Zookeeper源码部分 第2章 2.5 Follower和Leader状态同步源码

2022/6/5 1:20:15

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2.5 Follower和Leader状态同步源码

当选举结束后,每个节点都需要根据自己的角色更新自己的状态。选举出的Leader更新自己状态为Leader,其他节点更新自己状态为Follower。

Leader更新状态入口:leader.lead()

Follower更新状态入口:follower.followerLeader()

​ 注意:

(1)follower必须要让leader知道自己的状态:epoch、zxid、sid

必须要找出谁是leader;

发起请求连接leader;

发送自己的信息给leader;

leader接收到信息,必须要返回对应的信息给follower。

(2)当leader得知follower的状态了,就确定需要做何种方式的数据同步DIFF、TRUNC、SNAP

(3)执行数据同步

(4)当leader接收到超过半数follower的ack之后,进入正常工作状态,集群启动完成了

最终总结同步的方式:

(1)DIFF咱两一样,不需要做什么

(2)TRUNC follower的zxid比leader的zxid大,所以Follower要回滚

(3)COMMIT leader的zxid比follower的zxid大,发送Proposal给foloower提交执行

(4)如果follower并没有任何数据,直接使用SNAP的方式来执行数据同步(直接把数据全部序列到follower)

image-20220604230458476 image-20220604230524409

2.5.1 Leader.lead()等待接收follower的状态同步申请

1)在Leader.java种查找lead()方法

void lead() throws IOException, InterruptedException {
    self.end_fle = Time.currentElapsedTime();
    long electionTimeTaken = self.end_fle - self.start_fle;
    self.setElectionTimeTaken(electionTimeTaken);
    LOG.info("LEADING - LEADER ELECTION TOOK - {} {}", electionTimeTaken,
            QuorumPeer.FLE_TIME_UNIT);
    self.start_fle = 0;
    self.end_fle = 0;

    zk.registerJMX(new LeaderBean(this, zk), self.jmxLocalPeerBean);

    try {
        self.tick.set(0);
		// 恢复数据到内存,启动时,其实已经加载过了
        zk.loadData();

        leaderStateSummary = new StateSummary(self.getCurrentEpoch(), zk.getLastProcessedZxid());

        // Start thread that waits for connection requests from
        // new followers.
		// 等待其他follower节点向leader节点发送同步状态
        cnxAcceptor = new LearnerCnxAcceptor();
        cnxAcceptor.start();

        long epoch = getEpochToPropose(self.getId(), self.getAcceptedEpoch());

        … …
    } finally {
        zk.unregisterJMX(this);
    }
}
class LearnerCnxAcceptor extends ZooKeeperCriticalThread {
    private volatile boolean stop = false;

    public LearnerCnxAcceptor() {
        super("LearnerCnxAcceptor-" + ss.getLocalSocketAddress(), zk
                .getZooKeeperServerListener());
    }

    @Override
    public void run() {
        try {
            while (!stop) {
                Socket s = null;
                boolean error = false;
                try {
					// 等待接收follower的状态同步申请
                    s = ss.accept();

                    // start with the initLimit, once the ack is processed
                    // in LearnerHandler switch to the syncLimit
                    s.setSoTimeout(self.tickTime * self.initLimit);
                    s.setTcpNoDelay(nodelay);

                    BufferedInputStream is = new BufferedInputStream(
                            s.getInputStream());
					// 一旦接收到follower的请求,就创建LearnerHandler对象,处理请求
                    LearnerHandler fh = new LearnerHandler(s, is, Leader.this);
					// 启动线程
                    fh.start();
                } catch (SocketException e) {
                    ... ...
                } 
				... ...
            }
        } catch (Exception e) {
            LOG.warn("Exception while accepting follower", e.getMessage());
            handleException(this.getName(), e);
        }
    }

    public void halt() {
        stop = true;
    }
}

其中ss的初始化是在创建Leader对象时,创建的socket

private final ServerSocket ss;

Leader(QuorumPeer self,LeaderZooKeeperServer zk) throws IOException {
    this.self = self;
    this.proposalStats = new BufferStats();
    try {
        if (self.shouldUsePortUnification() || self.isSslQuorum()) {
            boolean allowInsecureConnection = self.shouldUsePortUnification();
            if (self.getQuorumListenOnAllIPs()) {
                ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection, self.getQuorumAddress().getPort());
            } else {
                ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection);
            }
        } else {
            if (self.getQuorumListenOnAllIPs()) {
                ss = new ServerSocket(self.getQuorumAddress().getPort());
            } else {
                ss = new ServerSocket();
            }
        }
        ss.setReuseAddress(true);
        if (!self.getQuorumListenOnAllIPs()) {
            ss.bind(self.getQuorumAddress());
        }
    } catch (BindException e) {
        ... ...
    }
    this.zk = zk;
    this.learnerSnapshotThrottler = createLearnerSnapshotThrottler(
            maxConcurrentSnapshots, maxConcurrentSnapshotTimeout);
}

2.5.2 Follower.lead()查找并连接Leader

1)在Follower.java种查找followLeader()方法

void followLeader() throws InterruptedException {
    self.end_fle = Time.currentElapsedTime();
    long electionTimeTaken = self.end_fle - self.start_fle;
    self.setElectionTimeTaken(electionTimeTaken);
    LOG.info("FOLLOWING - LEADER ELECTION TOOK - {} {}", electionTimeTaken,
            QuorumPeer.FLE_TIME_UNIT);
    self.start_fle = 0;
    self.end_fle = 0;
    fzk.registerJMX(new FollowerBean(this, zk), self.jmxLocalPeerBean);
    try {
		// 查找leader
        QuorumServer leaderServer = findLeader();            
        try {
            // 连接leader
            connectToLeader(leaderServer.addr, leaderServer.hostname);

            // 向leader注册
            long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);

            if (self.isReconfigStateChange())
               throw new Exception("learned about role change");
            //check to see if the leader zxid is lower than ours
            //this should never happen but is just a safety check
            long newEpoch = ZxidUtils.getEpochFromZxid(newEpochZxid);
            if (newEpoch < self.getAcceptedEpoch()) {
                LOG.error("Proposed leader epoch " + ZxidUtils.zxidToString(newEpochZxid)
                        + " is less than our accepted epoch " + ZxidUtils.zxidToString(self.getAcceptedEpoch()));
                throw new IOException("Error: Epoch of leader is lower");
            }
            syncWithLeader(newEpochZxid);                
            QuorumPacket qp = new QuorumPacket();
            while (this.isRunning()) {
                readPacket(qp);
                processPacket(qp);
            }
        } catch (Exception e) {
            LOG.warn("Exception when following the leader", e);
            try {
                sock.close();
            } catch (IOException e1) {
                e1.printStackTrace();
            }

            // clear pending revalidations
            pendingRevalidations.clear();
        }
    } finally {
        zk.unregisterJMX((Learner)this);
    }
}

/
 * Returns the address of the node we think is the leader.
 */
protected QuorumServer findLeader() {
    QuorumServer leaderServer = null;
    // Find the leader by id
	// 选举投票的时候记录的,最后推荐的leader的sid
    Vote current = self.getCurrentVote();
	
	// 如果这个sid在启动的所有服务器范围中
    for (QuorumServer s : self.getView().values()) {
        if (s.id == current.getId()) {
            // Ensure we have the leader's correct IP address before
            // attempting to connect.
			// 尝试连接leader的正确IP地址
            s.recreateSocketAddresses();
            leaderServer = s;
            break;
        }
    }
    if (leaderServer == null) {
        LOG.warn("Couldn't find the leader with id = "
                + current.getId());
    }
    return leaderServer;
}


protected void connectToLeader(InetSocketAddress addr, String hostname)
        throws IOException, InterruptedException, X509Exception {
    this.sock = createSocket();

    int initLimitTime = self.tickTime * self.initLimit;
    int remainingInitLimitTime = initLimitTime;
    long startNanoTime = nanoTime();

    for (int tries = 0; tries < 5; tries++) {
        try {
            // recalculate the init limit time because retries sleep for 1000 milliseconds
            remainingInitLimitTime = initLimitTime - (int)((nanoTime() - startNanoTime) / 1000000);
            if (remainingInitLimitTime <= 0) {
                LOG.error("initLimit exceeded on retries.");
                throw new IOException("initLimit exceeded on retries.");
            }

            sockConnect(sock, addr, Math.min(self.tickTime * self.syncLimit, remainingInitLimitTime));
            if (self.isSslQuorum())  {
                ((SSLSocket) sock).startHandshake();
            }
            sock.setTcpNoDelay(nodelay);
            break;
        } catch (IOException e) {
            ... ...
        }
        Thread.sleep(1000);
    }

    self.authLearner.authenticate(sock, hostname);

    leaderIs = BinaryInputArchive.getArchive(new BufferedInputStream(
            sock.getInputStream()));
    bufferedOutput = new BufferedOutputStream(sock.getOutputStream());
    leaderOs = BinaryOutputArchive.getArchive(bufferedOutput);
}

2.5.3 Leader.lead()创建LearnerHandler

void lead() throws IOException, InterruptedException {
    self.end_fle = Time.currentElapsedTime();
    long electionTimeTaken = self.end_fle - self.start_fle;
    self.setElectionTimeTaken(electionTimeTaken);
    LOG.info("LEADING - LEADER ELECTION TOOK - {} {}", electionTimeTaken,
            QuorumPeer.FLE_TIME_UNIT);
    self.start_fle = 0;
    self.end_fle = 0;

    zk.registerJMX(new LeaderBean(this, zk), self.jmxLocalPeerBean);

    try {
        self.tick.set(0);
        zk.loadData();

        leaderStateSummary = new StateSummary(self.getCurrentEpoch(), zk.getLastProcessedZxid());

        // Start thread that waits for connection requests from
        // new followers.
        cnxAcceptor = new LearnerCnxAcceptor();
        cnxAcceptor.start();

        ......
    } finally {
        zk.unregisterJMX(this);
    }
}

class LearnerCnxAcceptor extends ZooKeeperCriticalThread {
    private volatile boolean stop = false;
    ... ...

    @Override
    public void run() {
        try {
            while (!stop) {
                Socket s = null;
                boolean error = false;
                try {
                    s = ss.accept();

                    // start with the initLimit, once the ack is processed
                    // in LearnerHandler switch to the syncLimit
                    s.setSoTimeout(self.tickTime * self.initLimit);
                    s.setTcpNoDelay(nodelay);

                    BufferedInputStream is = new BufferedInputStream(
                            s.getInputStream());
                    LearnerHandler fh = new LearnerHandler(s, is, Leader.this);
                    fh.start();
                } catch (SocketException e) {
                    ... ...
				}
            }
        } catch (Exception e) {
            LOG.warn("Exception while accepting follower", e.getMessage());
            handleException(this.getName(), e);
        }
    }

    public void halt() {
        stop = true;
    }
}
由于public class LearnerHandler extends ZooKeeperThread{},说明LearnerHandler是一个线程。所以fh.start()执行的是LearnerHandler中的run()方法。
public void run() {
    try {
        leader.addLearnerHandler(this);
		// 心跳处理
        tickOfNextAckDeadline = leader.self.tick.get()
                + leader.self.initLimit + leader.self.syncLimit;

        ia = BinaryInputArchive.getArchive(bufferedInput);
        bufferedOutput = new BufferedOutputStream(sock.getOutputStream());
        oa = BinaryOutputArchive.getArchive(bufferedOutput);

		// 从网络中接收消息,并反序列化为packet
        QuorumPacket qp = new QuorumPacket();
        ia.readRecord(qp, "packet");

		// 选举结束后,observer和follower都应该给leader发送一个标志信息:FOLLOWERINFO或者OBSERVERINFO
        if(qp.getType() != Leader.FOLLOWERINFO && qp.getType() != Leader.OBSERVERINFO){
            LOG.error("First packet " + qp.toString()
                    + " is not FOLLOWERINFO or OBSERVERINFO!");
            return;
        }

        byte learnerInfoData[] = qp.getData();
        if (learnerInfoData != null) {
            ByteBuffer bbsid = ByteBuffer.wrap(learnerInfoData);
            if (learnerInfoData.length >= 8) {
                this.sid = bbsid.getLong();
            }
            if (learnerInfoData.length >= 12) {
                this.version = bbsid.getInt(); // protocolVersion
            }
            if (learnerInfoData.length >= 20) {
                long configVersion = bbsid.getLong();
                if (configVersion > leader.self.getQuorumVerifier().getVersion()) {
                    throw new IOException("Follower is ahead of the leader (has a later activated configuration)");
                }
            }
        } else {
            this.sid = leader.followerCounter.getAndDecrement();
        }

        if (leader.self.getView().containsKey(this.sid)) {
            LOG.info("Follower sid: " + this.sid + " : info : "
                    + leader.self.getView().get(this.sid).toString());
        } else {
            LOG.info("Follower sid: " + this.sid + " not in the current config " + Long.toHexString(leader.self.getQuorumVerifier().getVersion()));
        }
                    
        if (qp.getType() == Leader.OBSERVERINFO) {
              learnerType = LearnerType.OBSERVER;
        }
		// 读取Follower发送过来的lastAcceptedEpoch
		// 选举过程中,所使用的epoch,其实还是上一任leader的epoch
        long lastAcceptedEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());

        long peerLastZxid;
        StateSummary ss = null;
		
		// 读取follower发送过来的zxid
        long zxid = qp.getZxid();
		
		// Leader根据从Follower获取sid和旧的epoch,构建新的epoch 
        long newEpoch = leader.getEpochToPropose(this.getSid(), lastAcceptedEpoch);
        long newLeaderZxid = ZxidUtils.makeZxid(newEpoch, 0);

        if (this.getVersion() < 0x10000) {
            // we are going to have to extrapolate the epoch information
            long epoch = ZxidUtils.getEpochFromZxid(zxid);
            ss = new StateSummary(epoch, zxid);
            // fake the message
            leader.waitForEpochAck(this.getSid(), ss);
        } else {
            byte ver[] = new byte[4];
            ByteBuffer.wrap(ver).putInt(0x10000);
			// Leader向Follower发送信息(包含:zxid和newEpoch)
            QuorumPacket newEpochPacket = new QuorumPacket(Leader.LEADERINFO, newLeaderZxid, ver, null);
            oa.writeRecord(newEpochPacket, "packet");
			
            bufferedOutput.flush();
            QuorumPacket ackEpochPacket = new QuorumPacket();
            ia.readRecord(ackEpochPacket, "packet");
            if (ackEpochPacket.getType() != Leader.ACKEPOCH) {
                LOG.error(ackEpochPacket.toString()
                        + " is not ACKEPOCH");
                return;
			}
            ByteBuffer bbepoch = ByteBuffer.wrap(ackEpochPacket.getData());
            ss = new StateSummary(bbepoch.getInt(), ackEpochPacket.getZxid());
            leader.waitForEpochAck(this.getSid(), ss);
        }
        peerLastZxid = ss.getLastZxid();
       
        // Take any necessary action if we need to send TRUNC or DIFF
        // startForwarding() will be called in all cases
        boolean needSnap = syncFollower(peerLastZxid, leader.zk.getZKDatabase(), leader);
        
        /* if we are not truncating or sending a diff just send a snapshot */
        if (needSnap) {
            boolean exemptFromThrottle = getLearnerType() != LearnerType.OBSERVER;
            LearnerSnapshot snapshot = 
                    leader.getLearnerSnapshotThrottler().beginSnapshot(exemptFromThrottle);
            try {
                long zxidToSend = leader.zk.getZKDatabase().getDataTreeLastProcessedZxid();
                oa.writeRecord(new QuorumPacket(Leader.SNAP, zxidToSend, null, null), "packet");
                bufferedOutput.flush();

                LOG.info("Sending snapshot last zxid of peer is 0x{}, zxid of leader is 0x{}, "
                        + "send zxid of db as 0x{}, {} concurrent snapshots, " 
                        + "snapshot was {} from throttle",
                        Long.toHexString(peerLastZxid), 
                        Long.toHexString(leaderLastZxid),
                        Long.toHexString(zxidToSend), 
                        snapshot.getConcurrentSnapshotNumber(),
                        snapshot.isEssential() ? "exempt" : "not exempt");
                // Dump data to peer
                leader.zk.getZKDatabase().serializeSnapshot(oa);
                oa.writeString("BenWasHere", "signature");
                bufferedOutput.flush();
            } finally {
                snapshot.close();
            }
        }

        LOG.debug("Sending NEWLEADER message to " + sid);
        // the version of this quorumVerifier will be set by leader.lead() in case
        // the leader is just being established. waitForEpochAck makes sure that readyToStart is true if
        // we got here, so the version was set
        if (getVersion() < 0x10000) {
            QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER,
                    newLeaderZxid, null, null);
            oa.writeRecord(newLeaderQP, "packet");
        } else {
            QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER,
                    newLeaderZxid, leader.self.getLastSeenQuorumVerifier()
                            .toString().getBytes(), null);
            queuedPackets.add(newLeaderQP);
        }
        bufferedOutput.flush();

        // Start thread that blast packets in the queue to learner
        startSendingPackets();
        
        /*
         * Have to wait for the first ACK, wait until
         * the leader is ready, and only then we can
         * start processing messages.
         */
        qp = new QuorumPacket();
        ia.readRecord(qp, "packet");
        if(qp.getType() != Leader.ACK){
            LOG.error("Next packet was supposed to be an ACK,"
                + " but received packet: {}", packetToString(qp));
            return;
        }

        if(LOG.isDebugEnabled()){
        	LOG.debug("Received NEWLEADER-ACK message from " + sid);   
        }
        leader.waitForNewLeaderAck(getSid(), qp.getZxid());

        syncLimitCheck.start();
        
        // now that the ack has been processed expect the syncLimit
        sock.setSoTimeout(leader.self.tickTime * leader.self.syncLimit);

        /*
         * Wait until leader starts up
         */
        synchronized(leader.zk){
            while(!leader.zk.isRunning() && !this.isInterrupted()){
                leader.zk.wait(20);
            }
        }
        // Mutation packets will be queued during the serialize,
        // so we need to mark when the peer can actually start
        // using the data
        //
        LOG.debug("Sending UPTODATE message to " + sid);      
        queuedPackets.add(new QuorumPacket(Leader.UPTODATE, -1, null, null));

        while (true) {
            qp = new QuorumPacket();
            ia.readRecord(qp, "packet");

            long traceMask = ZooTrace.SERVER_PACKET_TRACE_MASK;
            if (qp.getType() == Leader.PING) {
                traceMask = ZooTrace.SERVER_PING_TRACE_MASK;
            }
            if (LOG.isTraceEnabled()) {
                ZooTrace.logQuorumPacket(LOG, traceMask, 'i', qp);
            }
            tickOfNextAckDeadline = leader.self.tick.get() + leader.self.syncLimit;


            ByteBuffer bb;
            long sessionId;
            int cxid;
            int type;

            switch (qp.getType()) {
            case Leader.ACK:
                if (this.learnerType == LearnerType.OBSERVER) {
                    if (LOG.isDebugEnabled()) {
                        LOG.debug("Received ACK from Observer  " + this.sid);
                    }
                }
                syncLimitCheck.updateAck(qp.getZxid());
                leader.processAck(this.sid, qp.getZxid(), sock.getLocalSocketAddress());
                break;
            case Leader.PING:
                // Process the touches
                ByteArrayInputStream bis = new ByteArrayInputStream(qp
                        .getData());
                DataInputStream dis = new DataInputStream(bis);
                while (dis.available() > 0) {
                    long sess = dis.readLong();
                    int to = dis.readInt();
                    leader.zk.touch(sess, to);
                }
                break;
            case Leader.REVALIDATE:
                bis = new ByteArrayInputStream(qp.getData());
                dis = new DataInputStream(bis);
                long id = dis.readLong();
                int to = dis.readInt();
                ByteArrayOutputStream bos = new ByteArrayOutputStream();
                DataOutputStream dos = new DataOutputStream(bos);
                dos.writeLong(id);
                boolean valid = leader.zk.checkIfValidGlobalSession(id, to);
                if (valid) {
                    try {
                        //set the session owner
                        // as the follower that
                        // owns the session
                        leader.zk.setOwner(id, this);
                    } catch (SessionExpiredException e) {
                        LOG.error("Somehow session " + Long.toHexString(id) +
                                " expired right after being renewed! (impossible)", e);
                    }
                }
                if (LOG.isTraceEnabled()) {
                    ZooTrace.logTraceMessage(LOG,
                                             ZooTrace.SESSION_TRACE_MASK,
                                             "Session 0x" + Long.toHexString(id)
                                             + " is valid: "+ valid);
                }
                dos.writeBoolean(valid);
                qp.setData(bos.toByteArray());
                queuedPackets.add(qp);
                break;
            case Leader.REQUEST:
                bb = ByteBuffer.wrap(qp.getData());
                sessionId = bb.getLong();
                cxid = bb.getInt();
                type = bb.getInt();
                bb = bb.slice();
                Request si;
                if(type == OpCode.sync){
                    si = new LearnerSyncRequest(this, sessionId, cxid, type, bb, qp.getAuthinfo());
                } else {
                    si = new Request(null, sessionId, cxid, type, bb, qp.getAuthinfo());
                }
                si.setOwner(this);
                leader.zk.submitLearnerRequest(si);
                break;
            default:
                LOG.warn("unexpected quorum packet, type: {}", packetToString(qp));
                break;
            }
        }
    } catch (IOException e) {
        ... ...
    } finally {
        ... ...
    }
}

2.5.4 Follower.lead()创建registerWithLeader

void followLeader() throws InterruptedException {
    self.end_fle = Time.currentElapsedTime();
    long electionTimeTaken = self.end_fle - self.start_fle;
    self.setElectionTimeTaken(electionTimeTaken);
    LOG.info("FOLLOWING - LEADER ELECTION TOOK - {} {}", electionTimeTaken,
            QuorumPeer.FLE_TIME_UNIT);
    self.start_fle = 0;
    self.end_fle = 0;
    fzk.registerJMX(new FollowerBean(this, zk), self.jmxLocalPeerBean);
    try {
        // 查找leader
        QuorumServer leaderServer = findLeader(); 

        try {
			// 连接leader
            connectToLeader(leaderServer.addr, leaderServer.hostname);

			// 向leader注册
            long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);
			
            if (self.isReconfigStateChange())
               throw new Exception("learned about role change");
            //check to see if the leader zxid is lower than ours
            //this should never happen but is just a safety check
            long newEpoch = ZxidUtils.getEpochFromZxid(newEpochZxid);
            if (newEpoch < self.getAcceptedEpoch()) {
                LOG.error("Proposed leader epoch " + ZxidUtils.zxidToString(newEpochZxid)
                        + " is less than our accepted epoch " + ZxidUtils.zxidToString(self.getAcceptedEpoch()));
                throw new IOException("Error: Epoch of leader is lower");
            }
            syncWithLeader(newEpochZxid);                
            QuorumPacket qp = new QuorumPacket();
			// 循环等待接收消息
            while (this.isRunning()) {
				// 读取packet信息
                readPacket(qp);
				// 处理packet消息
                processPacket(qp);
            }
        } catch (Exception e) {
            LOG.warn("Exception when following the leader", e);
            try {
                sock.close();
            } catch (IOException e1) {
                e1.printStackTrace();
            }

            // clear pending revalidations
            pendingRevalidations.clear();
        }
    } finally {
        zk.unregisterJMX((Learner)this);
    }
}

protected long registerWithLeader(int pktType) throws IOException{
    /*
     * Send follower info, including last zxid and sid
     */
	long lastLoggedZxid = self.getLastLoggedZxid();
    QuorumPacket qp = new QuorumPacket();                
    qp.setType(pktType);
    qp.setZxid(ZxidUtils.makeZxid(self.getAcceptedEpoch(), 0));
    
    /*
     * Add sid to payload
     */
    LearnerInfo li = new LearnerInfo(self.getId(), 0x10000, self.getQuorumVerifier().getVersion());
    ByteArrayOutputStream bsid = new ByteArrayOutputStream();
    BinaryOutputArchive boa = BinaryOutputArchive.getArchive(bsid);
    boa.writeRecord(li, "LearnerInfo");
    qp.setData(bsid.toByteArray());
    
	// 发送FollowerInfo给Leader
    writePacket(qp, true);
	
	// 读取Leader返回的结果:LeaderInfo
    readPacket(qp);    
    
    final long newEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());
	// 如果接收到LeaderInfo
	if (qp.getType() == Leader.LEADERINFO) {
    	// we are connected to a 1.0 server so accept the new epoch and read the next packet
    	leaderProtocolVersion = ByteBuffer.wrap(qp.getData()).getInt();
    	byte epochBytes[] = new byte[4];
    	final ByteBuffer wrappedEpochBytes = ByteBuffer.wrap(epochBytes);
		
		// 接收leader的epoch
    	if (newEpoch > self.getAcceptedEpoch()) {
			// 把自己原来的epoch保存在wrappedEpochBytes里
    		wrappedEpochBytes.putInt((int)self.getCurrentEpoch());
			// 把leader发送过来的epoch保存起来
    		self.setAcceptedEpoch(newEpoch);
    	} else if (newEpoch == self.getAcceptedEpoch()) {
    		// since we have already acked an epoch equal to the leaders, we cannot ack
    		// again, but we still need to send our lastZxid to the leader so that we can
    		// sync with it if it does assume leadership of the epoch.
    		// the -1 indicates that this reply should not count as an ack for the new epoch
            wrappedEpochBytes.putInt(-1);
    	} else {
    		throw new IOException("Leaders epoch, " + newEpoch + " is less than accepted epoch, " + self.getAcceptedEpoch());
    	}
		// 发送ackepoch给leader(包含了自己的:epoch和zxid)
    	QuorumPacket ackNewEpoch = new QuorumPacket(Leader.ACKEPOCH, lastLoggedZxid, epochBytes, null);
    	writePacket(ackNewEpoch, true);
		
        return ZxidUtils.makeZxid(newEpoch, 0);
    } else {
    	if (newEpoch > self.getAcceptedEpoch()) {
    		self.setAcceptedEpoch(newEpoch);
    	}
        if (qp.getType() != Leader.NEWLEADER) {
            LOG.error("First packet should have been NEWLEADER");
            throw new IOException("First packet should have been NEWLEADER");
        }
        return qp.getZxid();
    }
}

2.5.5 Leader.lead()接收Follwer状态,根据同步方式发送同步消息

public void run() {
    try {
        leader.addLearnerHandler(this);
		// 心跳处理
        tickOfNextAckDeadline = leader.self.tick.get()
                + leader.self.initLimit + leader.self.syncLimit;

        ia = BinaryInputArchive.getArchive(bufferedInput);
        bufferedOutput = new BufferedOutputStream(sock.getOutputStream());
        oa = BinaryOutputArchive.getArchive(bufferedOutput);

		// 从网络中接收消息,并反序列化为packet
        QuorumPacket qp = new QuorumPacket();
        ia.readRecord(qp, "packet");
		// 选举结束后,observer和follower都应该给leader发送一个标志信息:FOLLOWERINFO 或者OBSERVERINFO
        if(qp.getType() != Leader.FOLLOWERINFO && qp.getType() != Leader.OBSERVERINFO){
            LOG.error("First packet " + qp.toString()
                    + " is not FOLLOWERINFO or OBSERVERINFO!");
            return;
        }

        byte learnerInfoData[] = qp.getData();
        if (learnerInfoData != null) {
            ByteBuffer bbsid = ByteBuffer.wrap(learnerInfoData);
            if (learnerInfoData.length >= 8) {
                this.sid = bbsid.getLong();
            }
            if (learnerInfoData.length >= 12) {
                this.version = bbsid.getInt(); // protocolVersion
            }
            if (learnerInfoData.length >= 20) {
                long configVersion = bbsid.getLong();
                if (configVersion > leader.self.getQuorumVerifier().getVersion()) {
                    throw new IOException("Follower is ahead of the leader (has a later activated configuration)");
                }
            }
        } else {
            this.sid = leader.followerCounter.getAndDecrement();
        }

        if (leader.self.getView().containsKey(this.sid)) {
            LOG.info("Follower sid: " + this.sid + " : info : "
                    + leader.self.getView().get(this.sid).toString());
        } else {
            LOG.info("Follower sid: " + this.sid + " not in the current config " + Long.toHexString(leader.self.getQuorumVerifier().getVersion()));
        }
                    
        if (qp.getType() == Leader.OBSERVERINFO) {
              learnerType = LearnerType.OBSERVER;
        }
		// 读取Follower发送过来的lastAcceptedEpoch
		// 选举过程中,所使用的epoch,其实还是上一任leader的epoch
        long lastAcceptedEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());

        long peerLastZxid;
        StateSummary ss = null;
		
		// 读取follower发送过来的zxid
        long zxid = qp.getZxid();
		
		// 获取leader的最新epoch
		// 新的leader会构建一个新的epoch
        long newEpoch = leader.getEpochToPropose(this.getSid(), lastAcceptedEpoch);
        long newLeaderZxid = ZxidUtils.makeZxid(newEpoch, 0);

        if (this.getVersion() < 0x10000) {
            // we are going to have to extrapolate the epoch information
            long epoch = ZxidUtils.getEpochFromZxid(zxid);
            ss = new StateSummary(epoch, zxid);
            // fake the message
            leader.waitForEpochAck(this.getSid(), ss);
        } else {
            byte ver[] = new byte[4];
            ByteBuffer.wrap(ver).putInt(0x10000);
			
			// Leader向Follower发送信息(包含:zxid和newEpoch)
            QuorumPacket newEpochPacket = new QuorumPacket(Leader.LEADERINFO, newLeaderZxid, ver, null);
            oa.writeRecord(newEpochPacket, "packet");
			
            bufferedOutput.flush();
			// 接收到Follower应答的ackepoch
            QuorumPacket ackEpochPacket = new QuorumPacket();
            ia.readRecord(ackEpochPacket, "packet");
			
            if (ackEpochPacket.getType() != Leader.ACKEPOCH) {
                LOG.error(ackEpochPacket.toString()
                        + " is not ACKEPOCH");
                return;
			}
            ByteBuffer bbepoch = ByteBuffer.wrap(ackEpochPacket.getData());
			
			// 保存了对方follower或者observer的状态:epoch和zxid
            ss = new StateSummary(bbepoch.getInt(), ackEpochPacket.getZxid());
			
            leader.waitForEpochAck(this.getSid(), ss);
        }
        peerLastZxid = ss.getLastZxid();
       
        // Take any necessary action if we need to send TRUNC or DIFF
        // startForwarding() will be called in all cases
		// 方法判断Leader和Follower是否需要同步
        boolean needSnap = syncFollower(peerLastZxid, leader.zk.getZKDatabase(), leader);
        
        /* if we are not truncating or sending a diff just send a snapshot */
        if (needSnap) {
            boolean exemptFromThrottle = getLearnerType() != LearnerType.OBSERVER;
            LearnerSnapshot snapshot = 
                    leader.getLearnerSnapshotThrottler().beginSnapshot(exemptFromThrottle);
            try {
                long zxidToSend = leader.zk.getZKDatabase().getDataTreeLastProcessedZxid();
                oa.writeRecord(new QuorumPacket(Leader.SNAP, zxidToSend, null, null), "packet");
                bufferedOutput.flush();

                … …
                // Dump data to peer
                leader.zk.getZKDatabase().serializeSnapshot(oa);
                oa.writeString("BenWasHere", "signature");
                bufferedOutput.flush();
            } finally {
                snapshot.close();
            }
        }

        if (getVersion() < 0x10000) {
            QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER,
                        newLeaderZxid, null, null);
            oa.writeRecord(newLeaderQP, "packet");
        } else {
            QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER,
                        newLeaderZxid, leader.self.getLastSeenQuorumVerifier()
                                .toString().getBytes(), null);
            queuedPackets.add(newLeaderQP);
        }
        … …

        while (true) {
            … …
        }
    } catch (IOException e) {
        ... ...
    } finally {
        ... ...
    }
}


public boolean syncFollower(long peerLastZxid, ZKDatabase db, Leader leader) {
    /*
     * When leader election is completed, the leader will set its
     * lastProcessedZxid to be (epoch < 32). There will be no txn associated
     * with this zxid.
     *
     * The learner will set its lastProcessedZxid to the same value if
     * it get DIFF or SNAP from the leader. If the same learner come
     * back to sync with leader using this zxid, we will never find this
     * zxid in our history. In this case, we will ignore TRUNC logic and
     * always send DIFF if we have old enough history
     */
    boolean isPeerNewEpochZxid = (peerLastZxid & 0xffffffffL) == 0;
    // Keep track of the latest zxid which already queued
    long currentZxid = peerLastZxid;
    boolean needSnap = true;
    boolean txnLogSyncEnabled = db.isTxnLogSyncEnabled();
    ReentrantReadWriteLock lock = db.getLogLock();
    ReadLock rl = lock.readLock();
    try {
        rl.lock();
        long maxCommittedLog = db.getmaxCommittedLog();
        long minCommittedLog = db.getminCommittedLog();
        long lastProcessedZxid = db.getDataTreeLastProcessedZxid();

        LOG.info("Synchronizing with Follower sid: {} maxCommittedLog=0x{}"
                + " minCommittedLog=0x{} lastProcessedZxid=0x{}"
                + " peerLastZxid=0x{}", getSid(),
                Long.toHexString(maxCommittedLog),
                Long.toHexString(minCommittedLog),
                Long.toHexString(lastProcessedZxid),
                Long.toHexString(peerLastZxid));

        if (db.getCommittedLog().isEmpty()) {
            /*
             * It is possible that committedLog is empty. In that case
             * setting these value to the latest txn in leader db
             * will reduce the case that we need to handle
             *
             * Here is how each case handle by the if block below
             * 1. lastProcessZxid == peerZxid -> Handle by (2)
             * 2. lastProcessZxid < peerZxid -> Handle by (3)
             * 3. lastProcessZxid > peerZxid -> Handle by (5)
             */
            minCommittedLog = lastProcessedZxid;
            maxCommittedLog = lastProcessedZxid;
        }

        /*
         * Here are the cases that we want to handle
         *
         * 1. Force sending snapshot (for testing purpose)
         * 2. Peer and leader is already sync, send empty diff
         * 3. Follower has txn that we haven't seen. This may be old leader
         *    so we need to send TRUNC. However, if peer has newEpochZxid,
         *    we cannot send TRUNC since the follower has no txnlog
         * 4. Follower is within committedLog range or already in-sync.
         *    We may need to send DIFF or TRUNC depending on follower's zxid
         *    We always send empty DIFF if follower is already in-sync
         * 5. Follower missed the committedLog. We will try to use on-disk
         *    txnlog + committedLog to sync with follower. If that fail,
         *    we will send snapshot
         */

        if (forceSnapSync) {
            // Force leader to use snapshot to sync with follower
            LOG.warn("Forcing snapshot sync - should not see this in production");
        } else if (lastProcessedZxid == peerLastZxid) {
            // Follower is already sync with us, send empty diff
            LOG.info("Sending DIFF zxid=0x" + Long.toHexString(peerLastZxid) +
                     " for peer sid: " +  getSid());
            queueOpPacket(Leader.DIFF, peerLastZxid);
            needOpPacket = false;
            needSnap = false;
        } else if (peerLastZxid > maxCommittedLog && !isPeerNewEpochZxid) {
            // Newer than committedLog, send trunc and done
            LOG.debug("Sending TRUNC to follower zxidToSend=0x" +
                      Long.toHexString(maxCommittedLog) +
                      " for peer sid:" +  getSid());
            queueOpPacket(Leader.TRUNC, maxCommittedLog);
            currentZxid = maxCommittedLog;
            needOpPacket = false;
            needSnap = false;
        } else if ((maxCommittedLog >= peerLastZxid)
                && (minCommittedLog <= peerLastZxid)) {
            // Follower is within commitLog range
            LOG.info("Using committedLog for peer sid: " +  getSid());
            Iterator<Proposal> itr = db.getCommittedLog().iterator();
            currentZxid = queueCommittedProposals(itr, peerLastZxid,
                                                 null, maxCommittedLog);
            needSnap = false;
        } else if (peerLastZxid < minCommittedLog && txnLogSyncEnabled) {
            // Use txnlog and committedLog to sync

            // Calculate sizeLimit that we allow to retrieve txnlog from disk
            long sizeLimit = db.calculateTxnLogSizeLimit();
            // This method can return empty iterator if the requested zxid
            // is older than on-disk txnlog
            Iterator<Proposal> txnLogItr = db.getProposalsFromTxnLog(
                    peerLastZxid, sizeLimit);
            if (txnLogItr.hasNext()) {
                LOG.info("Use txnlog and committedLog for peer sid: " +  getSid());
                currentZxid = queueCommittedProposals(txnLogItr, peerLastZxid,
                                                     minCommittedLog, maxCommittedLog);

                LOG.debug("Queueing committedLog 0x" + Long.toHexString(currentZxid));
                Iterator<Proposal> committedLogItr = db.getCommittedLog().iterator();
                currentZxid = queueCommittedProposals(committedLogItr, currentZxid,
                                                     null, maxCommittedLog);
                needSnap = false;
            }
            // closing the resources
            if (txnLogItr instanceof TxnLogProposalIterator) {
                TxnLogProposalIterator txnProposalItr = (TxnLogProposalIterator) txnLogItr;
                txnProposalItr.close();
            }
        } else {
            LOG.warn("Unhandled scenario for peer sid: " +  getSid());
        }
        LOG.debug("Start forwarding 0x" + Long.toHexString(currentZxid) +
                  " for peer sid: " +  getSid());
        leaderLastZxid = leader.startForwarding(this, currentZxid);
    } finally {
        rl.unlock();
    }

    if (needOpPacket && !needSnap) {
        // This should never happen, but we should fall back to sending
        // snapshot just in case.
        LOG.error("Unhandled scenario for peer sid: " +  getSid() +
                 " fall back to use snapshot");
        needSnap = true;
    }

    return needSnap;
}

2.5.6 Follower.lead()应答Leader同步结果

protected void processPacket(QuorumPacket qp) throws Exception{
    switch (qp.getType()) {
    case Leader.PING:            
        ping(qp);            
        break;
    case Leader.PROPOSAL:           
        TxnHeader hdr = new TxnHeader();
        Record txn = SerializeUtils.deserializeTxn(qp.getData(), hdr);
        if (hdr.getZxid() != lastQueued + 1) {
            LOG.warn("Got zxid 0x"
                    + Long.toHexString(hdr.getZxid())
                    + " expected 0x"
                    + Long.toHexString(lastQueued + 1));
        }
        lastQueued = hdr.getZxid();
        
        if (hdr.getType() == OpCode.reconfig){
           SetDataTxn setDataTxn = (SetDataTxn) txn;       
           QuorumVerifier qv = self.configFromString(new String(setDataTxn.getData()));
           self.setLastSeenQuorumVerifier(qv, true);                               
        }
        
        fzk.logRequest(hdr, txn);
        break;
    case Leader.COMMIT:
        fzk.commit(qp.getZxid());
        break;
        
    … …
    case Leader.UPTODATE:
        LOG.error("Received an UPTODATE message after Follower started");
        break;
    case Leader.REVALIDATE:
        revalidate(qp);
        break;
    case Leader.SYNC:
        fzk.sync();
        break;
    default:
        LOG.warn("Unknown packet type: {}", LearnerHandler.packetToString(qp));
        break;
    }
}

public void commit(long zxid) {
    if (pendingTxns.size() == 0) {
        LOG.warn("Committing " + Long.toHexString(zxid)
                + " without seeing txn");
        return;
    }
    long firstElementZxid = pendingTxns.element().zxid;
    if (firstElementZxid != zxid) {
        LOG.error("Committing zxid 0x" + Long.toHexString(zxid)
                + " but next pending txn 0x"
                + Long.toHexString(firstElementZxid));
        System.exit(12);
    }
    Request request = pendingTxns.remove();
    commitProcessor.commit(request);
}

2.5.7 Leader.lead()应答Follower

由于public class LearnerHandler extends ZooKeeperThread{},说明LearnerHandler是一个线程。所以fh.start()执行的是LearnerHandler中的run()方法。

public void run() {
    … …
        //
        LOG.debug("Sending UPTODATE message to " + sid);      
        queuedPackets.add(new QuorumPacket(Leader.UPTODATE, -1, null, null));

        while (true) {
            … …
        }
    } catch (IOException e) {
        ... ...
    } finally {
        ... ...
    }
}



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