Spring Cloud Zookeeper Tutorial

Overview

Spring Cloud Zookeeper provides integration with Apache Zookeeper for service discovery and configuration management. Zookeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services.

Key Features of Spring Cloud Zookeeper

Spring Cloud Zookeeper offers several features that facilitate building and deploying microservices:

• Service Discovery: Automatically register and discover services within a microservices architecture.
• Configuration Management: Manage configuration for multiple applications in a centralized and flexible manner.
• Distributed Coordination: Use Zookeeper for distributed synchronization and coordination.

Setting Up Zookeeper

To set up a local Zookeeper instance, follow these steps:

$ wget https://apache.mirror.digitalpacific.com.au/zookeeper/zookeeper-3.7.0/apache-zookeeper-3.7.0-bin.tar.gz
$ tar -xzf apache-zookeeper-3.7.0-bin.tar.gz
$ cd apache-zookeeper-3.7.0-bin
$ ./bin/zkServer.sh start

This starts a Zookeeper server locally.

Setting Up Spring Cloud Zookeeper Client

To set up a Spring Cloud Zookeeper client, follow these steps:

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

// application.properties
spring.application.name=zookeeper-client
spring.cloud.zookeeper.connect-string=localhost:2181

The application.properties file configures the client to connect to the Zookeeper server and sets the application name.

Service Registration and Discovery

Once the Zookeeper client is configured, it will automatically register itself with the Zookeeper server and can discover other registered services.

// ExampleService.java
@Service
public class ExampleService {
    private final RestTemplate restTemplate;

    @Autowired
    public ExampleService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    public String callAnotherService() {
        return restTemplate.getForObject("http://another-service/endpoint", String.class);
    }
}

// RestTemplateConfig.java
@Configuration
public class RestTemplateConfig {
    @Bean
    @LoadBalanced
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

In this example, the ExampleService class uses a load-balanced RestTemplate to call another service registered with Zookeeper.

Configuration Management

Zookeeper can be used for centralized configuration management. Create a configuration in Zookeeper:

$ zkCli.sh
[zk: localhost:2181(CONNECTED) 0] create /config
[zk: localhost:2181(CONNECTED) 1] create /config/zookeeper-client
[zk: localhost:2181(CONNECTED) 2] create /config/zookeeper-client/message "Hello from Zookeeper"

Access the configuration in your Spring Boot application using the @Value annotation:

// ExampleController.java
@RestController
public class ExampleController {
    @Value("${message:Default Hello}")
    private String message;

    @GetMapping("/message")
    public String getMessage() {
        return message;
    }
}

// bootstrap.properties
spring.application.name=zookeeper-client
spring.cloud.zookeeper.config.root=config
spring.cloud.zookeeper.config.defaultContext=zookeeper-client

Distributed Coordination

Use Zookeeper for distributed coordination tasks such as leader election and distributed locks. Here’s an example of using Curator (a Zookeeper client library) for leader election:

// LeaderElectionConfig.java
@Configuration
public class LeaderElectionConfig {
    @Bean
    public CuratorFramework curatorFramework() {
        CuratorFramework client = CuratorFrameworkFactory.newClient("localhost:2181", new ExponentialBackoffRetry(1000, 3));
        client.start();
        return client;
    }

    @Bean
    public LeaderSelector leaderSelector(CuratorFramework client) {
        LeaderSelector leaderSelector = new LeaderSelector(client, "/leader", new LeaderSelectorListenerAdapter() {
            @Override
            public void takeLeadership(CuratorFramework client) throws Exception {
                // Leader logic here
                System.out.println("I am the leader");
                Thread.sleep(10000); // Simulate work
                System.out.println("Relinquishing leadership");
            }
        });
        leaderSelector.autoRequeue();
        leaderSelector.start();
        return leaderSelector;
    }
}

Key Points

Conclusion

Spring Cloud Zookeeper is a powerful tool for service discovery, configuration management, and distributed coordination in a microservices architecture. By leveraging its features, developers can build scalable and resilient microservices that can easily discover and communicate with each other. Happy coding!