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。