is theartifactId
. -
stubs
is the classifier. (Note thatstubs
is the default value.)
Because stubs
is the default classifier, the preceding example is equal to the following
example:
stubs: org.springframework:myApp
6.4. Configuring Spring Cloud Zookeeper Dependencies
You can set the following properties to enable or disable parts of Zookeeper Dependencies functionalities:
-
spring.cloud.zookeeper.dependencies
: If you do not set this property, you cannot use Zookeeper Dependencies. -
spring.cloud.zookeeper.dependency.ribbon.enabled
(enabled by default): Ribbon requires either explicit global configuration or a particular one for a dependency. By turning on this property, runtime load balancing strategy resolution is possible, and you can use theloadBalancerType
section of the Zookeeper Dependencies. The configuration that needs this property has an implementation ofLoadBalancerClient
that delegates to theILoadBalancer
presented in the next bullet. -
spring.cloud.zookeeper.dependency.ribbon.loadbalancer
(enabled by default): Thanks to this property, the customILoadBalancer
knows that the part of the URI passed to Ribbon might actually be the alias that has to be resolved to a proper path in Zookeeper. Without this property, you cannot register applications under nested paths. -
spring.cloud.zookeeper.dependency.headers.enabled
(enabled by default): This property registers aRibbonClient
that automatically appends appropriate headers and content types with their versions, as presented in the Dependency configuration. Without this setting, those two parameters do not work. -
spring.cloud.zookeeper.dependency.resttemplate.enabled
(enabled by default): When enabled, this property modifies the request headers of a@LoadBalanced
-annotatedRestTemplate
such that it passes headers and content type with the version set in dependency configuration. Without this setting, those two parameters do not work.
7. Spring Cloud Zookeeper Dependency Watcher
The Dependency Watcher mechanism lets you register listeners to your dependencies. The
functionality is, in fact, an implementation of the Observator
pattern. When a
dependency changes, its state (to either UP or DOWN), some custom logic can be applied.
7.1. Activating
Spring Cloud Zookeeper Dependencies functionality needs to be enabled for you to use the Dependency Watcher mechanism.
7.2. Registering a Listener
To register a listener, you must implement an interface called
org.springframework.cloud.zookeeper.discovery.watcher.DependencyWatcherListener
and
register it as a bean. The interface gives you one method:
void stateChanged(String dependencyName, DependencyState newState);
If you want to register a listener for a particular dependency, the dependencyName
would
be the discriminator for your concrete implementation. newState
provides you with
information about whether your dependency has changed to CONNECTED
or DISCONNECTED
.
7.3. Using the Presence Checker
Bound with the Dependency Watcher is the functionality called Presence Checker. It lets you provide custom behavior when your application boots, to react according to the state of your dependencies.
The default implementation of the abstract
org.springframework.cloud.zookeeper.discovery.watcher.presence.DependencyPresenceOnStartupVerifier
class is the
org.springframework.cloud.zookeeper.discovery.watcher.presence.DefaultDependencyPresenceOnStartupVerifier
,
which works in the following way.
-
If the dependency is marked us
required
and is not in Zookeeper, when your application boots, it throws an exception and shuts down. -
If the dependency is not
required
, theorg.springframework.cloud.zookeeper.discovery.watcher.presence.LogMissingDependencyChecker
logs that the dependency is missing at theWARN
level.
Because the DefaultDependencyPresenceOnStartupVerifier
is registered only when there is
no bean of type DependencyPresenceOnStartupVerifier
, this functionality can be
overridden.
8. Distributed Configuration with Zookeeper
Zookeeper provides a
hierarchical namespace
that lets clients store arbitrary data, such as configuration data. Spring Cloud Zookeeper
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
namespace by default. Multiple
PropertySource
instances are created, based on the application’s name and the active
profiles, to mimic the Spring Cloud Config order of resolving properties. For example, an
application with a name of testApp
and with the dev
profile has the following property
sources created for it:
-
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
namespace apply to all applications that use
zookeeper for configuration. Properties in the config/testApp
namespace are available
only to the instances of the service named testApp
.
Configuration is currently read on startup of the application. Sending a HTTP POST
request to /refresh
causes the configuration to be reloaded. Watching the configuration
namespace (which Zookeeper supports) is not currently implemented.
8.1. Activating
Including a dependency on
org.springframework.cloud:spring-cloud-starter-zookeeper-config
enables
autoconfiguration that sets up Spring Cloud Zookeeper Config.
When working with version 3.4 of Zookeeper you need to change the way you include the dependency as described here. |
8.2. Customizing
Zookeeper Config may be customized by setting the following properties:
spring:
cloud:
zookeeper:
config:
enabled: true
root: configuration
defaultContext: apps
profileSeparator: '::'
-
enabled
: Setting this value tofalse
disables Zookeeper Config. -
root
: Sets the base namespace for configuration values. -
defaultContext
: Sets the name used by all applications. -
profileSeparator
: Sets the value of the separator used to separate the profile name in property sources with profiles.
8.3. Access Control Lists (ACLs)
You can add authentication information for Zookeeper ACLs by calling the addAuthInfo
method of a CuratorFramework
bean. One way to accomplish this is to provide your own
CuratorFramework
bean, as shown in the following example:
@BoostrapConfiguration
public class CustomCuratorFrameworkConfig {
@Bean
public CuratorFramework curatorFramework() {
CuratorFramework curator = new CuratorFramework();
curator.addAuthInfo("digest", "user:password".getBytes());
return curator;
}
}
Consult
the ZookeeperAutoConfiguration class
to see how the CuratorFramework
bean’s default configuration.
Alternatively, you can add your credentials from a class that depends on the existing
CuratorFramework
bean, as shown in the following example:
@BoostrapConfiguration
public class DefaultCuratorFrameworkConfig {
public ZookeeperConfig(CuratorFramework curator) {
curator.addAuthInfo("digest", "user:password".getBytes());
}
}
The creation of this bean must occur during the boostrapping phase. You can register
configuration classes to run during this phase by annotating them with
@BootstrapConfiguration
and including them in a comma-separated list that you set as the
value of the org.springframework.cloud.bootstrap.BootstrapConfiguration
property in the
resources/META-INF/spring.factories
file, as shown in the following example:
org.springframework.cloud.bootstrap.BootstrapConfiguration=\ my.project.CustomCuratorFrameworkConfig,\ my.project.DefaultCuratorFrameworkConfig