zookeeper中session意味著一個物理連接，客戶端連接服務器成功之后，會發送一個連接型請求，此時就會有session?產生。

session由sessionTracker產生的，sessionTracker的實現有SessionTrackerImpl，LocalSessionTracker，LeaderSessionTracker(leader)，LearnerSessionTracker(follow and oberser)四種實現。它們的分支由各自的zookeeperServer.startup()開始。

1.sessionTrackerImpl

標準zookeeperServer的實現

public synchronized void startup() {if (sessionTracker == null) {createSessionTracker();}startSessionTracker();setupRequestProcessors();registerJMX();state = State.RUNNING;notifyAll();}

其實現由sessionTrackerImpl來實現，其官方說明

/**
* 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是一個線程，其run方法是真實邏輯：

@Overridepublic 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實現了session的各種操作：創建session，檢測session，刪除session等。我們以增加session為例，看一下session機制：

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 1.8SessionImpl 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;}

上文中出現了一個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;}

創建sessionImpl

　　紅色代碼所示。

更新過期時間并記錄

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);}

從上面的代碼可以看出，session都存放在一個sessionById的map里面，其定義為：

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

?

2.?LeaderSessionTracker

官方說明：

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

LeaderZooKeeperServer、LeaderZooKeeperServer、FollowerZooKeeperServer、ObserverZooKeeperServer均繼承自QuorumZooKeeperServer，

QuorumZooKeeperServer的startSessionTracker方法如下：

@Overrideprotected void startSessionTracker() {upgradeableSessionTracker = (UpgradeableSessionTracker) sessionTracker;upgradeableSessionTracker.start();} UpgradeableSessionTracker的實現類有兩個：LeaderSessionTracker和LearnerSessionTracker，很顯然，對leaderZookeeper的實現為LeaderSessionTracker，LearnerSessionTracker對應FollowerZooKeeperServer、ObserverZooKeeperServer。
LeaderSessionTracker的構造函數為：
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;}

其分為兩個sessionTracker，一個為globalSessionTracker，其實現為SessionTrackerImpl；另一個為localSessionTracker，其實現為：

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啟動時同時啟動global和local： public void start() {globalSessionTracker.start();if (localSessionTracker != null) {localSessionTracker.start();}}

創建session的過程：

public long createSession(int sessionTimeout) {if (localSessionsEnabled) {return localSessionTracker.createSession(sessionTimeout);}return globalSessionTracker.createSession(sessionTimeout);} globalSessionTracker的創建session上面已經論述，且看localSessionTracker的生成session，進一步代碼發現LocalSessionTracker繼承了SessionTrackerImpl，沒有重寫其創建session方法，即global和local創建session的方法相同。

3. LearnerSessionTracker

官方說明：

/**
* 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.
*/

其構造方法是： 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);}}

啟動時只啟動了localSessionTracker：

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

創建session時也僅僅由localSessionTracker生成：

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

?4 小結

? 　　根據服務器角色不同，ZooKeeperServer，LeaderZooKeeperServer，FollowerZooKeeperServer，ObserverZooKeeperServer分別代表單機服務器，集群leader服務器，集群Follower服務器，集群observer服務器，它們的sessionTracker實現是不同的。ZookeeperServer的對應sessionTracker實現是SessionTrackerImpl；LeaderZooKeeperServer的對應sessionTracker實現是LeaderSessionTracker，FollowerZooKeeperServer，ObserverZooKeeperServer的對應sessionTracker實現是LearnerSessionTracker。

　　有可以分為globalSessionTracker和LocalSessionTracker，其中單機只有一個標準的SessionTrackerImpl，集群leader開啟globalSessionTracker和LocalSessionTracker，follower和observer只開啟LocalSessionTracker。globalSessionTracker由SessionTrackerImpl實現，LocalSessionTracker繼承并擴展了SessionTrackerImpl。

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轉載于:https://www.cnblogs.com/davidwang456/p/5009659.html

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