zookeeper中有很多邏輯是等到超過一半選票才能繼續的場景。來看看怎麼實現的吧

　　Leader.getEpochToPropose

　　在選出來主之後，會進入到這個分支

　　QuorumPeer#

case LEADING:
                    LOG.info("LEADING");
                    try {
                        setLeader(makeLeader(logFactory));
                        leader.lead();
                        setLeader(
 
null);
                    } catch (Exception e) {
                        LOG.warn("Unexpected exception", e);
                    } finally {
                        if (leader != null) {
                            leader.shutdown("Forcing shutdown");
                            setLeader(null);
                        }
                        updateServerState();
                    }
                    
 
break;
                }

　　同時，follower也會進入自己的follow分支，簡單說下邏輯，就是和leader建立通訊，然後傳送訊息，表示要和leader進行資料同步。

　　訊息型別為Leader.FOLLOWERINFO

　　這裡忽略leader的網路通訊部分邏輯。直接看獲取epoch的邏輯。

　　protected final Set<Long> connectingFollowers = new HashSet<Long>();

public long getEpochToPropose(long sid, long lastAcceptedEpoch) throws
 
 InterruptedException, IOException {
        synchronized (connectingFollowers) {
            if (!waitingForNewEpoch) {
                return epoch;
            }
            if (lastAcceptedEpoch >= epoch) {//找followe傳過來的epoch最大值然後 +1
                epoch = lastAcceptedEpoch + 1;
            }
            if (isParticipant(sid)) {
                connectingFollowers.add(sid);
            }
            QuorumVerifier verifier = self.getQuorumVerifier();
            if (connectingFollowers.contains(self.getId()) && verifier.containsQuorum(connectingFollowers)) {//超過一半了
                waitingForNewEpoch = false;//這裡改為false 下面的while就會跳出了
                self.setAcceptedEpoch(epoch);
                connectingFollowers.notifyAll();//喚醒阻塞
            } else {
                long start = Time.currentElapsedTime();
                if (sid == self.getId()) {
                    timeStartWaitForEpoch = start;
                }
                long cur = start;
                long end = start + self.getInitLimit() * self.getTickTime();
                while (waitingForNewEpoch && cur < end && !quitWaitForEpoch) {
                    connectingFollowers.wait(end - cur);
                    cur = Time.currentElapsedTime();
                }
                if (waitingForNewEpoch) {
                    throw new InterruptedException("Timeout while waiting for epoch from quorum");
                }
            }
            return epoch;
        }
    }

　　QuorumVerifier 可以簡單地理解就是比較是否大於叢集伺服器數量的一半的工具類

　　向connectionFollowers中新增記錄的邏輯在LearnerHandler

@Override
    public void run() {
        try {
            ......long newEpoch = learnerMaster.getEpochToPropose(this.getSid(), lastAcceptedEpoch);//不斷向connectionFollowers中增加元素