zookeeper源码分析之六session机制

zookeeper中session意味着一个物理连接，客户端连接服务器成功之后，会发送一个连接型请求，此时就会有session 产生。

session由sessionTracker产生的，sessionTracker的实现有SessionTrackerImpl，LocalSessionTracker，LeaderSessionTracker(leader)，LearnerSessionTracker(follow and oberser)四种实现。它们的分支由各自的zookeeperServer.startup()开始。

1.sessionTrackerImpl

标准zookeeperServer的实现

    public synchronized void startup() {
        if (sessionTracker == null) {
            createSessionTracker();
        }
        startSessionTracker();
        setupRequestProcessors();

        registerJMX();

        state = State.RUNNING;
        notifyAll();
    }

其实现由sessionTrackerImpl来实现，其官方说明

/**
* This is a full featured SessionTracker. It tracks session in grouped by tick
* interval. It always rounds up the tick interval to provide a sort of grace
* period. Sessions are thus expired in batches made up of sessions that expire
* in a given interval.
*/

    protected void createSessionTracker() {
        sessionTracker = new SessionTrackerImpl(this, zkDb.getSessionWithTimeOuts(),
                tickTime, 1, getZooKeeperServerListener());
    }

    protected void startSessionTracker() {
        ((SessionTrackerImpl)sessionTracker).start();
    }

sessionTrackerImpl是一个线程，其run方法是真实逻辑：

    @Override
    public void run() {
        try {
            while (running) {
                long waitTime = sessionExpiryQueue.getWaitTime();
                if (waitTime > 0) {
                    Thread.sleep(waitTime);
                    continue;
                }

                for (SessionImpl s : sessionExpiryQueue.poll()) {
                    setSessionClosing(s.sessionId);
                    expirer.expire(s);
                }
            }
        } catch (InterruptedException e) {
            handleException(this.getName(), e);
        }
        LOG.info("SessionTrackerImpl exited loop!");
    }

sessionTrackerImpl实现了session的各种操作：创建session，检测session，删除session等。我们以增加session为例，看一下session机制：

    public long createSession(int sessionTimeout) {
        long sessionId = nextSessionId.getAndIncrement();
        addSession(sessionId, sessionTimeout);
        return sessionId;
    }
 public synchronized boolean addSession(long id, int sessionTimeout) {
        sessionsWithTimeout.put(id, sessionTimeout);

        boolean added = false;

        SessionImpl session = sessionsById.get(id);
        if (session == null){
            session = new SessionImpl(id, sessionTimeout);
        }

        // findbugs2.0.3 complains about get after put.
        // long term strategy would be use computeIfAbsent after JDK 1.8
        SessionImpl existedSession = sessionsById.putIfAbsent(id, session);

        if (existedSession != null) {
            session = existedSession;
        } else {
            added = true;
            LOG.debug("Adding session 0x" + Long.toHexString(id));
        }

        if (LOG.isTraceEnabled()) {
            String actionStr = added ? "Adding" : "Existing";
            ZooTrace.logTraceMessage(LOG, ZooTrace.SESSION_TRACE_MASK,
                    "SessionTrackerImpl --- " + actionStr + " session 0x"
                    + Long.toHexString(id) + " " + sessionTimeout);
        }

        updateSessionExpiry(session, sessionTimeout);
        return added;
    }

上文中出现了一个nextSessionId，看一下其的生成方式：

    /**
     * Generates an initial sessionId. High order byte is serverId, next 5
     * 5 bytes are from timestamp, and low order 2 bytes are 0s.
     */
    public static long initializeNextSession(long id) {
        long nextSid;
        nextSid = (Time.currentElapsedTime() << 24) >>> 8;
        nextSid =  nextSid | (id <<56);
        return nextSid;
    }

创建sessionImpl

　　红色代码所示。

更新过期时间并记录

    private void updateSessionExpiry(SessionImpl s, int timeout) {
        logTraceTouchSession(s.sessionId, timeout, "");
        sessionExpiryQueue.update(s, timeout);
    }

    private void logTraceTouchSession(long sessionId, int timeout, String sessionStatus){
        if (!LOG.isTraceEnabled())
            return;

        String msg = MessageFormat.format(
                "SessionTrackerImpl --- Touch {0}session: 0x{1} with timeout {2}",
                sessionStatus, Long.toHexString(sessionId), Integer.toString(timeout));

        ZooTrace.logTraceMessage(LOG, ZooTrace.CLIENT_PING_TRACE_MASK, msg);
    }

从上面的代码可以看出，session都存放在一个sessionById的map里面，其定义为：

protected final ConcurrentHashMap<Long, SessionImpl> sessionsById =
new ConcurrentHashMap<Long, SessionImpl>();

2. LeaderSessionTracker

官方说明：

/**
* The leader session tracker tracks local and global sessions on the leader.
*/

LeaderZooKeeperServer、LeaderZooKeeperServer、FollowerZooKeeperServer、ObserverZooKeeperServer均继承自QuorumZooKeeperServer，

QuorumZooKeeperServer的startSessionTracker方法如下：

    @Override
    protected void startSessionTracker() {
        upgradeableSessionTracker = (UpgradeableSessionTracker) sessionTracker;
        upgradeableSessionTracker.start();
    }

UpgradeableSessionTracker的实现类有两个：LeaderSessionTracker和LearnerSessionTracker，很显然，对leaderZookeeper的实现为LeaderSessionTracker，LearnerSessionTracker对应FollowerZooKeeperServer、ObserverZooKeeperServer。
LeaderSessionTracker的构造函数为：

    public LeaderSessionTracker(SessionExpirer expirer,
            ConcurrentMap<Long, Integer> sessionsWithTimeouts,
            int tickTime, long id, boolean localSessionsEnabled,
            ZooKeeperServerListener listener) {

        this.globalSessionTracker = new SessionTrackerImpl(
            expirer, sessionsWithTimeouts, tickTime, id, listener);

        this.localSessionsEnabled = localSessionsEnabled;
        if (this.localSessionsEnabled) {
            createLocalSessionTracker(expirer, tickTime, id, listener);
        }
        serverId = id;
    }

其分为两个sessionTracker，一个为globalSessionTracker，其实现为SessionTrackerImpl；另一个为localSessionTracker，其实现为：

    public void createLocalSessionTracker(SessionExpirer expirer,
            int tickTime, long id, ZooKeeperServerListener listener) {
        this.localSessionsWithTimeouts =
            new ConcurrentHashMap<Long, Integer>();
        this.localSessionTracker = new LocalSessionTracker(
            expirer, this.localSessionsWithTimeouts, tickTime, id, listener);
    }

LeaderSessionTracker启动时同时启动global和local：

    public void start() {
        globalSessionTracker.start();
        if (localSessionTracker != null) {
            localSessionTracker.start();
        }
    }

创建session的过程：

    public long createSession(int sessionTimeout) {
        if (localSessionsEnabled) {
            return localSessionTracker.createSession(sessionTimeout);
        }
        return globalSessionTracker.createSession(sessionTimeout);
    }

globalSessionTracker的创建session上面已经论述，且看localSessionTracker的生成session，进一步代码发现LocalSessionTracker继承了SessionTrackerImpl，没有重写其创建session方法，即global和local创建session的方法相同。

3. LearnerSessionTracker

官方说明：

/**
* The learner session tracker is used by learners (followers and observers) to
* track zookeeper sessions which may or may not be echoed to the leader. When
* a new session is created it is saved locally in a wrapped
* LocalSessionTracker. It can subsequently be upgraded to a global session
* as required. If an upgrade is requested the session is removed from local
* collections while keeping the same session ID. It is up to the caller to
* queue a session creation request for the leader.
* A secondary function of the learner session tracker is to remember sessions
* which have been touched in this service. This information is passed along
* to the leader with a ping.
*/

其构造方法是：

    public LearnerSessionTracker(SessionExpirer expirer,
            ConcurrentMap<Long, Integer> sessionsWithTimeouts,
            int tickTime, long id, boolean localSessionsEnabled,
            ZooKeeperServerListener listener) {
        this.expirer = expirer;
        this.touchTable.set(new ConcurrentHashMap<Long, Integer>());
        this.globalSessionsWithTimeouts = sessionsWithTimeouts;
        this.serverId = id;
        nextSessionId.set(SessionTrackerImpl.initializeNextSession(serverId));

        this.localSessionsEnabled = localSessionsEnabled;
        if (this.localSessionsEnabled) {
            createLocalSessionTracker(expirer, tickTime, id, listener);
        }
    }

启动时只启动了localSessionTracker：

    public void start() {
        if (localSessionTracker != null) {
            localSessionTracker.start();
        }
    }

创建session时也仅仅由localSessionTracker生成：

    public long createSession(int sessionTimeout) {
        if (localSessionsEnabled) {
            return localSessionTracker.createSession(sessionTimeout);
        }
        return nextSessionId.getAndIncrement();
    }

 4 小结

　　根据服务器角色不同，ZooKeeperServer，LeaderZooKeeperServer，FollowerZooKeeperServer，ObserverZooKeeperServer分别代表单机服务器，集群leader服务器，集群Follower服务器，集群observer服务器，它们的sessionTracker实现是不同的。ZookeeperServer的对应sessionTracker实现是SessionTrackerImpl；LeaderZooKeeperServer的对应sessionTracker实现是LeaderSessionTracker，FollowerZooKeeperServer，ObserverZooKeeperServer的对应sessionTracker实现是LearnerSessionTracker。

　　有可以分为globalSessionTracker和LocalSessionTracker，其中单机只有一个标准的SessionTrackerImpl，集群leader开启globalSessionTracker和LocalSessionTracker，follower和observer只开启LocalSessionTracker。globalSessionTracker由SessionTrackerImpl实现，LocalSessionTracker继承并扩展了SessionTrackerImpl。