一、Eureka架构
从下面的架构图中可以看出，不管是服务的调用者还是服务的提供者都可以认为是一个EurekaClient，在启动的过程中会将自身注册到EurekaServer中（也就是Eureka注册中心），并通过心跳保持服务的续约。
springcloud高可用架构图
二、Eureka的启动注册
注册是InstanceId的生成可以查看另一篇SpringCloud学习笔记(四)-InstanceId的生成
通常我们通过@EnableDiscoveryClient来声明一个Eureka客户端。源码注释这样描述EnableDiscoveryClient
Annotation to enable a DiscoveryClient implementation
即通过该注释会开启一个DiscoveryClient实例，DiscoveryClient类源码可以看到，DiscoveryClient的构造方法中有如下代码段。
// 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
而通过DiscoveryClient的源码可以看到，在服务启动时，会启动两个线程池，一个用于心跳的保持，一个用于缓存的刷新。进一步跟踪源码，在initScheduledTasks中，开启心跳定时器，同时会启动一个线程InstanceInfoReplicator，这个线程会将本地实例信息更新到远程服务器，也就是将当前服务注册到注册中心。源码如下：
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);
}
}
这里有个小细节，instanceInfoReplicator在启动的时候，会有一定时间的延迟，默认是40秒，也就是client在启动时，不是马上向注册中心注册，而是会延迟40秒再注册。
instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());
public int getInitialInstanceInfoReplicationIntervalSeconds() {
return configInstance.getIntProperty(
namespace + INITIAL_REGISTRATION_REPLICATION_DELAY_KEY, 40).get();
}
接着看register方法, 该方法会将实例元数据信息（instanceInfo）通过http方式注册到Eureka服务端。
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;
}
三、EurekaClient的心跳保持
上一小节已经提到了在启动过程中会初始化心跳定时器
//服务刷新默认30秒，可通过eureka.instance.lease-renewal-interval-in-seconds修改
int renewalIntervalInSecs = instanceInfo.getLeaseInfo().getRenewalIntervalInSecs();
int expBackOffBound = clientConfig.getHeartbeatExecutorExponentialBackOffBound();
logger.info("Starting heartbeat executor: " + "renew interval is: " + renewalIntervalInSecs);
// Heartbeat timer
scheduler.schedule(
new TimedSupervisorTask(
"heartbeat",
scheduler,
heartbeatExecutor,
renewalIntervalInSecs,
TimeUnit.SECONDS,
expBackOffBound,
new HeartbeatThread()
),
renewalIntervalInSecs, TimeUnit.SECONDS);
心跳线程，定时向注册中心发送http请求，如果返回404，会重新注册。
private class HeartbeatThread implements Runnable {
public void run() {
if (renew()) {
lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
}
}
}
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;
}
}
四、总结
在spring-cloud-netflix-eureka-client包下spring.factories可以查看配置入口（EurekaClientAutoConfiguration）
1：项目启动
2：初始化配置EurekaClientAutoConfiguration->eurekaInstanceConfigBean
3：构造EurekaClient对象（内部类EurekaClientAutoConfiguration：：RefreshableEurekaClientConfiguration）
3.1：构造心跳任务线程池
3.2：构造缓存刷新任务线程池
4：启动定时任务（心跳+缓存刷新）
4.1：启动缓存刷新定时任务
4.2：启动心跳定时任务
4.3：启动instanceInfoReplicator线程，执行注册任务
5：服务启动时，会延迟40秒向注册中心注册
6：心跳时间默认是30秒，可通过eureka.instance.lease-renewal-interval-in-seconds修改
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