elasticsearch的zendiscovery和master选举机制原理分析-mile米乐体育
目录
- 前言
- join的代码
- findmaster方法
- 总结
前言
上一篇通过electmasterservice源码，分析了master选举的原理的大部分内容：master候选节点id排序保证选举一致性及通过设置最小可见候选节点数目避免brain split。节点排序后选举只能保证局部一致性，如果发生节点接收到了错误的集群状态就会选举出错误的master，因此必须有其它措施来保证选举的一致性。这就是上一篇所提到的第二点：被选举的数量达到一定的数目同时自己也选举自己，这个节点才能成为master。这一点体现在zendiscovery中，本篇将结合节点的发现过程进一步介绍master选举机制。
节点启动后首先启动join线程，join线程会寻找cluster的master节点，如果集群之前已经启动，并且运行良好，则试图连接集群的master节点，加入集群。否则（集群正在启动）选举master节点，如果自己被选为master，则向集群中其它节点发送一个集群状态更新的task，如果master是其它节点则试图加入该集群。
join的代码
private void innerjoincluster() { discoverynode masternode = null; final thread currentthread = thread.currentthread(); //一直阻塞直到找到master节点，在集群刚刚启动，或者集群master丢失的情况，这种阻塞能够保证集群一致性 while (masternode == null && jointhreadcontrol.jointhreadactive(currentthread)) { masternode = findmaster(); } //有可能自己会被选举为master（集群启动，或者加入时正在选举） if (clusterservice.localnode().equals(masternode)) { //如果本身是master，则需要向其它所有节点发送集群状态更新 clusterservice.submitstateupdatetask("zen-disco-join (elected_as_master)", priority.immediate, new processedclusterstatenonmasterupdatetask() { @override public clusterstate execute(clusterstate currentstate) { //选举时错误的，之前的master状态良好，则不更新状态，仍旧使用之前状态。 if (currentstate.nodes().masternode() != null) { return currentstate; } discoverynodes.builder builder = new discoverynodes.builder(currentstate.nodes()).masternodeid(currentstate.nodes().localnode().id()); // update the fact that we are the master... clusterblocks clusterblocks = clusterblocks.builder().blocks(currentstate.blocks()).removeglobalblock(qwnardiscoverysettings.getnomasterblock()).build(); currentstate = clusterstate.builder(currentstate).nodes(builder).blocks(clusterblocks).build(); // eagerly run reroute to remove dead nodes from routing table routingallocation.result result = allocationservice.reroute(currentstate); return clusterstate.builder(currentstate).routingresult(result).build(); } @override public void onfailure(string source, throwable t) { logger.error("unexpected failure during [{}]", t, source); jointhreadcontrol.markthreadasdoneandstartnew(currentthread); } @override public void clusterstatephttp://www.cppcns.comrocessed(string source, clusterstate oldstate, clusterstate newstate) { if (newstate.nodes().localnodemaster()) { // we only starts nodesfd if we are master (it may be that we received a cluster state while pinging) jointhreadcontrol.markthreadasdone(currentthread); nodesfd.updatenodesandping(newstate); // start the nodes fd } else { // if we're not a master it means another node published a cluster state while we were pinging // make sure we go through another pinging round and actively join it jointhreadcontrol.markthreadasdoneandstartnew(currentthread); } sendinitialstateeventifneeded(); long count = clusterjoinscounter.incrementandget(); logger.trace("cluster joins counter set to [{}] (elected as master)", count); } }); } else { // 找到的节点不是我，试图连接该master final boolean success = joinelectedmaster(masternode); // finalize join through the cluster state update thread final discoverynode finalmasternode = masternode; clusterservice.submitstateupdatetask("finalize_join (" masternode ")", new clusterstatenonmasterupdatetask() { @override public clusterstate execute(clusterstate currentstate) throws exception { if (!success) { // failed to join. try again... jointhreadcontrol.markthreadasdoneandstartnew(currentthread); return currentstate; } if (currentstate.getnodes().masternode() == null) { // post 1.3.0, the master should publish a new cluster state before acking our join request. we now should have // a valid master. logger.debug("no master node is set, despite of join request completing. retrying pings."); jointhreadcontrol.markthreadasdoneandstartnew(currentthread); return currentstate; } if (!currentstate.getnodes().masternode().equals(finalmasternode)) { return jointhreadcontrol.stoprunningthreadandrejoin(currentstate, "master_switched_while_finalizing_join"); } // note: we do not have to start master fault detection here because it's set at {@link #handlenewclusterstatefrommaster } // when the first cluster state arrives. jointhreadcontrol.markthreadasdone(currentthread); return currentstate; } @override public void onfailure(string source, @nullable throwable t) { logger.error("unexpected error while trying to finalize cluster join", t); jointhreadcontrol.markthreadasdoneandstartnew(currentthread); } }); } }
以上就是join的过程。zendiscovery在启动时会启动一个join线程，这个线程调用了该方法。同时在节点离开，master丢失等情况下也会重启这一线程仍然运行join方法。
findmaster方法
这个方法体现了master选举的机制。代码如下：
private discoverynode findmaster() { //ping集群中的节点 zenping.pingresponse[] fullpingresponses = pingservice.pingandwait(pingtimeout); if (fullpingresponses == null) {return null; }// 过滤所得到的ping响应，虑除client节点，单纯的data节点 list
//本节点暂时是master也要加入候选节点进行选举 if (localnode.masternode()) { activenodes.add(localnode); long joinscounter = clusterjoinscounter.get(); if (joinscounter > 0) { logger.trace("adding local node to the list of active nodes who has previously joined the cluster (joins counter is [{}})", joinscounter); joinedonceactivenodes.add(localnode); } } //pingmasters为空，则本节点是master节点， if (pingmasters.isempty()) { if (electmaster.hasenoughmasternodes(activenodes)) {//保证选举数量,说明有足够多的节点选举本节点为master，但是这还不够，本节点还需要再选举一次，如果 本次选举节点仍旧是自己，那么本节点才能成为master。这里就体现了master选举的第二条原则。 discoverynode master = electmaster.electmaster(joinedonceactivenodes); if (master != null) { return master; } return electmaster.electmaster(activenodes); } else { // if we don't have enough master nodes, we bail, because there are not enough master to elect from logger.trace("not enough master nodes [{}]", activenodes); return null; } } else { //phttp://www.cppcns.comingmasters不为空（pingmasters列表中应该都是同一个节点），本节点没有被选举为master，那就接受之前的选举。 return electmaster.electmaster(pingmasters); } }
上面的重点部分都做了标注，就不再分析。除了findmaster方法，还有一个方法也体现了master选举，那就是handlemastergone。下面是它的部分代码，提交master丢失task部分，
clusterservice.submitstateupdatetask("zen-disco-master_failed (" masternode ")", priority.immediate,newprocessedclusterstatenonmasterupdatetask() { @override public clusterstate execute(clusterstate currentstate) { //获取到当前集群状态下的所有节点 discoverynodes discoverynodes = discoverynodes.builder(currentstate.nodes()) // make sure the old master node, which has failed, is not part of the nodes we publish .remove(masternode.id()) .masternodeid(null).build(); //rejoin过程仍然是重复findmaster过程 if (rejoin) { return rejoin(clusterstate.builder(currentstate).nodes(discoverynodes).build(), "master left (reason = " reason ")"); } //无法达到选举数量，进行findmaster过程 if (!electmaster.hasenoughmasternodes(discoverynodes)) { return rejoin(clusterstate.builder(currentstate).nodes(discoverynodes).build(), "not enough master nodes after master left (reason = " reason ")"); } //在当前集群状态下，如果候选节点数量达到预期数量，那么选举出来的节点一定是同一个节点，因为所有的节点看到的集群states是一致的 final discoverynode electedmaster = electmaster.electmaster(discoverynodes); // elect master final discoverynode localnode = currentstate.nodes().localnode(); .... }
从以上的代码可以看到master选举节点的应用场景，无论是findmaster还是handlemastergone,他们都保证了选举一致性。那就是所选节点数量必须要达到一定的数量，否则不能认为选举成功，进入等待环境。如果当前节点被其它节点选举为master，仍然要进行选举一次以保证选举的一致性。这样在保证了选举数量同时对候选节点排序从而保证选举的一致性。
发现和加入集群是zendiscovery的主要功能，当然它还有一些其它功能，如处理节点离开（handleleaverequest），处理master发送的最小clustersates（handlenewclusterstatefrommaster）等功能。这里就不一一介绍，有兴趣请参考相关源码。
总结
本节结合zendiscovery，分析了master选举的另外一部分内容。同时zendiscovery是节点发现集群功能的集合，它主要功能是发现（选举）出集群的master节点，并试图加入集群。同时如果 本机是master还会处理节点的离开和节点丢失，如果不是master则会处理来自master的节点状态更新。
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