今天搭建了一个zookeeper集群，分别为server1,server2,server3。

# The number of milliseconds of each tick

tickTime=2000

# 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

# the directory where the snapshot is stored.

dataDir=/tmp/zookeeper

# the port at which the clients will connect

clientPort=2181

server.1=172.16.1.159:2888:3888
server.2=172.16.1.123:2888:3888

server.3=172.16.1.22:2888:3888

在启动zkCli.sh 的时候就出现了日志信息无限重复打印，内容如下：

2017-10-23 10:57:14,816 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:15,036 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:15,036 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:15,037 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:15,663 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:15,663 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:15,664 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:16,190 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:16,190 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:16,191 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:16,358 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:16,359 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:16,359 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:16,773 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:16,773 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:16,774 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:17,025 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:17,025 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:17,026 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:17,300 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:17,300 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:17,301 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:18,229 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:18,230 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:18,231 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:18,747 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:18,747 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:18,748 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:19,451 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:19,452 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:19,453 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:20,012 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:20,012 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:20,013 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:20,947 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:20,947 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:20,948 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:21,907 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:21,907 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:21,908 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:22,545 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:22,545 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:22,546 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:22,764 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:22,765 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:22,765 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:23,370 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:23,371 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session
2017-10-23 10:57:23,372 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:24,093 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181
2017-10-23 10:57:24,094 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
2017-10-23 10:57:24,094 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1183] - Unable to read additional data from server sessionid 0x0, likely server has closed socket, closing socket connection and attempting reconnect
2017-10-23 10:57:25,148 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1058] - Opening socket connection to server localhost/127.0.0.1:2181
2017-10-23 10:57:25,148 - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@947] - Socket connection established to localhost/127.0.0.1:2181, initiating session

使用dubbo添加服务的时候也添加不上，在网上查找原因，说是可能jar包版本与zookeeper版本不一致，然后去核对版本，发现版本号一致的情况下也无法注册服务。之后又去核对myid文件，发现也没有错误。

后来才搞明白，由于我在zoo.cfg中配置了3台机器，但是只启动了1台，zookeeper就会认为服务处于不可用状态。
zookeeper有个选举算法，当整个集群超过半数机器宕机，zookeeper会认为集群处于不可用状态。
所以，3台机器启动1台无法连接，如果启动2台及以上就可以连接了。