【zookeeper核心源码解析】第三课：leader与follower何时开始同步，如何同步数据

系列文章目录

【zookeeper核心源码解析】第一课：zk启动类核心流程序列图
【zookeeper核心源码解析】第二课：俯瞰QuorumPeer启动核心流程，实现选举关键流程
【zookeeper核心源码解析】第三课：leader与follower何时开始同步，如何同步数据
【zookeeper核心源码解析】第四课：客户端与服务端读写的io核心流程

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1. leader准备好同步线程

leader.lead() 方法内部初始化LearnerCnxAcceptor后台线程，该线程用于与flollower同步数据：
 
            cnxAcceptor = new LearnerCnxAcceptor();
            cnxAcceptor.start();
            

2. follower开始同步

follower选举成功后，就开始同步，在上一节中，得知follower选举成功后执行follower.followLeader() 该方法会同步

源码如下：

  /**
     * the main method called by the follower to follow the leader
     *
     * @throws InterruptedException
     */
    void followLeader() throws InterruptedException {
        self.end_fle = System.currentTimeMillis();
        LOG.info("FOLLOWING - LEADER ELECTION TOOK - " +
              (self.end_fle - self.start_fle));
        self.start_fle = 0;
        self.end_fle = 0;
        fzk.registerJMX(new FollowerBean(this, zk), self.jmxLocalPeerBean);
        try {
            InetSocketAddress addr = findLeader();            
            try {
                connectToLeader(addr);//连接到leader
                long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);

                //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");
                }
                //跟leader进行数据同步  集群恢复后 leader-follower的数据同步 leader重新选举 进行数据同步
                syncWithLeader(newEpochZxid);    //跟leader进行数据同步
                QuorumPacket qp = new QuorumPacket();
                while (self.isRunning()) {
                    readPacket(qp);//不停从leader接收同步数据
                    processPacket(qp);
                }
            } catch (IOException 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);
        }
    }