1-elasticsearch之discovery节点探测
在es的设计中，一个集群必须有一个主节点（master node）。用来处理请求、索引的创建、修改、节点管理等。
当有了master节点，该节点就要对各子节点进行周期性（心跳机制）的探测，保证整个集群的健康。
主节点和各节点之间都会进行心跳检测，比如mater要确保各节点健康状况、是否宕机等，而子节点也要要确保master的健康状况，一旦master宕机，各子节点要重新选举新的master。这种相互间的心跳检测就是cluster的faultdetection。下图展示了faultdetection继承关系。
faultdetection有两种实现方式，分别是master探测其他节点和其他节点对master的探测。faultdetection抽象了方法handleTransportDisconnect，该方法在内部类FDConnectionListener 中被调用。es中大量使用了listener的异步方式，因此可以大大的提升系统性能：
private class FDConnectionListener implements TransportConnectionListener {
@Override
public void onNodeConnected(DiscoveryNode node) {
}
@Override
public void onNodeDisconnected(DiscoveryNode node) {
handleTransportDisconnect(node);
}
}
当faultdetection启动时就会注册相应的FDConnectionListener，在周期性检测时，发现有节点失联，会通过onNodeDisconnected方法回调handleTransportDisconnect进行处理。先来看masterFaultdetection的启动代码：
private void innerStart(final DiscoveryNode masterNode) {
this.masterNode = masterNode;
this.retryCount = 0;
this.notifiedMasterFailure.set(false);
// 尝试连接master节点
try {
transportService.connectToNode(masterNode);
} catch (final Exception e) {
// 连接失败通知masterNode失败
notifyMasterFailure(masterNode, "failed to perform initial connect [" + e.getMessage() + "]");
return;
}
//关闭之前的masterping，重启新的masterping
if (masterPinger != null) {
masterPinger.stop();
}
this.masterPinger = new MasterPinger();
// 周期之后启动masterPing，这里并没有周期启动masterPing，只是设定了延迟时间。
threadPool.schedule(pingInterval, ThreadPool.Names.SAME, masterPinger);
}
再来看master连接失败的处理逻辑：
private void notifyMasterFailure(final DiscoveryNode masterNode, final String reason) {
if (notifiedMasterFailure.compareAndSet(false, true)) {
threadPool.generic().execute(new Runnable() {
@Override
public void run() {
//通知所有listener master丢失
for (Listener listener : listeners) {
listener.onMasterFailure(masterNode, reason);
}
}
});
stop("master failure, " + reason);
}
}
zen discovery机制实现了listener.onMasterFailure接口，处理master失联的相关问题。下面是部分示例代码：
private class MasterPinger implements Runnable {
private volatile boolean running = true;
public void stop() {
this.running = false;
}
@Override
public void run() {
if (!running) {
// return and don't spawn...
return;
}
final DiscoveryNode masterToPing = masterNode;
final MasterPingRequest request = new MasterPingRequest(clusterService.localNode().id(), masterToPing.id(), clusterName);
final TransportRequestOptions options = options().withType(TransportRequestOptions.Type.PING).withTimeout(pingRetryTimeout);
transportService.sendRequest(masterToPing, MASTER_PING_ACTION_NAME, request, options, new BaseTransportResponseHandler<MasterPingResponseResponse>() {
@Override
public MasterPingResponseResponse newInstance() {
return new MasterPingResponseResponse();
}
@Override
public void handleResponse(MasterPingResponseResponse response) {
if (!running) {
return;
}
// reset the counter, we got a good result
MasterFaultDetection.this.retryCount = 0;
// check if the master node did not get switched on us..., if it did, we simply return with no reschedule
if (masterToPing.equals(MasterFaultDetection.this.masterNode())) {
// 启动新的ping周期
threadPool.schedule(pingInterval, ThreadPool.Names.SAME, MasterPinger.this);
}
}
@Override
public void handleException(TransportException exp) {
if (!running) {
return;
}
synchronized (masterNodeMutex) {
// check if the master node did not get switched on us...
if (masterToPing.equals(MasterFaultDetection.this.masterNode())) {
if (exp instanceof ConnectTransportException || exp.getCause() instanceof ConnectTransportException) {
handleTransportDisconnect(masterToPing);
return;
} else if (exp.getCause() instanceof NoLongerMasterException) {
logger.debug("[master] pinging a master {} that is no longer a master", masterNode);
notifyMasterFailure(masterToPing, "no longer master");
return;
} else if (exp.getCause() instanceof NotMasterException) {
logger.debug("[master] pinging a master {} that is not the master", masterNode);
notifyMasterFailure(masterToPing, "not master");
return;
} else if (exp.getCause() instanceof NodeDoesNotExistOnMasterException) {
logger.debug("[master] pinging a master {} but we do not exists on it, act as if its master failure", masterNode);
notifyMasterFailure(masterToPing, "do not exists on master, act as master failure");
return;
}
int retryCount = ++MasterFaultDetection.this.retryCount;
logger.trace("[master] failed to ping [{}], retry [{}] out of [{}]", exp, masterNode, retryCount, pingRetryCount);
if (retryCount >= pingRetryCount) {
logger.debug("[master] failed to ping [{}], tried [{}] times, each with maximum [{}] timeout", masterNode, pingRetryCount, pingRetryTimeout);
// not good, failure
notifyMasterFailure(masterToPing, "failed to ping, tried [" + pingRetryCount + "] times, each with maximum [" + pingRetryTimeout + "] timeout");
} else {
// resend the request, not reschedule, rely on send timeout
transportService.sendRequest(masterToPing, MASTER_PING_ACTION_NAME, request, options, this);
}
}
}
}
);
}
}
masterPing是一个线程，在innerStart的方法中没有设定周期启动masterPing，但是由于masterPing需要进行心跳检测，这个问题就交给了上例的run方法。如果ping成功就会重启一个新的ping，这样既保证了ping线程的唯一性同时也保证了ping的顺序和间隔。ping的方式同样是通过transport发送一个masterPingRequest进行连接，节点收到该请求后，如果该节点已不再是master就会抛出一个NotMasterException。否则会响应notifyMasterFailure方法。对于网络问题导致的无响应情况，会调用handleTransportDisconnect(masterToPing)方法处理：
protected void handleTransportDisconnect(DiscoveryNode node) {
//这里需要同步
synchronized (masterNodeMutex) {
//master 已经换成其它节点，就没必要再连接
if (!node.equals(this.masterNode)) {
return;
}
if (connectOnNetworkDisconnect) {
try {
//尝试再次连接
transportService.connectToNode(node);
// if all is well, make sure we restart the pinger
if (masterPinger != null) {
masterPinger.stop();
}
//连接成功启动新的masterping
this.masterPinger = new MasterPinger();
// we use schedule with a 0 time value to run the pinger on the pool as it will run on later
threadPool.schedule(TimeValue.timeValueMillis(0), ThreadPool.Names.SAME, masterPinger);
} catch (Exception e) {
//连接出现异常，启动master节点丢失通知
logger.trace("[master] [{}] transport disconnected (with verified connect)", masterNode);
notifyMasterFailure(masterNode, "transport disconnected (with verified connect)");
}
} else {
//不需要重连，通知master丢失。
logger.trace("[master] [{}] transport disconnected", node);
notifyMasterFailure(node, "transport disconnected");
}
}
}
就是masterfaultDetection的整个流程：
启动中如果master节点失联则通知节点丢失，否则在一定延迟（3s）后启动masterPing，masterPing线程尝试连接master节点，如果master节点仍然失联，则再次尝试连接。master节点收到masterPingRequest请求后首先看一下自己还是不是master，如果不是则抛出异常，否则正常回应。节点如果收到响应式异常则启动master丢失通知，否则此次ping结束。在一定时间后重新启动新的masterPing线程。
这里只是说master的faultdetection，而node的faultdetection跟master逻辑相似。区别主要在于ping异常处理上。
在node的faultdetection中，当某个node出现异常或者没有响应，会启动node丢失机制，只是具体的处理逻辑不同。
欢迎斧正，that's all see also：[cluster discovery概述及FaultDetection分析](https://www.cnblogs.com/zziawanblog/p/6533731.html)
