Spring Cloud Eureka（七）：DiscoveryClient 源码分析
1、本节概要
上一节文章主要介绍了Eureka Client 的服务注册的流程，没有对服务治理进行介绍，本文目的就是从源码角度来学习服务实例的治理机制，主要包括以下内容：
- 服务注册（register）
- 服务续约（renew）
- 服务下线（unregister）
- 服务拉取（fetchRegistry）
- 缓存刷新（refreshRegistry）
eureka client 与 eureka server 的交互式通过REST API 来完成的，那么这时使用的HttpClient工具在，Netflix eureka和 spring cloud eureka 中是有区别的，前者使用的是 JerseyReplicationClient，后者使用的是 RestTemplateEurekaHttpClient
2、服务注册（register）
服务注册的方式是通过rest api 进行注册的，注册成功之后返回的正常状态码是 204
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;
}
3、服务续约（renew）
服务先向server端发送一个心跳，如果返回状态是 200，则表示续约成功；如果返回状态码是404，则向服务端重新注册自己。
续约服务是由一个守护线程每隔 30秒向服务端发送心跳来完成的
/**
* 续约方法
*/
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;
}
}
// 心跳定时器
scheduler.schedule(
new TimedSupervisorTask(
"heartbeat",
scheduler,
heartbeatExecutor,
renewalIntervalInSecs,
TimeUnit.SECONDS,
expBackOffBound,
new HeartbeatThread()
),
renewalIntervalInSecs, TimeUnit.SECONDS);
private class HeartbeatThread implements Runnable {
public void run() {
if (renew()) {
lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
}
}
}
4、服务下线（unregister）
通过rest api 向server端发送取消请求，那么在server端将会注销掉该服务，前提是请求注销的服务在注册中心已经存在了。
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);
}
}
}
5、服务获取（fetchRegistry）
如果增量拉取被禁用或是第一次拉取，则全量拉取server端已经注册的服务实例信息，否则只拉取增量服务实例数据
private boolean fetchRegistry(boolean forceFullRegistryFetch) {
Stopwatch tracer = FETCH_REGISTRY_TIMER.start();
try {
// If the delta is disabled or if it is the first time, get all
// applications
Applications applications = getApplications();
if (clientConfig.shouldDisableDelta()
|| (!Strings.isNullOrEmpty(clientConfig.getRegistryRefreshSingleVipAddress()))
|| forceFullRegistryFetch
|| (applications == null)
|| (applications.getRegisteredApplications().size() == 0)
|| (applications.getVersion() == -1)) //Client application does not have latest library supporting delta
{
logger.info("Disable delta property : {}", clientConfig.shouldDisableDelta());
logger.info("Single vip registry refresh property : {}", clientConfig.getRegistryRefreshSingleVipAddress());
logger.info("Force full registry fetch : {}", forceFullRegistryFetch);
logger.info("Application is null : {}", (applications == null));
logger.info("Registered Applications size is zero : {}",
(applications.getRegisteredApplications().size() == 0));
logger.info("Application version is -1: {}", (applications.getVersion() == -1));
//全量拉取服务实例数据
getAndStoreFullRegistry();
} else {
//增量拉取服务实例
getAndUpdateDelta(applications);
}
applications.setAppsHashCode(applications.getReconcileHashCode());
logTotalInstances();
} catch (Throwable e) {
logger.error(PREFIX + "{} - was unable to refresh its cache! status = {}", appPathIdentifier, e.getMessage(), e);
return false;
} finally {
if (tracer != null) {
tracer.stop();
}
}
// 刷新本地缓存
onCacheRefreshed();
// 基于缓存中的实例数据更新远程实例状态
updateInstanceRemoteStatus();
// 注册表拉取成功后返回true
return true;
}
6、缓存刷新（refreshRegistry）
系统默认是每隔30秒刷新本地存储的注册表
private void initScheduledTasks() {
if (clientConfig.shouldFetchRegistry()) {
// registry cache refresh timer
int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds();
int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound();
scheduler.schedule(
new TimedSupervisorTask(
"cacheRefresh",
scheduler,
cacheRefreshExecutor,
registryFetchIntervalSeconds,
TimeUnit.SECONDS,
expBackOffBound,
new CacheRefreshThread()
),
registryFetchIntervalSeconds, TimeUnit.SECONDS);
}
...................................
}
class CacheRefreshThread implements Runnable {
public void run() {
refreshRegistry();
}
}
@VisibleForTesting
void refreshRegistry() {
try {
boolean isFetchingRemoteRegionRegistries = isFetchingRemoteRegionRegistries();
boolean remoteRegionsModified = false;
// This makes sure that a dynamic change to remote regions to fetch is honored.
String latestRemoteRegions = clientConfig.fetchRegistryForRemoteRegions();
if (null != latestRemoteRegions) {
String currentRemoteRegions = remoteRegionsToFetch.get();
if (!latestRemoteRegions.equals(currentRemoteRegions)) {
// Both remoteRegionsToFetch and AzToRegionMapper.regionsToFetch need to be in sync
synchronized (instanceRegionChecker.getAzToRegionMapper()) {
if (remoteRegionsToFetch.compareAndSet(currentRemoteRegions, latestRemoteRegions)) {
String[] remoteRegions = latestRemoteRegions.split(",");
remoteRegionsRef.set(remoteRegions);
instanceRegionChecker.getAzToRegionMapper().setRegionsToFetch(remoteRegions);
remoteRegionsModified = true;
} else {
logger.info("Remote regions to fetch modified concurrently," +
" ignoring change from {} to {}", currentRemoteRegions, latestRemoteRegions);
}
}
} else {
// Just refresh mapping to reflect any DNS/Property change
instanceRegionChecker.getAzToRegionMapper().refreshMapping();
}
}
boolean success = fetchRegistry(remoteRegionsModified);
if (success) {
registrySize = localRegionApps.get().size();
lastSuccessfulRegistryFetchTimestamp = System.currentTimeMillis();
}
if (logger.isDebugEnabled()) {
StringBuilder allAppsHashCodes = new StringBuilder();
allAppsHashCodes.append("Local region apps hashcode: ");
allAppsHashCodes.append(localRegionApps.get().getAppsHashCode());
allAppsHashCodes.append(", is fetching remote regions? ");
allAppsHashCodes.append(isFetchingRemoteRegionRegistries);
for (Map.Entry<String, Applications> entry : remoteRegionVsApps.entrySet()) {
allAppsHashCodes.append(", Remote region: ");
allAppsHashCodes.append(entry.getKey());
allAppsHashCodes.append(" , apps hashcode: ");
allAppsHashCodes.append(entry.getValue().getAppsHashCode());
}
logger.debug("Completed cache refresh task for discovery. All Apps hash code is {} ",
allAppsHashCodes);
}
} catch (Throwable e) {
logger.error("Cannot fetch registry from server", e);
}
}
文末总结：
1、Eureka client从注册中心拉取服务列表，然后自身会做缓存
2、作为服务消费者，就是从这些缓存信息中获取的服务提供者的信息
3、增量更新的服务以30秒为周期循环更新
4、增量更新数据在服务端保存时间为3分钟，因此Eureka client取得的数据虽然是增量更新，仍然可能和30秒前取的数据一样，所以Eureka client要自己来处理重复信息
5、Eureka client的增量更新，其实获取的是Eureka server最近三分钟内的变更，如果Eureka client有超过三分钟没有做增量更新的话（例如网络问题），这就造成了Eureka server和Eureka client之间的数据不一致。正常情况下，Eureka client多次增量更新后，最终的服务列表数据应该Eureka server保持一致，但如果期间发生异常，可能导致和Eureka server的数据不一致，为了暴露这个问题，Eureka server每次返回的增量更新数据中，会带有一致性哈希码，Eureka client用本地服务列表数据算出的一致性哈希码应该和Eureka server返回的一致，若不一致就证明增量更新出了问题导致Eureka client和Eureka server上的服务列表信息不一致了，此时需要全量更新
博主文章同步发布：https://blog.csdn.net/u010647035/article/details/83959552
