Zookeeper状态

    public enum ServerState {
        // 初始状态
        LOOKING,
        FOLLOWING,
        LEADING,
        // 不参与选举
        OBSERVING
    }

    /**
     * (Used for monitoring) shows the current phase of
     * Zab protocol that peer is running.
     */
    public enum ZabState {
        //集群进入选举状态
        ELECTION,
        // 第一：leader收集follower的lastProcessedZxid，这个主要用来通过和leader的lastProcessedZxid对比来确认follower需要同步的数据范围
        // 第二：选举出一个新的peerEpoch，主要用于防止旧的leader来进行提交操作（旧leader向follower发送命令的时候，follower发现zxid所在的peerEpoch比现在的小，则直接拒绝，防止出现不一致性）
        DISCOVERY,
        // 同步数据
        SYNCHRONIZATION,
        // 提供服务
        BROADCAST
    }

    /**
     * (Used for monitoring) When peer is in synchronization phase, this shows
     * which synchronization mechanism is being used
     */
    public enum SyncMode {
        NONE,
        // 直接差异化同步, Leader直接把数据发给learner
        DIFF,
        // Leader 服务器将本机上的全量内存数据都同步给 Learner 
        SNAP,
        // Learner上回滚Leader上不存在的提交
        TRUNC
    }

    /*
     * A peer can either be participating, which implies that it is willing to
     * both vote in instances of consensus and to elect or become a Leader, or
     * it may be observing in which case it isn't.
     *
     * We need this distinction to decide which ServerState to move to when
     * conditions change (e.g. which state to become after LOOKING).
     */
    
    public enum LearnerType {
        // FOLLOWING
        PARTICIPANT,
        // OBSERVING
        OBSERVER
    }

    /*
     * To enable observers to have no identifier, we need a generic identifier
     * at least for QuorumCnxManager. We use the following constant to as the
     * value of such a generic identifier.
     */

    static final long OBSERVER_ID = Long.MAX_VALUE;