Spring Cloud Consul实现选举机制

1.什么是Spring Cloud Consul？

1. 服务注册与发现：通过 Consul 的服务注册功能，Spring Cloud Consul 可以实现微服务的动态注册和发现，简化服务间通信。
2. 分布式配置管理：通过 Consul 的 Key/Value 存储机制，提供对分布式配置的管理。
3. 健康检查：支持服务实例的健康检查，确保只有健康的实例可供其他服务调用。
4. 选举与分布式锁：通过 Consul 的会话机制，支持分布式锁和领导选举。

Spring Cloud Consul 的选举机制

工作原理：

1. 会话创建：
   • 服务实例向 Consul 创建一个会话（Session），这是一个临时的、与实例绑定的对象。
   • 会话带有 TTL（生存时间），需要定期续约，保持活跃状态。
2. 获取锁（Lock）：
   • 通过将一个 Key 的值设置为当前会话 ID，服务尝试获取该 Key 的锁。
   • Consul 使用 CAS（Compare and Swap）操作来确保只有一个服务实例可以成功获取锁。
3. 锁定成功：
   • 成功获取锁的服务实例被视为领导者（Leader）。
   • 其他实例会定期尝试获取锁，但只能等待当前锁被释放或超时。
4. 锁释放或失效：
   • 如果领导实例未能及时续约会话（例如宕机或网络中断），Consul 会释放与该会话相关联的锁，其他实例可以竞争成为新的领导者。

2.环境搭建

run Consul Agent

docker run -d --name=dev-consul -p 8500:8500 consul

web ui

3.代码工程

实验目标

• 使用 Consul 提供的会话机制和键值存储来实现 分布式领导选举。
• 通过 @InboundChannelAdapter 和 @ServiceActivator 实现周期性检查领导身份并执行领导任务。

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>springcloud-demo</artifactId>
        <groupId>com.et</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>LeaderElection</artifactId>

    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
    </properties>
    <dependencies>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <!-- Spring Cloud Starter Consul Discovery -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-consul-discovery</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.integration</groupId>
            <artifactId>spring-integration-core</artifactId>
        </dependency>

    </dependencies>

</project>

LeaderElectionConfig.java

package com.et;

import jakarta.annotation.PreDestroy;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpMethod;
import org.springframework.http.ResponseEntity;
import org.springframework.integration.annotation.InboundChannelAdapter;
import org.springframework.integration.annotation.Poller;
import org.springframework.integration.annotation.ServiceActivator;
import org.springframework.integration.core.MessageSource;
import org.springframework.integration.support.MessageBuilder;
import org.springframework.messaging.Message;
import org.springframework.messaging.MessageHandler;
import org.springframework.messaging.MessagingException;
import org.springframework.web.client.RestTemplate;

@Configuration
public class LeaderElectionConfig {

    private static final String LEADER_KEY = "service/leader";
    private static final String CONSUL_URL = "http://localhost:8500";
    private String sessionId;


    @Bean
    @InboundChannelAdapter(value = "leaderChannel", poller = @Poller(fixedDelay = "5000"))
    public MessageSource<String> leaderMessageSource() {
        return () -> {
            // Implement logic to check if this instance is the leader
            boolean isLeader = checkLeadership();
            return MessageBuilder.withPayload(isLeader ? "I am the leader" : "I am not the leader").build();
        };
    }

    @Bean
    @ServiceActivator(inputChannel = "leaderChannel")
    public MessageHandler leaderMessageHandler() {
        return new MessageHandler() {
            @Override
            public void handleMessage(Message<?> message) throws MessagingException {
                System.out.println(message.getPayload());
                // Implement logic to perform leader-specific tasks
            }
        };
    }


    private final RestTemplate restTemplate = new RestTemplate();

    public LeaderElectionConfig() {
        this.sessionId = createSession();
    }

    private String createSession() {
        String url = CONSUL_URL + "/v1/session/create";
        HttpHeaders headers = new HttpHeaders();
        HttpEntity<String> entity = new HttpEntity<>("{\"Name\": \"leader-election-session\"}", headers);
        //ResponseEntity<String> response = restTemplate.postForEntity(url, entity, String.class);
        //  PUT
        ResponseEntity<String> response = restTemplate.exchange(url, HttpMethod.PUT, entity, String.class);
        // Extract session ID from response
        return response.getBody().split("\"")[3]; // This is a simple way to extract the session ID
    }

    public boolean checkLeadership() {
        String url = CONSUL_URL + "/v1/kv/" + LEADER_KEY + "?acquire=" + sessionId;
        HttpHeaders headers = new HttpHeaders();
        HttpEntity<String> entity = new HttpEntity<>(headers);
        ResponseEntity<Boolean> response = restTemplate.exchange(url, HttpMethod.PUT, entity, Boolean.class);
        return Boolean.TRUE.equals(response.getBody());
    }
    public void releaseLeadership() {
        String url = CONSUL_URL + "/v1/kv/" + LEADER_KEY + "?release=" + sessionId;
        HttpHeaders headers = new HttpHeaders();
        HttpEntity<String> entity = new HttpEntity<>(headers);
        ResponseEntity<Boolean> response = restTemplate.exchange(url, HttpMethod.PUT, entity, Boolean.class);
        if (Boolean.TRUE.equals(response.getBody())) {
            System.out.println("Released leadership successfully");
        } else {
            System.out.println("Failed to release leadership");
        }
    }
    @PreDestroy
    public void onExit() {
        releaseLeadership();
    }
}

代码解释

1. 初始化：
   • 启动时通过 createSession() 向 Consul 注册会话。
2. 周期性任务：
   • 每 5 秒通过 checkLeadership() 检查领导身份。
   • 如果是领导者，执行特定任务（如打印日志、执行业务逻辑）。
3. 释放资源：
   • 应用关闭时，通过 releaseLeadership() 释放锁。

LeaderElectionApplication.java

package com.et;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.integration.config.EnableIntegration;

@SpringBootApplication
@EnableDiscoveryClient
@EnableIntegration
public class LeaderElectionApplication {

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

配置文件

server.port=8081
spring.cloud.consul.discovery.enabled=true
spring.cloud.consul.discovery.register=true
spring.application.name=leader-election-example
spring.cloud.consul.host=localhost
spring.cloud.consul.port=8500
spring.cloud.consul.discovery.instance-id=${spring.application.name}:${spring.application.instance_id:${random.value}}

server.port=8082
spring.cloud.consul.discovery.enabled=true
spring.cloud.consul.discovery.register=true
spring.application.name=leader-election-example
spring.cloud.consul.host=localhost
spring.cloud.consul.port=8500
spring.cloud.consul.discovery.instance-id=${spring.application.name}:${spring.application.instance_id:${random.value}}

以上只是一些关键代码，所有代码请参见下面代码仓库

代码仓库

• https://github.com/Harries/springcloud-demo(LeaderElection)

4.测试

java -jar myapp.jar --spring.profiles.active=node1

java -jar myapp.jar --spring.profiles.active=node2

5.引用

• https://developer.hashicorp.com/consul/docs/dynamic-app-config/kv
• https://www.liuhaihua.cn/archives/711803.html