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zookeeper的Leader选举源码解析

2023-03-29 10:03:47 来源：博客园

作者：京东物流梁吉超

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。

【资料图】

01Leader选举机制

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。

02Leader选举集群配置

重命名zoo_sample.cfg文件为zoo1.cfg ，zoo2.cfg，zoo3.cfg，zoo4.cfg
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

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。

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

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。

03Leader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：

sid=1节点：初始选票为自己，将选票vote(1,0)发送给sid=2节点；
sid=2节点：初始选票为自己，将选票vote(2,0)发送给sid=1节点；
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)；
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；
第一轮投票选举结束。

第二轮投票：

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；
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；
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；
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

04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

05选举流程代码解析

图3 选举代码实现流程图

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;    }

QuorumPeerMain.runFromConfig(config) 启动服务；
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.createElectionAlgorithmprotected 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;}

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();     }}

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();    //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)；}

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#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 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】//WorkerReceiverpublic 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);         //...      }    }//...}

06选举核心逻辑

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 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 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#containsQuorumpublic 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)。

07总结

通过对Leader选举源码的解析，可以了解到：

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。
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。
为BIO在多线程技术上的实践带来了宝贵的经验。

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