深入Spring Cloud源码设计－eureka一篇就够

- 日理万妓 2022-06-12 04:36 93阅读 0赞

# What is eureka ? #

> Eureka is a REST (Representational State Transfer) based service that is primarily used in the AWS cloud for locating services for the purpose of load balancing and failover of middle-tier servers. We call this service, the Eureka Server. Eureka also comes with a Java-based client component,the Eureka Client, which makes interactions with the service much easier. The client also has a built-in load balancer that does basic round-robin load balancing.

*eureka 发音 \[juˈri:kə\] 伊瑞克*

翻译后可简单概括为：

Eureka是一个基于 REST 的服务，用于定位服务，以实现云端中间层服务发现和故障转移的中间件。它有两个重要组成部分，Eureka服务端和基于JAVA的客户端组件。

# eureka核心模块 #

![eureka基本架构][eureka]

由上图可以看出，eureka主要有三种角色：

1.  Eureka Server  
    eureka服务端，主要用来做服务的注册与发现
2.  Service Provider  
    服务的实际提供方，会将服务注册到Eureka Server上
3.  Service Consumer  
    服务消费方，从Eureka Server获取服务列表，向Service Provider发起真实调用请求

TIP：这三个角色是逻辑上的划分，可能在使用时，这几个角色可以是同一个实例；  
一个Service Provider既可以是Service Consumer，也可以是Service Provider

![这里写图片描述][SouthEast]

上图进一步展示了3个角色之间的交互。

1.  ServiceProvider会向Eureka Server做Register（服务注册）、Renew（服务续约）、Cancel（服务下线）等操作。
2.  EurekaServer之间会做注册服务的同步，从而保证状态一致
3.  ServiceConsumer会向Eureka Server获取注册服务列表，并消费服务

# eureka源码阅读入口 #

## eureka的服务端 ##

**A：** EurekaBootStrap.java 实现了ServletContextListener接口，当项目启动时会初始化该类，触发contextInitialized方法的执行。

**B：** contextInitialized中调用了initEurekaServerContext方法。

**C：**  
initEurekaServerContext方法依次调用了三个方法  
PeerAwareInstanceRegistryImpl初始化  
PeerEurekaNodes初始化  
DefaultEurekaServerContext初始化，该类有@Singleton注解，并含有@PostConstruct注解（构造方法执行后执行）的initialize方法

**D：**  
initialize调用了两个方法  
PeerEurekaNodes的start方法  
PeerAwareInstanceRegistryImpl的init方法

**E：**  
init方法依次调用三个方法  
\- **initializedResponseCache方法，该方法继续调用ResponseCacheImpl类，最后使用的是guava cache**  
\- **scheduleRenewalThresholdUpdateTask使用Timer做定时任务定时更新新注册服务配置参数更新**  
\- **initRemoteRegionRegistry初始化注册中心**

## eureka的客户端 ##

DiscoveryClient.java这个类中含有了client侧的很多操作：  
register()  
renew()  
unregister()  
fetchRegistry() 等等

### register()方法调用过程 ###

最为主要的register方法，是在DiscoveryClient初始化的过程中被调用，如下DiscoveryClient构造方法中调用了initScheduledTasks()

@Inject
        DiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs args,
                        Provider<BackupRegistry> backupRegistryProvider) {
            ...
    
            if (clientConfig.shouldFetchRegistry() && !fetchRegistry(false)) {
                fetchRegistryFromBackup();
            }
    
            // call and execute the pre registration handler before all background tasks (inc registration) is started
            if (this.preRegistrationHandler != null) {
                this.preRegistrationHandler.beforeRegistration();
            }
            initScheduledTasks();
    
            try {
                Monitors.registerObject(this);
            } catch (Throwable e) {
                logger.warn("Cannot register timers", e);
            }
    
            // This is a bit of hack to allow for existing code using DiscoveryManager.getInstance()
            // to work with DI'd DiscoveryClient DiscoveryManager.getInstance().setDiscoveryClient(this); DiscoveryManager.getInstance().setEurekaClientConfig(config); }

initScheduledTasks 方法中启动了InstanceInfoReplicator线程，  
instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds())

private void initScheduledTasks() {
                // Heartbeat timer
                scheduler.schedule(
                        new TimedSupervisorTask(
                                "heartbeat",
                                scheduler,
                                heartbeatExecutor,
                                renewalIntervalInSecs,
                                TimeUnit.SECONDS,
                                expBackOffBound,
                                new HeartbeatThread()
                        ),
                        renewalIntervalInSecs, TimeUnit.SECONDS);
    
                // InstanceInfo replicator
                instanceInfoReplicator = new InstanceInfoReplicator(
                        this,
                        instanceInfo,
                    clientConfig.getInstanceInfoReplicationIntervalSeconds(),
                        2); // burstSize
    
                statusChangeListener = new ApplicationInfoManager.StatusChangeListener() {
                    @Override
                    public String getId() {
                        return "statusChangeListener";
                    }
              ...
                instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());
            } else {
                logger.info("Not registering with Eureka server per configuration");
            }
        }

InstanceInfoReplicator实现了Runnable接口是一个线程，其run方法逻辑如下，可以看到其调用了 discoveryClient.register();

public void run() {
            try {
                discoveryClient.refreshInstanceInfo();
    
                Long dirtyTimestamp = instanceInfo.isDirtyWithTime();
                if (dirtyTimestamp != null) {
                    discoveryClient.register();
                    instanceInfo.unsetIsDirty(dirtyTimestamp);
                }
            } catch (Throwable t) {
                logger.warn("There was a problem with the instance info replicator", t);
            } finally {
                Future next = scheduler.schedule(this, replicationIntervalSeconds, TimeUnit.SECONDS);
                scheduledPeriodicRef.set(next);
            }
        }

再来看看DiscoveryClient的register方法

/** * Register with the eureka service by making the appropriate REST call. */
        boolean register() throws Throwable {
            logger.info(PREFIX + appPathIdentifier + ": registering service...");
            EurekaHttpResponse<Void> httpResponse;
            try {
                httpResponse = eurekaTransport.registrationClient.register(instanceInfo);
            } catch (Exception e) {
                logger.warn("{} - registration failed {}", PREFIX + appPathIdentifier, e.getMessage(), e);
                throw e;
            }
            if (logger.isInfoEnabled()) {
                logger.info("{} - registration status: {}", PREFIX + appPathIdentifier, httpResponse.getStatusCode());
            }
            return httpResponse.getStatusCode() == 204;
        }

### renew ()方法调用过程 ###

Renew（服务续约）操作由Service Provider定期调用，类似于heartbeat。主要是用来告诉Eureka Server Service Provider还活着，避免服务被剔除掉

/** * Renew with the eureka service by making the appropriate REST call */
        boolean renew() {
            EurekaHttpResponse<InstanceInfo> httpResponse;
            try {
                httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null);
                logger.debug("{} - Heartbeat status: {}", PREFIX + appPathIdentifier, httpResponse.getStatusCode());
                if (httpResponse.getStatusCode() == 404) {
                    REREGISTER_COUNTER.increment();
                    logger.info("{} - Re-registering apps/{}", PREFIX + appPathIdentifier, instanceInfo.getAppName());
                    return register();
                }
                return httpResponse.getStatusCode() == 200;
            } catch (Throwable e) {
                logger.error("{} - was unable to send heartbeat!", PREFIX + appPathIdentifier, e);
                return false;
            }
        }

renew方法在HeartbeatThread线程中被调用

/** * The heartbeat task that renews the lease in the given intervals. */
        private class HeartbeatThread implements Runnable { 
    
            public void run() {
                if (renew()) {
                    lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
                }
            }
        }

HeartbeatThread线程在initScheduledTasks方法中被调用

private void initScheduledTasks() {
       ...
                // Heartbeat timer
                scheduler.schedule(
                        new TimedSupervisorTask(
                                "heartbeat",
                                scheduler,
                                heartbeatExecutor,
                                renewalIntervalInSecs,
                                TimeUnit.SECONDS,
                                expBackOffBound,
                                new HeartbeatThread()
                        ),
                        renewalIntervalInSecs, TimeUnit.SECONDS);
    
                // InstanceInfo replicator
                instanceInfoReplicator = new InstanceInfoReplicator(
                        this,
                        instanceInfo,
                        clientConfig.getInstanceInfoReplicationIntervalSeconds(),
                        2); // burstSize
    ...
    ｝

initScheduledTasks是在DiscoveryClient构造函数初始化过程中被调用。

通过register方法和renew方法的分析，相信大家已经摸索出eureka代码的套路，其他的方法，大家可以自己去深入理解了。

# 参考资料 #

[http://xujin.org/categories/Spring-Cloud-Eureka/][http_xujin.org_categories_Spring-Cloud-Eureka]

[http://blog.csdn.net/jenny8080/article/details/52448403][http_blog.csdn.net_jenny8080_article_details_52448403]

[https://github.com/Netflix/eureka][https_github.com_Netflix_eureka]

[https://github.com/spring-cloud/spring-cloud-netflix/tree/v1.2.2.RELEASE][https_github.com_spring-cloud_spring-cloud-netflix_tree_v1.2.2.RELEASE]

[https://springcloud.cc/][https_springcloud.cc]

[eureka]: /images/20220612/1e6d1eecf58847b6b6ea3a14e738a652.png
[SouthEast]: /images/20220612/c1e7fcad1f404a1eb8ca2913ae897380.png
[http_xujin.org_categories_Spring-Cloud-Eureka]: http://xujin.org/categories/Spring-Cloud-Eureka/
[http_blog.csdn.net_jenny8080_article_details_52448403]: http://blog.csdn.net/jenny8080/article/details/52448403
[https_github.com_Netflix_eureka]: https://github.com/Netflix/eureka
[https_github.com_spring-cloud_spring-cloud-netflix_tree_v1.2.2.RELEASE]: https://github.com/spring-cloud/spring-cloud-netflix/tree/v1.2.2.RELEASE
[https_springcloud.cc]: https://springcloud.cc/