2. Service Discovery with Zookeeper

Including a dependency on org.springframework.cloud:spring-cloud-starter-zookeeper-discovery enables autoconfiguration that sets up Spring Cloud Zookeeper Discovery.

	Note
	For web functionality, you still need to include org.springframework.boot:spring-boot-starter-web.

	Caution
	When working with version 3.4 of Zookeeper you need to change the way you include the dependency as described here.

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);
    }

}

	Note
	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:

application.yml. 

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

	Caution
	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.

Spring Cloud has support for Feign (a REST client builder) and Spring RestTemplate, 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;
}