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
由上图可以看出，eureka主要有三种角色：
- Eureka Server
eureka服务端，主要用来做服务的注册与发现 - Service Provider
服务的实际提供方，会将服务注册到Eureka Server上 - 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
上图进一步展示了3个角色之间的交互。
- ServiceProvider会向Eureka Server做Register（服务注册）、Renew（服务续约）、Cancel（服务下线）等操作。
- EurekaServer之间会做注册服务的同步，从而保证状态一致
- 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://blog.csdn.net/jenny8080/article/details/52448403
https://github.com/Netflix/eureka
https://github.com/spring-cloud/spring-cloud-netflix/tree/v1.2.2.RELEASE
