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;
}

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　　很正常的，特别是当一些分枝之间的遗传距离很相近，没办法确定分枝之间的进化关系。我不知道你清不清楚bootstrap的算法。如果你用的是序列的话，简单的讲就是把序列的位点都重排，重排后的序列再用相同的办法构树