Eureka（一）： Eureka Client

简介

Eureka是一种基于REST（Representational State Transfer）的服务，主要用于AWS云，用于定位服务，以实现中间层服务器的负载平衡和故障转移。我们将此服务称为Eureka Server。Eureka还附带了一个基于Java的客户端组件Eureka Client，它使与服务的交互变得更加容易。客户端还有一个内置的负载均衡器，可以进行基本的循环负载均衡。在Netflix，一个更复杂的负载均衡器包含Eureka基于流量，资源使用，错误条件等多种因素提供加权负载平衡，以提供卓越的弹性。
先看一张 github 上 Netflix Eureka 的一架构图，如下：

从图可以看出在这个体系中，有2个角色，即Eureka Server和Eureka Client。而Eureka Client又分为Applicaton Service和Application Client，即服务提供者何服务消费者。每个区域有一个Eureka集群，并且每个区域至少有一个eureka服务器可以处理区域故障，以防服务器瘫痪。

Eureka Client 在 Eureka Server 注册，然后Eureka Client 每30秒向 Eureka Server 发送一次心跳来更新一次租约。如果 Eureka Client 无法续订租约几次，则会在大约90秒内 Eureka Server 将其从服务器注册表中删除。注册信息和续订将复制到群集中的所有 Eureka Server 节点。来自任何区域的客户端都可以查找注册表信息（每30秒发生一次）根据这些注册表信息，Application Client 可以远程调用 Applicaton Service 来消费服务。

源码分析

基于Spring Cloud的 eureka 的 client 端在启动类上加上 @EnableDiscoveryClient 注解，就可以用 NetFlix 提供的 Eureka client。下面就以 @EnableDiscoveryClient 为入口，进行Eureka Client的源码分析。

@EnableDiscoveryClient，通过源码可以发现这是一个标记注解：

/*** Annotation to enable a DiscoveryClient implementation.* @author Spencer Gibb*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableDiscoveryClientImportSelector.class)
public @interface EnableDiscoveryClient {boolean autoRegister() default true;
}

通过注释可以知道 @EnableDiscoveryClient 注解是用来启用 DiscoveryClient 的实现，DiscoveryClient接口代码如下：

public interface DiscoveryClient {String description();List<ServiceInstance> getInstances(String serviceId);List<String> getServices();}

接口说明：

description()：实现描述。
getInstances(String serviceId)：获取与特定serviceId关联的所有ServiceInstance
getServices()：返回所有已知的服务ID

DiscoveryClient 接口的实现结构图：

EurekaDiscoveryClient：Eureka 的 DiscoveryClient 实现类。
CompositeDiscoveryClient：用于排序可用客户端的发现客户端的顺序。
NoopDiscoveryClient：什么都不做的服务发现实现类，已经被废弃。
SimpleDiscoveryClient：简单的服务发现实现类 SimpleDiscoveryClient，具体的服务实例从 SimpleDiscoveryProperties 配置中获取。

EurekaDiscoveryClient 是 Eureka 对 DiscoveryClient接口的实现，代码如下：

public class EurekaDiscoveryClient implements DiscoveryClient {public static final String DESCRIPTION = "Spring Cloud Eureka Discovery Client";private final EurekaInstanceConfig config;private final EurekaClient eurekaClient;public EurekaDiscoveryClient(EurekaInstanceConfig config, EurekaClient eurekaClient) {this.config = config;this.eurekaClient = eurekaClient;}@Overridepublic String description() {return DESCRIPTION;}@Overridepublic List<ServiceInstance> getInstances(String serviceId) {List<InstanceInfo> infos = this.eurekaClient.getInstancesByVipAddress(serviceId,false);List<ServiceInstance> instances = new ArrayList<>();for (InstanceInfo info : infos) {instances.add(new EurekaServiceInstance(info));}return instances;}@Overridepublic List<String> getServices() {Applications applications = this.eurekaClient.getApplications();if (applications == null) {return Collections.emptyList();}List<Application> registered = applications.getRegisteredApplications();List<String> names = new ArrayList<>();for (Application app : registered) {if (app.getInstances().isEmpty()) {continue;}names.add(app.getName().toLowerCase());}return names;}}

从代码可以看出 EurekaDiscoveryClient 实现了 DiscoveryClient 定义的规范接口，真正实现发现服务的是 EurekaClient，下面是 EurekaClient 依赖结构图：

EurekaClient 唯一实现类 DiscoveryClient，DiscoveryClient 的构造方法如下：

@Inject
DiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs args,Provider<BackupRegistry> backupRegistryProvider) {//省略...try {// default size of 2 - 1 each for heartbeat and cacheRefreshscheduler = Executors.newScheduledThreadPool(2,new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-%d").setDaemon(true).build());heartbeatExecutor = new ThreadPoolExecutor(1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(),new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-HeartbeatExecutor-%d").setDaemon(true).build());  // use direct handoffcacheRefreshExecutor = new ThreadPoolExecutor(1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(),new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d").setDaemon(true).build());  // use direct handoff//省略...initScheduledTasks();try {Monitors.registerObject(this);} catch (Throwable e) {logger.warn("Cannot register timers", e);}//省略...
}

可以看到这个构造方法里面，主要做了下面几件事：

创建了scheduler定时任务的线程池，heartbeatExecutor心跳检查线程池(服务续约)，cacheRefreshExecutor(服务获取)
然后initScheduledTasks()开启上面三个线程池，往上面3个线程池分别添加相应任务。然后创建了一个instanceInfoReplicator(Runnable任务)，然后调用InstanceInfoReplicator.start方法，把这个任务放进上面scheduler定时任务线程池(服务注册并更新)。

服务注册(Registry)

上面说了，initScheduledTasks()方法中调用了InstanceInfoReplicator.start()方法，InstanceInfoReplicator 的 run()方法代码如下：

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);}
}

发现 InstanceInfoReplicator的run方法，run方法中会调用DiscoveryClient的register方法。DiscoveryClient 的 register方法代码如下：

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

最终又经过一系列调用，最终会调用到AbstractJerseyEurekaHttpClient的register方法，代码如下：

public EurekaHttpResponse<Void> register(InstanceInfo info) {String urlPath = "apps/" + info.getAppName();ClientResponse response = null;try {Builder resourceBuilder = jerseyClient.resource(serviceUrl).path(urlPath).getRequestBuilder();addExtraHeaders(resourceBuilder);response = resourceBuilder.header("Accept-Encoding", "gzip").type(MediaType.APPLICATION_JSON_TYPE).accept(MediaType.APPLICATION_JSON).post(ClientResponse.class, info);return anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();} finally {if (logger.isDebugEnabled()) {logger.debug("Jersey HTTP POST {}/{} with instance {}; statusCode={}", serviceUrl, urlPath, info.getId(),response == null ? "N/A" : response.getStatus());}if (response != null) {response.close();}}
}

可以看到最终通过http rest请求eureka server端，把应用自身的InstanceInfo实例注册给server端，我们再来完整梳理一下服务注册流程：

Renew服务续约

服务续约和服务注册非常类似，HeartbeatThread 代码如下：

private class HeartbeatThread implements Runnable {public void run() {if (renew()) {//更新最后一次心跳的时间lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();}}
}
// 续约的主方法
boolean renew() {EurekaHttpResponse<InstanceInfo> httpResponse;try {httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null);logger.debug(PREFIX + "{} - Heartbeat status: {}", appPathIdentifier, httpResponse.getStatusCode());if (httpResponse.getStatusCode() == 404) {REREGISTER_COUNTER.increment();logger.info(PREFIX + "{} - Re-registering apps/{}", appPathIdentifier, instanceInfo.getAppName());long timestamp = instanceInfo.setIsDirtyWithTime();boolean success = register();if (success) {instanceInfo.unsetIsDirty(timestamp);}return success;}return httpResponse.getStatusCode() == 200;} catch (Throwable e) {logger.error(PREFIX + "{} - was unable to send heartbeat!", appPathIdentifier, e);return false;}
}

发送心跳，请求eureka server 端，如果接口返回值为404，就是说服务不存在，那么重新走注册流程。

如果接口返回值为404，就是说不存在，从来没有注册过，那么重新走注册流程。

服务续约流程如下图：

服务下线cancel

在服务shutdown的时候，需要及时通知服务端把自己剔除，以避免客户端调用已经下线的服务，shutdown()方法代码如下：

public synchronized void shutdown() {if (isShutdown.compareAndSet(false, true)) {logger.info("Shutting down DiscoveryClient ...");if (statusChangeListener != null && applicationInfoManager != null) {applicationInfoManager.unregisterStatusChangeListener(statusChangeListener.getId());}// 关闭各种定时任务// 关闭刷新实例信息/注册的定时任务// 关闭续约(心跳)的定时任务// 关闭获取注册信息的定时任务cancelScheduledTasks();// If APPINFO was registeredif (applicationInfoManager != null&& clientConfig.shouldRegisterWithEureka()&& clientConfig.shouldUnregisterOnShutdown()) {// 更改实例状态，使实例不再接收流量applicationInfoManager.setInstanceStatus(InstanceStatus.DOWN);//向EurekaServer端发送下线请求unregister();}if (eurekaTransport != null) {eurekaTransport.shutdown();}heartbeatStalenessMonitor.shutdown();registryStalenessMonitor.shutdown();logger.info("Completed shut down of DiscoveryClient");}
}private void cancelScheduledTasks() {if (instanceInfoReplicator != null) {instanceInfoReplicator.stop();}if (heartbeatExecutor != null) {heartbeatExecutor.shutdownNow();}if (cacheRefreshExecutor != null) {cacheRefreshExecutor.shutdownNow();}if (scheduler != null) {scheduler.shutdownNow();}
}void unregister() {// It can be null if shouldRegisterWithEureka == falseif(eurekaTransport != null && eurekaTransport.registrationClient != null) {try {logger.info("Unregistering ...");EurekaHttpResponse<Void> httpResponse = eurekaTransport.registrationClient.cancel(instanceInfo.getAppName(), instanceInfo.getId());logger.info(PREFIX + "{} - deregister  status: {}", appPathIdentifier, httpResponse.getStatusCode());} catch (Exception e) {logger.error(PREFIX + "{} - de-registration failed{}", appPathIdentifier, e.getMessage(), e);}}
}

先关闭各种定时任务，然后向eureka server 发送服务下线通知。服务下线流程如下图：

参考

https://github.com/Netflix/eureka/wiki
http://yeming.me/2016/12/01/eureka1/
http://blog.didispace.com/springcloud-sourcecode-eureka/
https://www.jianshu.com/p/71a8bdbf03f4