ZooKeeper 状态机

ZAB状态机

QuorumPeer这个线程负责状态机的维护

@Override
public void run() {
    ...
    try {
        /*
         * Main loop
         */
        while (running) {
            switch (getPeerState()) {
            //初始状态
            case LOOKING:
                LOG.info("LOOKING");

                //允许只读服务
                if (Boolean.getBoolean("readonlymode.enabled")) {
                } else {
                    try {
                        setBCVote(null);
                        //发起leader选举
                        setCurrentVote(makeLEStrategy().lookForLeader());
                    } catch (Exception e) {
                        LOG.warn("Unexpected exception", e);
                        setPeerState(ServerState.LOOKING);
                    }
                }
                break;
            case OBSERVING:
                try {
                    LOG.info("OBSERVING");
                    setObserver(makeObserver(logFactory));
                    observer.observeLeader();
                } catch (Exception e) {
                    LOG.warn("Unexpected exception",e );
                } finally {
                    observer.shutdown();
                    setObserver(null);
                    setPeerState(ServerState.LOOKING);
                }
                break;
            case FOLLOWING:
                try {
                    LOG.info("FOLLOWING");
                    setFollower(makeFollower(logFactory));
                    //陷入发现，同步和广播阶段
                    follower.followLeader();
                } catch (Exception e) {
                    LOG.warn("Unexpected exception",e);
                } finally {
                    follower.shutdown();
                    setFollower(null);
                    setPeerState(ServerState.LOOKING);
                }
                break;
            case LEADING:
                LOG.info("LEADING");
                try {
                    setLeader(makeLeader(logFactory));
                    //同步，广播
                    leader.lead();
                    setLeader(null);
                } catch (Exception e) {
                    LOG.warn("Unexpected exception",e);
                } finally {
                    if (leader != null) {
                        leader.shutdown("Forcing shutdown");
                        setLeader(null);
                    }
                    setPeerState(ServerState.LOOKING);
                }
                break;
            }
        }
    }
}