zookeeper的Leader选举源码解析
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Tech 导读
zookeeper是一个分布式服务框架，主要解决分布式应用中常见的多种数据问题，例如集群管理，状态同步等。为解决这些问题zookeeper需要Leader选举进行保障数据的强一致性机制和稳定性。本文通过集群的配置，对leader选举源进行解析，让读者们了解如何利用BIO通信机制，多线程多层队列实现高性能架构。
导读
每一个zookeeper服务端称之为一个节点，每个节点都有投票权，把其选票投向每一个有选举权的节点，当其中一个节点选举出票数过半，这个节点就会成为Leader，其它节点成为Follower。
1. 重命名zoo_sample.cfg文件为zoo1.cfg ，zoo2.cfg，zoo3.cfg，zoo4.cfg
2. 修改zoo.cfg文件，修改值如下：
【plain】zoo1.cfg文件内容：dataDir=/export/data/zookeeper-1clientPort=2181server.1=127.0.0.1:2001:3001server.2=127.0.0.1:2002:3002:participantserver.3=127.0.0.1:2003:3003:participantserver.4=127.0.0.1:2004:3004:observerzoo2.cfg文件内容：dataDir=/export/data/zookeeper-2clientPort=2182server.1=127.0.0.1:2001:3001server.2=127.0.0.1:2002:3002:participantserver.3=127.0.0.1:2003:3003:participantserver.4=127.0.0.1:2004:3004:observerzoo3.cfg文件内容：dataDir=/export/data/zookeeper-3clientPort=2183server.1=127.0.0.1:2001:3001server.2=127.0.0.1:2002:3002:participantserver.3=127.0.0.1:2003:3003:participantserver.4=127.0.0.1:2004:3004:observerzoo4.cfg文件内容：dataDir=/export/data/zookeeper-4clientPort=2184server.1=127.0.0.1:2001:3001server.2=127.0.0.1:2002:3002:participantserver.3=127.0.0.1:2003:3003:participantserver.4=127.0.0.1:2004:3004:observer
3. server.第几号服务器(对应myid文件内容)=ip:数据同步端口:选举端口:选举标识
participant默认参与选举标识，可不写. observer不参与选举
4.在/export/data/zookeeper-1,/export/data/zookeeper-2,/export/data/zookeeper-3,/export/data/zookeeper-4目录下创建myid文件，文件内容分别写1 ，2，3，4，用于标识sid(全称：Server ID)赋值。
5. 启动三个zookeeper实例：
bin/zkServer.sh start conf/zoo1.cfg
bin/zkServer.sh start conf/zoo2.cfg
bin/zkServer.sh start conf/zoo3.cfg
图1 第一轮到第二轮投票流程
前提：
设定票据数据格式vote(sid,zxid,epoch)
sid是Server ID每台服务的唯一标识，是myid文件内容；
zxid是数据事务id号；
epoch为选举周期，为方便理解下面讲解内容暂定为1初次选举，不写入下面内容里。
按照顺序启动sid=1，sid=2节点
第一轮投票：
1. sid=1节点：初始选票为自己，将选票vote(1,0)发送给sid=2节点；
2. sid=2节点：初始选票为自己，将选票vote(2,0)发送给sid=1节点；
3. sid=1节点：收到sid=2节点选票vote(2,0)和当前自己的选票vote(1,0)，首先比对zxid值，zxid越大代表数据最新，优先选择zxid最大的选票，如果zxid相同，选举最大sid。当前投票选举结果为vote(2,0)，sid=1节点的选票变为vote(2,0)；
4. sid=2节点：收到sid=1节点选票vote(1,0)和当前自己的选票vote(2,0),参照上述选举方式，选举结果为vote(2,0),sid=2节点的选票不变；
5. 第一轮投票选举结束。
第二轮投票：
1. sid=1节点：当前自己的选票为vote(2,0),将选票vote(2,0)发送给sid=2节点；
2. sid=2节点：当前自己的选票为vote(2,0),将选票vote(2,0)发送给sid=1节点；
3. sid=1节点：收到sid=2节点选票vote(2,0)和自己的选票vote(2,0), 按照半数选举算法，总共3个节点参与选举，已有2个节点选举出相同选票,推举sid=2节点为Leader，自己角色变为Follower；
4. sid=2节点：收到sid=1节点选票vote(2,0)和自己的选票vote(2,0)，按照半数选举算法推举sid=2节点为Leader,自己角色变为Leader。
这时启动sid=3节点后，集群里已经选举出leader，sid=1和sid=2节点会将自己的leader选票发回给sid=3节点，通过半数选举结果还是sid=2节点为leader。
3.1 Leader选举采用多层队列架构
通过查看zkServer.sh文件内容找到服务启动类：
图3 选举代码实现流程图
1. 加载配置文件QuorumPeerConfig.parse(path)；
针对 Leader选举关键配置信息如下：
读取dataDir目录找到myid文件内容，设置当前应用sid标识，做为投票人身份信息。下面遇到myid变量为当前节点自己sid标识。
设置peerType当前应用是否参与选举
new QuorumMaj()解析server.前缀加载集群成员信息，加载allMembers所有成员，votingMembers参与选举成员，observingMembers观察者成员，设置half值votingMembers.size()/2.
【Java】public QuorumMaj(Properties props) throws ConfigException {for (Entry<Object, Object> entry : props.entrySet()) {String key = entry.getKey().toString();String value = entry.getValue().toString();//读取集群配置文件中的server.开头的应用实例配置信息if (key.startsWith("server.")) {int dot = key.indexOf('.');long sid = Long.parseLong(key.substring(dot + 1));QuorumServer qs = new QuorumServer(sid, value);allMembers.put(Long.valueOf(sid), qs);if (qs.type == LearnerType.PARTICIPANT)//应用实例绑定的角色为PARTICIPANT意为参与选举votingMembers.put(Long.valueOf(sid), qs);else {//观察者成员observingMembers.put(Long.valueOf(sid), qs);}} else if (key.equals("version")) {version = Long.parseLong(value, 16);}}//过半基数half = votingMembers.size() / 2;}
2. QuorumPeerMain.runFromConfig(config) 启动服务；
3. QuorumPeer.startLeaderElection() 开启选举服务；
设置当前选票new Vote(sid,zxid,epoch)
【plain】synchronized public void startLeaderElection(){try {if (getPeerState() == ServerState.LOOKING) {//首轮：当前节点默认投票对象为自己currentVote = new Vote(myid, getLastLoggedZxid(), getCurrentEpoch());}} catch(IOException e) {RuntimeException re = new RuntimeException(e.getMessage());re.setStackTrace(e.getStackTrace());throw re;}//........}
创建选举管理类：QuorumCnxnManager；
初始化recvQueue<Message(sid,ByteBuffer)>接收投票队列(第二层传输队列)；
初始化queueSendMap<sid,queue>按sid发送投票队列(第二层传输队列)；
初始化senderWorkerMap<sid,SendWorker>发送投票工作线程容器，表示着与sid投票节点已连接；
初始化选举监听线程类QuorumCnxnManager.Listener。
【Java】//QuorumPeer.createCnxnManager()public QuorumCnxManager(QuorumPeer self,final long mySid,Map<Long,QuorumPeer.QuorumServer> view,QuorumAuthServer authServer,QuorumAuthLearner authLearner,int socketTimeout,boolean listenOnAllIPs,int quorumCnxnThreadsSize,boolean quorumSaslAuthEnabled) {//接收投票队列(第二层传输队列)this.recvQueue = new ArrayBlockingQueue<Message>(RECV_CAPACITY);//按sid发送投票队列(第二层传输队列)this.queueSendMap = new ConcurrentHashMap<Long, ArrayBlockingQueue<ByteBuffer>>();//发送投票工作线程容器，表示着与sid投票节点已连接this.senderWorkerMap = new ConcurrentHashMap<Long, SendWorker>();this.lastMessageSent = new ConcurrentHashMap<Long, ByteBuffer>();String cnxToValue = System.getProperty("zookeeper.cnxTimeout");if(cnxToValue != null){this.cnxTO = Integer.parseInt(cnxToValue);}this.self = self;this.mySid = mySid;this.socketTimeout = socketTimeout;this.view = view;this.listenOnAllIPs = listenOnAllIPs;initializeAuth(mySid, authServer, authLearner, quorumCnxnThreadsSize,quorumSaslAuthEnabled);// Starts listener thread that waits for connection requests//创建选举监听线程 接收选举投票请求listener = new Listener();listener.setName("QuorumPeerListener");}//QuorumPeer.createElectionAlgorithmprotected Election createElectionAlgorithm(int electionAlgorithm){Election le=null;//TODO: use a factory rather than a switchswitch (electionAlgorithm) {case 0:le = new LeaderElection(this);break;case 1:le = new AuthFastLeaderElection(this);break;case 2:le = new AuthFastLeaderElection(this, true);break;case 3:qcm = createCnxnManager();// new QuorumCnxManager(... new Listener())QuorumCnxManager.Listener listener = qcm.listener;if(listener != null){listener.start();//启动选举监听线程FastLeaderElection fle = new FastLeaderElection(this, qcm);fle.start();le = fle;} else {LOG.error("Null listener when initializing cnx manager");}break;default:assert false;}return le;}
4. 开启选举监听线程QuorumCnxnManager.Listener；
创建ServerSockket等待大于自己sid节点连接，连接信息存储到senderWorkerMap<sid,SendWorker>；
sid>self.sid才可以连接过来。
【Java】//上面的listener.start()执行后，选择此方法public void run() {int numRetries = 0;InetSocketAddress addr;Socket client = null;while((!shutdown) && (numRetries < 3)){try {ss = new ServerSocket();ss.setReuseAddress(true);if (self.getQuorumListenOnAllIPs()) {int port = self.getElectionAddress().getPort();addr = new InetSocketAddress(port);} else {// Resolve hostname for this server in case the// underlying ip address has changed.self.recreateSocketAddresses(self.getId());addr = self.getElectionAddress();}LOG.info("My election bind port: " + addr.toString());setName(addr.toString());ss.bind(addr);while (!shutdown) {client = ss.accept();setSockOpts(client);LOG.info("Received connection request "+ client.getRemoteSocketAddress());// Receive and handle the connection request// asynchronously if the quorum sasl authentication is// enabled. This is required because sasl server// authentication process may take few seconds to finish,// this may delay next peer connection requests.if (quorumSaslAuthEnabled) {receiveConnectionAsync(client);} else {//接收连接信息receiveConnection(client);}numRetries = 0;}} catch (IOException e) {if (shutdown) {break;}LOG.error("Exception while listening", e);numRetries++;try {ss.close();Thread.sleep(1000);} catch (IOException ie) {LOG.error("Error closing server socket", ie);} catch (InterruptedException ie) {LOG.error("Interrupted while sleeping. " +"Ignoring exception", ie);}closeSocket(client);}}LOG.info("Leaving listener");if (!shutdown) {LOG.error("As I'm leaving the listener thread, "+ "I won't be able to participate in leader "+ "election any longer: "+ self.getElectionAddress());} else if (ss != null) {// Clean up for shutdown.try {ss.close();} catch (IOException ie) {// Don't log an error for shutdown.LOG.debug("Error closing server socket", ie);}}}//代码执行路径：receiveConnection()->handleConnection(...)private void handleConnection(Socket sock, DataInputStream din)throws IOException {//...省略if (sid < self.getId()) {/** This replica might still believe that the connection to sid is* up, so we have to shut down the workers before trying to open a* new connection.*/SendWorker sw = senderWorkerMap.get(sid);if (sw != null) {sw.finish();}/** Now we start a new connection*/LOG.debug("Create new connection to server: {}", sid);closeSocket(sock);if (electionAddr != null) {connectOne(sid, electionAddr);} else {connectOne(sid);}} else { // Otherwise start worker threads to receive data.SendWorker sw = new SendWorker(sock, sid);RecvWorker rw = new RecvWorker(sock, din, sid, sw);sw.setRecv(rw);SendWorker vsw = senderWorkerMap.get(sid);if (vsw != null) {vsw.finish();}//存储连接信息<sid,SendWorker>senderWorkerMap.put(sid, sw);queueSendMap.putIfAbsent(sid,new ArrayBlockingQueue<ByteBuffer>(SEND_CAPACITY));sw.start();rw.start();}}
5. 创建FastLeaderElection快速选举服务；
初始选票发送队列sendqueue（第一层队列）
初始选票接收队列recvqueue（第一层队列）
创建线程WorkerSender
创建线程WorkerReceiver
【Java】//FastLeaderElection.starterprivate void starter(QuorumPeer self, QuorumCnxManager manager) {this.self = self;proposedLeader = -1;proposedZxid = -1;//发送队列sendqueue（第一层队列）sendqueue = new LinkedBlockingQueue<ToSend>();//接收队列recvqueue（第一层队列）recvqueue = new LinkedBlockingQueue<Notification>();this.messenger = new Messenger(manager);}//new Messenger(manager)Messenger(QuorumCnxManager manager) {//创建线程WorkerSenderthis.ws = new WorkerSender(manager);this.wsThread = new Thread(this.ws,"WorkerSender[myid=" + self.getId() + "]");this.wsThread.setDaemon(true);//创建线程WorkerReceiverthis.wr = new WorkerReceiver(manager);this.wrThread = new Thread(this.wr,"WorkerReceiver[myid=" + self.getId() + "]");this.wrThread.setDaemon(true)；}
6. 开启WorkerSender和WorkerReceiver线程。
WorkerSender线程自旋获取sendqueue第一层队列元素
sendqueue队列元素内容为相关选票信息详见ToSend类；
首先判断选票sid是否和自己sid值相同，相等直接放入到recvQueue队列中；
不相同将sendqueue队列元素转储到queueSendMap<sid,queue>第二层传输队列中。
【Java】//FastLeaderElection.Messenger.WorkerSenderclass WorkerSender extends ZooKeeperThread{//...public void run() {while (!stop) {try {ToSend m = sendqueue.poll(3000, TimeUnit.MILLISECONDS);if(m == null) continue;//将投票信息发送出去process(m);} catch (InterruptedException e) {break;}}LOG.info("WorkerSender is down");}}//QuorumCnxManager#toSendpublic void toSend(Long sid, ByteBuffer b) {/** If sending message to myself, then simply enqueue it (loopback).*/if (this.mySid == sid) {b.position(0);addToRecvQueue(new Message(b.duplicate(), sid));/** Otherwise send to the corresponding thread to send.*/} else {/** Start a new connection if doesn't have one already.*/ArrayBlockingQueue<ByteBuffer> bq = new ArrayBlockingQueue<ByteBuffer>(SEND_CAPACITY);ArrayBlockingQueue<ByteBuffer> oldq = queueSendMap.putIfAbsent(sid, bq);//转储到queueSendMap<sid,queue>第二层传输队列中if (oldq != null) {addToSendQueue(oldq, b);} else {addToSendQueue(bq, b);}connectOne(sid);}}
WorkerReceiver线程自旋获取recvQueue第二层传输队列元素转存到recvqueue第一层队列中。
【Java】//WorkerReceiverpublic void run() {Message response;while (!stop) {// Sleeps on receivetry {//自旋获取recvQueue第二层传输队列元素response = manager.pollRecvQueue(3000, TimeUnit.MILLISECONDS);if(response == null) continue;// The current protocol and two previous generations all send at least 28 bytesif (response.buffer.capacity() < 28) {LOG.error("Got a short response: " + response.buffer.capacity());continue;}//...if(self.getPeerState() == QuorumPeer.ServerState.LOOKING){//第二层传输队列元素转存到recvqueue第一层队列中recvqueue.offer(n);//...}}//...}
1. 启动线程QuorumPeer
开始Leader选举投票makeLEStrategy().lookForLeader()；
sendNotifications()向其它节点发送选票信息，选票信息存储到sendqueue队列中。sendqueue队列由WorkerSender线程处理。
【plain】//QuorunPeer.run//...try {reconfigFlagClear();if (shuttingDownLE) {shuttingDownLE = false;startLeaderElection();}//makeLEStrategy().lookForLeader() 发送投票setCurrentVote(makeLEStrategy().lookForLeader());} catch (Exception e) {LOG.warn("Unexpected exception", e);setPeerState(ServerState.LOOKING);}//...//FastLeaderElection.lookLeaderpublic Vote lookForLeader() throws InterruptedException {//...//向其他应用发送投票sendNotifications();//...}private void sendNotifications() {//获取应用节点for (long sid : self.getCurrentAndNextConfigVoters()) {QuorumVerifier qv = self.getQuorumVerifier();ToSend notmsg = new ToSend(ToSend.mType.notification,proposedLeader,proposedZxid,logicalclock.get(),QuorumPeer.ServerState.LOOKING,sid,proposedEpoch, qv.toString().getBytes());if(LOG.isDebugEnabled()){LOG.debug("Sending Notification: " + proposedLeader + " (n.leader), 0x" +Long.toHexString(proposedZxid) + " (n.zxid), 0x" + Long.toHexString(logicalclock.get()) +" (n.round), " + sid + " (recipient), " + self.getId() +" (myid), 0x" + Long.toHexString(proposedEpoch) + " (n.peerEpoch)");}//储存投票信息sendqueue.offer(notmsg);}}class WorkerSender extends ZooKeeperThread {//...public void run() {while (!stop) {try {//提取已储存的投票信息ToSend m = sendqueue.poll(3000, TimeUnit.MILLISECONDS);if(m == null) continue;process(m);} catch (InterruptedException e) {break;}}LOG.info("WorkerSender is down");}//...}
自旋recvqueue队列元素获取投票过来的选票信息：
【Java】public Vote lookForLeader() throws InterruptedException {//.../** Loop in which we exchange notifications until we find a leader*/while ((self.getPeerState() == ServerState.LOOKING) &&(!stop)){/** Remove next notification from queue, times out after 2 times* the termination time*///提取投递过来的选票信息Notification n = recvqueue.poll(notTimeout,TimeUnit.MILLISECONDS);/** Sends more notifications if haven't received enough.* Otherwise processes new notification.*/if(n == null){if(manager.haveDelivered()){//已全部连接成功，并且前一轮投票都完成，需要再次发起投票sendNotifications();} else {//如果未收到选票信息，manager.contentAll()自动连接其它socket节点manager.connectAll();}/** Exponential backoff*/int tmpTimeOut = notTimeout*2;notTimeout = (tmpTimeOut < maxNotificationInterval?tmpTimeOut : maxNotificationInterval);LOG.info("Notification time out: " + notTimeout);}//....}//...}
【Java】//manager.connectAll()->connectOne(sid)->initiateConnection(...)->startConnection(...)private boolean startConnection(Socket sock, Long sid)throws IOException {DataOutputStream dout = null;DataInputStream din = null;try {// Use BufferedOutputStream to reduce the number of IP packets. This is// important for x-DC scenarios.BufferedOutputStream buf = new BufferedOutputStream(sock.getOutputStream());dout = new DataOutputStream(buf);// Sending id and challenge// represents protocol version (in other words - message type)dout.writeLong(PROTOCOL_VERSION);dout.writeLong(self.getId());String addr = self.getElectionAddress().getHostString() + ":" + self.getElectionAddress().getPort();byte[] addr_bytes = addr.getBytes();dout.writeInt(addr_bytes.length);dout.write(addr_bytes);dout.flush();din = new DataInputStream(new BufferedInputStream(sock.getInputStream()));} catch (IOException e) {LOG.warn("Ignoring exception reading or writing challenge: ", e);closeSocket(sock);return false;}// authenticate learnerQuorumPeer.QuorumServer qps = self.getVotingView().get(sid);if (qps != null) {// TODO - investigate why reconfig makes qps null.authLearner.authenticate(sock, qps.hostname);}// If lost the challenge, then drop the new connection//保证集群中所有节点之间只有一个通道连接if (sid > self.getId()) {LOG.info("Have smaller server identifier, so dropping the " +"connection: (" + sid + ", " + self.getId() + ")");closeSocket(sock);// Otherwise proceed with the connection} else {SendWorker sw = new SendWorker(sock, sid);RecvWorker rw = new RecvWorker(sock, din, sid, sw);sw.setRecv(rw);SendWorker vsw = senderWorkerMap.get(sid);if(vsw != null)vsw.finish();senderWorkerMap.put(sid, sw);queueSendMap.putIfAbsent(sid, new ArrayBlockingQueue<ByteBuffer>(SEND_CAPACITY));sw.start();rw.start();return true;}return false;}
图4 节点之间连接方式
【Java】public Vote lookForLeader() throws InterruptedException {//...if (n.electionEpoch > logicalclock.get()) {//当前选举周期小于选票周期，重置recvset选票池//大于当前周期更新当前选票信息，再次发送投票logicalclock.set(n.electionEpoch);recvset.clear();if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,getInitId(), getInitLastLoggedZxid(), getPeerEpoch())) {updateProposal(n.leader, n.zxid, n.peerEpoch);} else {updateProposal(getInitId(),getInitLastLoggedZxid(),getPeerEpoch());}sendNotifications();} else if (n.electionEpoch < logicalclock.get()) {if(LOG.isDebugEnabled()){LOG.debug("Notification election epoch is smaller than logicalclock. n.electionEpoch = 0x"+ Long.toHexString(n.electionEpoch)+ ", logicalclock=0x" + Long.toHexString(logicalclock.get()));}break;} else if (totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,proposedLeader, proposedZxid, proposedEpoch)) {//相同选举周期//接收的选票与当前选票PK成功后，替换当前选票updateProposal(n.leader, n.zxid, n.peerEpoch);sendNotifications();}//...}
在上代码中，自旋从recvqueue队列中获取到选票信息。开始进行选举：
判断当前选票和接收过来的选票周期是否一致
大于当前周期更新当前选票信息，再次发送投票
周期相等：当前选票信息和接收的选票信息进行PK
【Java】//接收的选票与当前选票PKprotected boolean totalOrderPredicate(long newId, long newZxid, long newEpoch, long curId, long curZxid, long curEpoch) {LOG.debug("id: " + newId + ", proposed id: " + curId + ", zxid: 0x" +Long.toHexString(newZxid) + ", proposed zxid: 0x" + Long.toHexString(curZxid));if(self.getQuorumVerifier().getWeight(newId) == 0){return false;}/** We return true if one of the following three cases hold:* 1- New epoch is higher* 2- New epoch is the same as current epoch, but new zxid is higher* 3- New epoch is the same as current epoch, new zxid is the same* as current zxid, but server id is higher.*/return ((newEpoch > curEpoch) ||((newEpoch == curEpoch) &&((newZxid > curZxid) || ((newZxid == curZxid) && (newId > curId)))));wId > curId)))));}
在上述代码中的totalOrderPredicate方法逻辑如下：
竞选周期大于当前周期为true
竞选周期相等，竞选zxid大于当前zxid为true
竞选周期相等，竞选zxid等于当前zxid，竞选sid大于当前sid为true
经过上述条件判断为true将当前选票信息替换为竞选成功的选票，同时再次将新的选票投出去。
【Java】public Vote lookForLeader() throws InterruptedException {//...//存储节点对应的选票信息// key:选票来源sid value:选票推举的Leader sidrecvset.put(n.sid, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch));//半数选举开始if (termPredicate(recvset,new Vote(proposedLeader, proposedZxid,logicalclock.get(), proposedEpoch))) {// Verify if there is any change in the proposed leaderwhile((n = recvqueue.poll(finalizeWait,TimeUnit.MILLISECONDS)) != null){if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,proposedLeader, proposedZxid, proposedEpoch)){recvqueue.put(n);break;}}/*WorkerSender* This predicate is true once we don't read any new* relevant message from the reception queue*/if (n == null) {//已选举出leader 更新当前节点是否为leaderself.setPeerState((proposedLeader == self.getId()) ?ServerState.LEADING: learningState());Vote endVote = new Vote(proposedLeader,proposedZxid, proposedEpoch);leaveInstance(endVote);return endVote;}}//...}/*** Termination predicate. Given a set of votes, determines if have* sufficient to declare the end of the election round.** @param votes* Set of votes* @param vote* Identifier of the vote received last PK后的选票*/private boolean termPredicate(HashMap<Long, Vote> votes, Vote vote) {SyncedLearnerTracker voteSet = new SyncedLearnerTracker();voteSet.addQuorumVerifier(self.getQuorumVerifier());if (self.getLastSeenQuorumVerifier() != null&& self.getLastSeenQuorumVerifier().getVersion() > self.getQuorumVerifier().getVersion()) {voteSet.addQuorumVerifier(self.getLastSeenQuorumVerifier());}/** First make the views consistent. Sometimes peers will have different* zxids for a server depending on timing.*///votes 来源于recvset 存储各个节点推举出来的选票信息for (Map.Entry<Long, Vote> entry : votes.entrySet()) {//选举出的sid和其它节点选择的sid相同存储到voteSet变量中。if (vote.equals(entry.getValue())) {//保存推举出来的sidvoteSet.addAck(entry.getKey());}}//判断选举出来的选票数量是否过半return voteSet.hasAllQuorums();}//QuorumMaj#containsQuorumpublic boolean containsQuorum(Set<Long> ackSet) {return (ackSet.size() > half);}
在上述代码中：recvset是存储每个sid推举的选票信息。
第一轮 sid1:vote(1,0,1) ,sid2:vote(2,0,1)；
第二轮 sid1:vote(2,0,1) ,sid2:vote(2,0,1)。
最终经过选举信息vote(2,0,1)为推荐leader,并用推荐leader在recvset选票池里比对持相同票数量为2个。因为总共有3个节点参与选举，sid1和sid2都选举sid2为leader，满足票数过半要求，故确认sid2为leader。
setPeerState更新当前节点角色；
proposedLeader选举出来的sid和自己sid相等，设置为Leader；
上述条件不相等，设置为Follower或Observing；
更新currentVote当前选票为Leader的选票vote(2,0,1)。
通过对Leader选举源码的解析，可以了解到：
1. 多个应用节点之间网络通信采用BIO方式进行相互投票，同时保证每个节点之间只使用一个通道，减少网络资源的消耗，足以见得在BIO分布式中间件开发中的技术重要性。
2. 基于BIO的基础上，灵活运用多线程和内存消息队列完好实现多层队列架构，每层队列由不同的线程分工协作，提高快速选举性能目的。
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