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心跳，eureka client每隔一定的时间，会给eureka server发送心跳，保持心跳，让eureka server知道自己还活着
（1）DiscoveryClient初始化的时候，会去调度一堆定时任务，其中有一个就是HeartbeatThread，心跳线程
@Singleton
public class DiscoveryClient implements EurekaClient {
//构造函数
@Inject
DiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs args,
Provider<BackupRegistry> backupRegistryProvider) {
//初始化调度任务
initScheduledTasks();
}
private void initScheduledTasks() {
// Heartbeat timer
scheduler.schedule(
new TimedSupervisorTask(
"heartbeat",
scheduler,
heartbeatExecutor,
renewalIntervalInSecs,
TimeUnit.SECONDS,
expBackOffBound,
new HeartbeatThread()
),
renewalIntervalInSecs, TimeUnit.SECONDS);
}
}（2）在这里可以看到，默认是每隔30秒去发送一次心跳，每隔30秒执行一次HeartbeatTHread线程的逻辑，发送心跳
@JsonRootName("leaseInfo")
public class LeaseInfo {
public static final int DEFAULT_LEASE_RENEWAL_INTERVAL = 30;
}（3）这边的话就是去发送这个心跳，走的是EurekaHttpClient的sendHeartbeat()方法，http://localhost:8080/v2/apps/ServiceA/i-000000-1，走的是put请求
/**
* The heartbeat task that renews the lease in the given intervals.
* 在给定间隔更新租约的心跳任务。
* HeartbeatThread线程类
*/
private class HeartbeatThread implements Runnable {
public void run() {
if (renew()) {
lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
}
}
}
/**
* 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());
long timestamp = instanceInfo.setIsDirtyWithTime();
boolean success = register();
if (success) {
instanceInfo.unsetIsDirty(timestamp);
}
return success;
}
return httpResponse.getStatusCode() == 200;
} catch (Throwable e) {
logger.error("{} - was unable to send heartbeat!", PREFIX + appPathIdentifier, e);
return false;
}
}public class Jersey2ReplicationClient extends AbstractJersey2EurekaHttpClient implements HttpReplicationClient {
@Override
public EurekaHttpResponse<InstanceInfo> sendHeartBeat(String appName, String id, InstanceInfo info, InstanceStatus overriddenStatus) {
String urlPath = "apps/" + appName + '/' + id;
}
}（4）负责承接服务实例的心跳相关的这些操作的，是ApplicationsResource，服务相关的controller，eureka用的是jersey的mvc框架。找到ApplicationResource，再次找到InstanceResource，通过PUT请求，可以找到renewLease方法。
@Produces({"application/xml", "application/json"})
public class InstanceResource {
@PUT
public Response renewLease(
@HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication,
@QueryParam("overriddenstatus") String overriddenStatus,
@QueryParam("status") String status,
@QueryParam("lastDirtyTimestamp") String lastDirtyTimestamp) {
boolean isFromReplicaNode = "true".equals(isReplication);
boolean isSuccess = registry.renew(app.getName(), id, isFromReplicaNode);
// Not found in the registry, immediately ask for a register
if (!isSuccess) {
logger.warn("Not Found (Renew): {} - {}", app.getName(), id);
return Response.status(Status.NOT_FOUND).build();
}
// Check if we need to sync based on dirty time stamp, the client
// instance might have changed some value
Response response = null;
if (lastDirtyTimestamp != null && serverConfig.shouldSyncWhenTimestampDiffers()) {
response = this.validateDirtyTimestamp(Long.valueOf(lastDirtyTimestamp), isFromReplicaNode);
// Store the overridden status since the validation found out the node that replicates wins
if (response.getStatus() == Response.Status.NOT_FOUND.getStatusCode()
&& (overriddenStatus != null)
&& !(InstanceStatus.UNKNOWN.name().equals(overriddenStatus))
&& isFromReplicaNode) {
registry.storeOverriddenStatusIfRequired(app.getAppName(), id, InstanceStatus.valueOf(overriddenStatus));
}
} else {
response = Response.ok().build();
}
logger.debug("Found (Renew): {} - {}; reply status={}" + app.getName(), id, response.getStatus());
return response;
}
}（5）通过注册表的renew()方法，进去完成服务续约，实际进入AbstractInstanceRegistry的renew()方法
public abstract class AbstractInstanceRegistry implements InstanceRegistry {
public boolean renew(String appName, String id, boolean isReplication) {
RENEW.increment(isReplication);
Map<String, Lease<InstanceInfo>> gMap = registry.get(appName);
Lease<InstanceInfo> leaseToRenew = null;
if (gMap != null) {
leaseToRenew = gMap.get(id);
}
if (leaseToRenew == null) {
RENEW_NOT_FOUND.increment(isReplication);
logger.warn("DS: Registry: lease doesn't exist, registering resource: {} - {}", appName, id);
return false;
} else {
InstanceInfo instanceInfo = leaseToRenew.getHolder();
if (instanceInfo != null) {
// touchASGCache(instanceInfo.getASGName());
InstanceStatus overriddenInstanceStatus = this.getOverriddenInstanceStatus(
instanceInfo, leaseToRenew, isReplication);
if (overriddenInstanceStatus == InstanceStatus.UNKNOWN) {
logger.info("Instance status UNKNOWN possibly due to deleted override for instance {}"
+ "; re-register required", instanceInfo.getId());
RENEW_NOT_FOUND.increment(isReplication);
return false;
}
if (!instanceInfo.getStatus().equals(overriddenInstanceStatus)) {
Object[] args = {
instanceInfo.getStatus().name(),
instanceInfo.getOverriddenStatus().name(),
instanceInfo.getId()
};
logger.info(
"The instance status {} is different from overridden instance status {} for instance {}. "
+ "Hence setting the status to overridden status", args);
instanceInfo.setStatusWithoutDirty(overriddenInstanceStatus);
}
}
renewsLastMin.increment();
//调用Lease类的renew方法更新lastUpdateTimestamp时间戳
leaseToRenew.renew();
return true;
}
}
}（6）从注册表的map中，根据服务名和实例id，获取一个Lease<InstanceInfo>。实际的服务续约的逻辑，其实就是在Lease对象中，更新一下lastUpdateTimestamp这个时间戳，每次续约，就更新一下这个时间戳就ok了。
public class Lease<T> {
public void renew() {
lastUpdateTimestamp = System.currentTimeMillis() + duration;
}
}总结：
所谓发送心跳，其实就是client端请求了server端一个类似于http://localhost:8080/v2/apps/ServiceA/i-000000-1的restful接口，然后根据appName和appId找到server端注册表中需要更新的服务实例（其实就是发送心跳的服务对应的服务实例），然后更新一下它的lastUpdateTimestamp为当前时间即可。