Including a dependency on org.springframework.cloud:spring-cloud-starter-zookeeper-discovery will enable auto-configuration that will setup Spring Cloud Zookeeper Discovery.

	Note
	You still need to include org.springframework.boot:spring-boot-starter-web for web functionality.

When a client registers with Zookeeper, it provides meta-data about itself such as host and port, id and name.

Example 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);
    }

}

(i.e. utterly normal Spring Boot app). If Zookeeper is located somewhere other than localhost:2181, the configuration is required to locate the server. Example:

application.yml. 

spring:
  cloud:
    zookeeper:
      connect-string: localhost:2181

	Caution
	If you use Spring Cloud Zookeeper Config, 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.

Having spring-cloud-starter-zookeeper-discovery on the classpath makes the app into both a Zookeeper "service" (i.e. it registers itself) and a "client" (i.e. 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.

Spring Cloud has support for Feign (a REST client builder) and also Spring RestTemplate using the 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, 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().toString();
    }
    return null;
}

Spring Cloud Zookeeper provides an implementation of Ribbon’s ServerList. When the spring-cloud-starter-zookeeper-discovery is used, Ribbon is auto-configured to use the ZookeeperServerList by default.

Netflix Eureka supports having instances registered with the server that are OUT_OF_SERVICE and not returned as active service instances. This is very useful for behaviors such as blue/green deployments. 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 UP for backwards compatibility. To change the status of an instance POST OUT_OF_SERVICE to the ServiceRegistry instance status actuator endpoint.

----
$ echo -n OUT_OF_SERVICE | http POST http://localhost:8081/service-registry/instance-status
----

NOTE: The above example uses the `http` command from https://httpie.org

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 via Feign (a REST client builder) and also Spring RestTemplate.

You can also benefit from the Zookeeper Dependency Watchers functionality that lets you control and monitor what is the state of your dependencies and decide what to do with that.

Let’s take a closer look at an example of dependencies representation:

application.yml. 

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

Let’s now go through each part of the dependency one by one. The root property name is spring.cloud.zookeeper.dependencies.

@FeignClient("newsletter")
public interface NewsletterService {
        @RequestMapping(method = RequestMethod.GET, value = "/newsletter")
        String getNewsletters();
}

application/vnd.newsletter.$version+json

v1

application/vnd.newsletter.v1+json

headers:
    Accept:
        - text/html
        - application/xhtml+xml
    Cache-Control:
        - no-cache

stubs: org.springframework:foo:stubs

stubs: org.springframework:foo

There is a bunch of properties that you can set to enable / disable parts of Zookeeper Dependencies functionalities.

Spring Cloud Zookeeper Dependencies functionality needs to be enabled to profit from Dependency Watcher mechanism.

In order to register a listener you have to implement an interface 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 then the dependencyName would be the discriminator for your concrete implementation. newState will provide you with information whether your dependency has changed to CONNECTED or DISCONNECTED.

Bound with Dependency Watcher is the functionality called Presence Checker. It allows you to provide custom behaviour upon booting of your application to react accordingly 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.

The functionality can be overriden since the DefaultDependencyPresenceOnStartupVerifier is registered only when there is no bean of DependencyPresenceOnStartupVerifier.

Including a dependency on org.springframework.cloud:spring-cloud-starter-zookeeper-config will enable auto-configuration that will setup Spring Cloud Zookeeper Config.

Zookeeper Config may be customized using the following properties:

bootstrap.yml. 

spring:
  cloud:
    zookeeper:
      config:
        enabled: true
        root: configuration
        defaultContext: apps
        profileSeparator: '::'

You can add authentication information for Zookeeper ACLs by calling the addAuthInfo method of a CuratorFramework bean. One way to accomplish this is by providing your own CuratorFramework bean:

@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 is configured by default.

Alternatively, you can add your credentials from a class that depends on the existing CuratorFramework bean:

@BoostrapConfiguration
public class DefaultCuratorFrameworkConfig {

  public ZookeeperConfig(CuratorFramework curator) {
    curator.addAuthInfo("digest", "user:password".getBytes());
  }

}

This 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 set as the value of the property org.springframework.cloud.bootstrap.BootstrapConfiguration in the file resources/META-INF/spring.factories:

resources/META-INF/spring.factories. 

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
my.project.CustomCuratorFrameworkConfig,\
my.project.DefaultCuratorFrameworkConfig