zookeeper的Leader选举源码解析|视图|代码|key|leader

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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。

Leader选举机制

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Leader选举机制采用半数选举算法。

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。

Leader选举集群配置

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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-1 clientPort=2181 server.1=127.0.0.1:2001:3001 server.2=127.0.0.1:2002:3002:participant server.3=127.0.0.1:2003:3003:participant server.4=127.0.0.1:2004:3004:observer

zoo2.cfg文件内容： dataDir=/export/data/zookeeper-2 clientPort=2182 server.1=127.0.0.1:2001:3001 server.2=127.0.0.1:2002:3002:participant server.3=127.0.0.1:2003:3003:participant server.4=127.0.0.1:2004:3004:observer

zoo3.cfg文件内容： dataDir=/export/data/zookeeper-3 clientPort=2183 server.1=127.0.0.1:2001:3001 server.2=127.0.0.1:2002:3002:participant server.3=127.0.0.1:2003:3003:participant server.4=127.0.0.1:2004:3004:observer

zoo4.cfg文件内容： dataDir=/export/data/zookeeper-4 clientPort=2184 server.1=127.0.0.1:2001:3001 server.2=127.0.0.1:2002:3002:participant server.3=127.0.0.1:2003:3003:participant server.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

6. 每启动一个实例，都会读取启动参数配置zoo.cfg文件，这样实例就可以知道其作为服务端身份信息sid以及集群中有多少个实例参与选举。

Leader选举流程

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

zookeeper选举底层主要分为选举应用层和消息传输队列层，第一层应用层队列统一接收和发送选票，而第二层传输层队列，是按照服务端sid分成了多个队列，是为了避免给每台服务端发送消息互相影响。比如对某台机器发送不成功不会影响正常服务端的发送。

图2 多层队列上下关系交互流程图

解析代码入口类

通过查看zkServer.sh文件内容找到服务启动类：

org.apache.zookeeper.server.quorum.QuorumPeerMain

选举流程代码解析

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 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接收投票队列(第二层传输队列)；
初始化queueSendMap按sid发送投票队列(第二层传输队列)；
初始化senderWorkerMap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 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(RECV_CAPACITY); //按sid发送投票队列(第二层传输队列) this.queueSendMap = new ConcurrentHashMap>(); //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(); this.lastMessageSent = new ConcurrentHashMap();

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.createElectionAlgorithm protected Election createElectionAlgorithm(int electionAlgorithm){ Election le=null; //TODO: use a factory rather than a switch switch (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>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 senderWorkerMap.put(sid, sw);

queueSendMap.putIfAbsent(sid, new ArrayBlockingQueue(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.starter private 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(); //接收队列recvqueue（第一层队列） recvqueue = new LinkedBlockingQueue(); this.messenger = new Messenger(manager); } //new Messenger(manager) Messenger(QuorumCnxManager manager) { //创建线程WorkerSender this.ws = new WorkerSender(manager);

this.wsThread = new Thread(this.ws, "WorkerSender[myid=" + self.getId() + "]"); this.wsThread.setDaemon(true); //创建线程WorkerReceiver this.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#toSend public 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 bq = new ArrayBlockingQueue( SEND_CAPACITY); ArrayBlockingQueue oldq = queueSendMap.putIfAbsent(sid, bq); //转储到queueSendMap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】 //WorkerReceiver public void run() { Message response; while (!stop) { // Sleeps on receive try { //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 bytes if (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); //... } } //... }

选举核心逻辑

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.lookLeader public 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 learner QuorumPeer.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( SEND_CAPACITY));

sw.start(); rw.start();

return true;

} return false; }

如上述代码中所示，sid>self.sid才可以创建连接Socket和SendWorker,RecvWorker线程，存储到senderWorkerMap中。对应第2步中的sid

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PK

protected 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 sid recvset.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 leader while((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leader self.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 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 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sid voteSet.addAck(entry.getKey()); } } //判断选举出来的选票数量是否过半 return voteSet.hasAllQuorums(); } //QuorumMaj#containsQuorum public boolean containsQuorum(Set 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)。

总结

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。

3. 为BIO在多线程技术上的实践带来了宝贵的经验。

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