zookeeper客戶端連不上伺服器，檢視服務端日誌 檔案zookeeper.out（目錄見 /usr/local/zookeeper/conf/zoo.cfg中的dataLogDir配置項）如下：

2016-02-16 11:17:44,503 [myid:2] - WARN  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:[email protected]] - Exception when following the leader

java.net.SocketTimeoutException: Read timed out
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.read(SocketInputStream.java:129)
        at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
        at java.io.BufferedInputStream.read(BufferedInputStream.java:237)
        at java.io.DataInputStream.readInt(DataInputStream.java:370)
        at org.apache.jute.BinaryInputArchive.readInt(BinaryInputArchive.java:63)
        at org.apache.zookeeper.server.quorum.QuorumPacket.deserialize(QuorumPacket.java:83)
        at org.apache.jute.BinaryInputArchive.readRecord(BinaryInputArchive.java:108)
        at org.apache.zookeeper.server.quorum.Learner.readPacket(Learner.java:152)
        at org.apache.zookeeper.server.quorum.Learner.registerWithLeader(Learner.java:272)
        at org.apache.zookeeper.server.quorum.Follower.followLeader(Follower.java:72)
        at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:740)
2016-02-16 11:17:44,503 [myid:2] - INFO  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:

[email protected]] - shutdown called
java.lang.Exception: shutdown Follower
        at org.apache.zookeeper.server.quorum.Follower.shutdown(Follower.java:166)
        at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:744)
2016-02-16 11:17:44,504 [myid:2] - INFO  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:

[email protected]] - Shutting down
2016-02-16 11:17:44,504 [myid:2] - INFO  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:[email protected]] - shutting down
2016-02-16 11:17:44,504 [myid:2] - INFO  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:[email protected]] - LOOKING
2016-02-16 11:17:44,507 [myid:2] - INFO  [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:

[email protected]] - Reading snapshot /web/applog/zookeeper/data/version-2/snapshot.119f00000001
2016-02-16 11:17:46,752 [myid:2] - INFO  [WorkerReceiver[myid=2]:[email protected]] - Notification: 4 (n.leader), 0x119f00000001 (n.zxid), 0x139b (n.round), LOOKING (n.state), 4 (n.sid), 0x14bf (n.peerEPoch), LOOKING (my state)
2016-02-16 11:17:52,916 [myid:2] - INFO  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:[email protected]] - Accepted socket connection from /10.21.34.240:49942
2016-02-16 11:17:52,916 [myid:2] - WARN  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:[email protected]] - Exception causing close of session 0x0 due to java.io.IOException: ZooKeeperServer not running
2016-02-16 11:17:52,916 [myid:2] - INFO  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:[email protected]] - Closed socket connection for client /10.21.34.240:49942 (no session established for client)
2016-02-16 11:17:52,920 [myid:2] - INFO  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:[email protected]] - Accepted socket connection from /10.21.33.25:21546

2016-02-16 11:17:52,920 [myid:2] - WARN  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:[email protected]] - Exception causing close of session 0x0 due to java.io.IOException: ZooKeeperServer not running

起初以為是防火牆或者其它網路問題，搜尋引擎上沒找到有過類似經驗的貼子（這也是為什麼我寫下來的原因），對zookeeper不熟，無奈只能自己研究配置項的含義，發現它提到幾個超時項可能與上述異常有關

# The number of ticks that the initial 
# synchronization phase can take
initLimit=10

# The number of ticks that can pass between 
# sending a request and getting an acknowledgement
syncLimit=5

登入和同步超時時間，當資料量比較大時，這個值要相應增大，不然會超時，故我將這兩項調大（10倍），重啟所有server，果然不再報錯，客戶端連上了

再核實此推斷，發現確實有一個節點的子節點數量很大：

[zk: localhost:2181(CONNECTED) 2] stat /scheduler/trigger/formatnotify/_SCND
cZxid = 0x8023698c2
ctime = Thu Jul 30 17:19:11 CST 2015
mZxid = 0x8023698c2
mtime = Thu Jul 30 17:19:11 CST 2015
pZxid = 0xd01406c09
cversion = 2216863
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 0
numChildren = 2216675

2百多萬，所以使用zookeeper一定要注意做好清理工作，否則做一個協調指揮者，一個小小的失誤，將導致整個生產系統癱瘓！