[ZooKeeper]分布式Session超时

云中漫步

　　前面分析知道session超时由leader负责，假设某个session长时间没心跳超时，SessionTrackImpl入口

if (set != null) {
for (SessionImpl s : set.sessions) {
setSessionClosing(s.sessionId);
expirer.expire(s);
}
}
　　session失效是一个proposal过程

    private void close(long sessionId) {
submitRequest(null, sessionId, OpCode.closeSession, 0, null, null);
}
　　leader段执行处理链，PrepRequestProcessor执行，closeSession的request没有txn内容

case OpCode.closeSession:
// We don't want to do this check since the session expiration thread
// queues up this operation without being the session owner.
// this request is the last of the session so it should be ok
//zks.sessionTracker.checkSession(request.sessionId, request.getOwner());
//session下的所有临时节点，需要删除，添加到outstandingChanges队列
HashSet<String> es = zks.getZKDatabase()
.getEphemerals(request.sessionId);
synchronized (zks.outstandingChanges) {
for (ChangeRecord c : zks.outstandingChanges) {
if (c.stat == null) {
// Doing a delete
es.remove(c.path);
} else if (c.stat.getEphemeralOwner() == request.sessionId) {
es.add(c.path);
}
}
for (String path2Delete : es) {
addChangeRecord(new ChangeRecord(request.hdr.getZxid(),
path2Delete, null, 0, null));
}
//设置状态为closing
zks.sessionTracker.setSessionClosing(request.sessionId);
}
LOG.info("Processed session termination for sessionid: 0x"
+ Long.toHexString(request.sessionId));
break;
　　之后leader通过ProposalRequestProcessor发起投票，并写入log，follower处理投票，写入log并ack。然后leader commit请求，leader和follower都进入FinalRequestProcessor处理。先是datatree处理

case OpCode.closeSession:
killSession(header.getClientId(), header.getZxid());
break;

void killSession(long session, long zxid) {
// the list is already removed from the ephemerals
// so we do not have to worry about synchronizing on
// the list. This is only called from FinalRequestProcessor
// so there is no need for synchronization. The list is not
// changed here. Only create and delete change the list which
// are again called from FinalRequestProcessor in session下所有临时节点删除.
//sequence
HashSet<String> list = ephemerals.remove(session);
if (list != null) {
for (String path : list) {
try {
deleteNode(path, zxid);
if (LOG.isDebugEnabled()) {
LOG
.debug("Deleting ephemeral node " + path
+ " for session 0x"
+ Long.toHexString(session));
}
} catch (NoNodeException e) {
LOG.warn("Ignoring NoNodeException for path " + path
+ " while removing ephemeral for dead session 0x"
+ Long.toHexString(session));
}
}
}
}
　　然后FinalRequestProcessor删除session

else if (opCode == OpCode.closeSession) {
sessionTracker.removeSession(sessionId);
}
　　然后FinalRequestProcessor关闭连接

if (request.hdr != null && request.hdr.getType() == OpCode.closeSession) {
ServerCnxnFactory scxn = zks.getServerCnxnFactory();
// this might be possible since
// we might just be playing diffs from the leader
if (scxn != null && request.cnxn == null) {
// calling this if we have the cnxn results in the client's
// close session response being lost - we've already closed
// the session/socket here before we can send the closeSession
// in the switch block below
scxn.closeSession(request.sessionId);
return;
}
}
　　closeSession就是把session从内存数据结构中删除。
　　遍历连接集合，关闭sessionid对应的那个连接

   private void closeSessionWithoutWakeup(long sessionId) {
HashSet<NIOServerCnxn> cnxns;
synchronized (this.cnxns) {
cnxns = (HashSet<NIOServerCnxn>)this.cnxns.clone();
}
for (NIOServerCnxn cnxn : cnxns) {
if (cnxn.getSessionId() == sessionId) {
try {
cnxn.close();
} catch (Exception e) {
LOG.warn("exception during session close", e);
}
break;
}
}
}
　　以上就完成了server端session超时的处理
1.leader发起closeSession的Proposal
2.该session对应的server的FinalRequestProcessor中关闭连接
3.server端发起的session超时处理不需要返回client信息，如果是client主动关闭session，则需要返回closeConn的信息，然后再关闭连接