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地址:https://mp.weixin.qq.com/s/47…
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;
}
@Override
public String description() {return DESCRIPTION;}
@Override
public 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;
}
@Override
public 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 cacheRefresh
scheduler = 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 handoff
cacheRefreshExecutor = 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 registered
if (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 == false
if(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/eu…
http://yeming.me/2016/12/01/e…
http://blog.didispace.com/spr…
https://www.jianshu.com/p/71a…
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