前两篇博客我们介绍了Spring Cloud Eureka的服务注册与续约保活实现机制，这篇博客我们简单来介绍一下服务下线的实现机制。Eureka Server对外暴露下线接口，Eureka Client在服务关闭时通过远程调用接口来实现服务下线操作。
Eureka Client 服务下线
在Eureka Client关闭时会调用DiscoveryClient的shutdown方法，在shutdown方法中会完成一系列的线程关闭操作，并通过http远程调用Eureka Server对外暴露的下线接口进行服务下线操作。
@PreDestroy
@Override
public synchronized void shutdown() {
if (isShutdown.compareAndSet(false, true)) {
logger.info("Shutting down DiscoveryClient ...");
if (statusChangeListener != null && applicationInfoManager != null) {
applicationInfoManager.unregisterStatusChangeListener(statusChangeListener.getId());
}
//停止相关定时任务线程
cancelScheduledTasks();
// If APPINFO was registered
if (applicationInfoManager != null && clientConfig.shouldRegisterWithEureka()) {
applicationInfoManager.setInstanceStatus(InstanceStatus.DOWN);
//进行服务下线操作
unregister();
}
if (eurekaTransport != null) {
eurekaTransport.shutdown();
}
heartbeatStalenessMonitor.shutdown();
registryStalenessMonitor.shutdown();
logger.info("Completed shut down of DiscoveryClient");
}
}
在unregister中通过http请求远程调用Eureka Server的服务下线接口进行服务下线操作。
void unregister() {
// It can be null if shouldRegisterWithEureka == false
if(eurekaTransport != null && eurekaTransport.registrationClient != null) {
try {
logger.info("Unregistering ...");
//主动调用下线接口，进行服务下线操作
EurekaHttpResponse<Void> httpResponse = eurekaTransport.registrationClient.cancel(instanceInfo.getAppName(), instanceInfo.getId());
logger.info(PREFIX + appPathIdentifier + " - deregister status: " + httpResponse.getStatusCode());
} catch (Exception e) {
logger.error(PREFIX + appPathIdentifier + " - de-registration failed" + e.getMessage(), e);
}
}
}
Eureka Server 服务下线
Eureka Server对外提供DELETE方法cancelLease来接收服务的下线处理操作，具体实现在InstanceResource类中。
@DELETE
public Response cancelLease(
@HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication) {
//进行服务下线操作
boolean isSuccess = registry.cancel(app.getName(), id,
"true".equals(isReplication));
if (isSuccess) {
logger.debug("Found (Cancel): " + app.getName() + " - " + id);
return Response.ok().build();
} else {
logger.info("Not Found (Cancel): " + app.getName() + " - " + id);
return Response.status(Status.NOT_FOUND).build();
}
}
在InstanceRegistry中调用cancel方法首先会通知本地服务下线，然后调用父类PeerAwareInstanceRegistryImpl的cancel实现服务下线操作。
@Override
public boolean cancel(String appName, String serverId, boolean isReplication) {
//利用spring 事件机制广播服务下线操作
handleCancelation(appName, serverId, isReplication);
//父类中完成服务下线操作
return super.cancel(appName, serverId, isReplication);
}
在PeerAwareInstanceRegistryImpl的cancel中首先调用父类AbstractInstanceRegistry的cancel完成服务下线的操作，然后通知其他服务服务进行下线操作。
public boolean cancel(final String appName, final String id,
final boolean isReplication) {
//调用父类的服务下线实现
if (super.cancel(appName, id, isReplication)) {
//通知服务下线
replicateToPeers(Action.Cancel, appName, id, null, null, isReplication);
synchronized (lock) {
if (this.expectedNumberOfRenewsPerMin > 0) {
// Since the client wants to cancel it, reduce the threshold (1 for 30 seconds, 2 for a minute)
this.expectedNumberOfRenewsPerMin = this.expectedNumberOfRenewsPerMin - 2;
this.numberOfRenewsPerMinThreshold =
(int) (this.expectedNumberOfRenewsPerMin * serverConfig.getRenewalPercentThreshold());
}
}
return true;
}
return false;
}
在抽象类AbstractInstanceRegistry中调用cancel完成服务实例信息的删除操作。
@Override
public boolean cancel(String appName, String id, boolean isReplication) {
return internalCancel(appName, id, isReplication);
}
protected boolean internalCancel(String appName, String id, boolean isReplication) {
try {
read.lock();
CANCEL.increment(isReplication);
//根据appName获取所有服务名
Map<String, Lease<InstanceInfo>> gMap = registry.get(appName);
Lease<InstanceInfo> leaseToCancel = null;
if (gMap != null) {
//根据id删除服务实例
leaseToCancel = gMap.remove(id);
}
synchronized (recentCanceledQueue) {
recentCanceledQueue.add(new Pair<Long, String>(System.currentTimeMillis(), appName + "(" + id + ")"));
}
//根据id删除服务实例
InstanceStatus instanceStatus = overriddenInstanceStatusMap.remove(id);
if (instanceStatus != null) {
logger.debug("Removed instance id {} from the overridden map which has value {}", id, instanceStatus.name());
}
if (leaseToCancel == null) {
CANCEL_NOT_FOUND.increment(isReplication);
logger.warn("DS: Registry: cancel failed because Lease is not registered for: {}/{}", appName, id);
return false;
} else {
leaseToCancel.cancel();
InstanceInfo instanceInfo = leaseToCancel.getHolder();
String vip = null;
String svip = null;
if (instanceInfo != null) {
instanceInfo.setActionType(ActionType.DELETED);
recentlyChangedQueue.add(new RecentlyChangedItem(leaseToCancel));
instanceInfo.setLastUpdatedTimestamp();
vip = instanceInfo.getVIPAddress();
svip = instanceInfo.getSecureVipAddress();
}
invalidateCache(appName, vip, svip);
logger.info("Cancelled instance {}/{} (replication={})", appName, id, isReplication);
return true;
}
} finally {
read.unlock();
}
}
总结：服务下线实现机制分析起来也是比较简单的，Eureka Server通过对外暴露DELETE接口，Eureka Client通过主动调用接口来完成服务下线操作。
