Zookeeper的一次迁移故障
前阶段同事迁移Zookeeper（是给Kafka使用的以及flume使用）后发现所有Flume-producer/consumer端集体报错：07 Jan 2014 01:19:32,571 INFO (org.apache.zookeeper.ClientCnxn$SendThread.startConnect:1058)- Opening socket connection to server xxx:2181
07 Jan 2014 01:19:32,572 INFO (org.apache.zookeeper.ClientCnxn$SendThread.primeConnection:947)- Socket connection established to xxx:2181, initiating session
07 Jan 2014 01:19:32,573 INFO (org.apache.zookeeper.ClientCnxn$SendThread.run:1183)- Unable to read additional data from server sessionid 0x142f42b91871911, likely server has closed socket, closing socket connection and attempting reconnect
07 Jan 2014 01:19:32,845 INFO (org.apache.zookeeper.ClientCnxn$SendThread.startConnect:1058)- Opening socket connection to server xxx:2181 一直在不断的重试连接失败再重试，问同事说：网路连通性早就验证过，然后查看server端日志发现：
2014-01-06 23:59:59,987 - INFO - Accepted socket connection from /xxx:45282
2014-01-06 23:59:59,987 - WARN - Connection request from old client xxx:45282; will
be dropped if server is in r-o mode
2014-01-06 23:59:59,987 - INFO - Refusing session request for client xxx:45282 as it
has seen zxid 0x60fd15564 our last zxid is 0x10000000f client must try another server
2014-01-06 23:59:59,987 - INFO - Closed socket connection for client xxx:45282 (no se
ssion established for client)
2014-01-06 23:59:59,989 - INFO - Accepted socket connection from xxx:45285 发现Flume还是保留原来的zxid，但是现在的zxid竟然是0，所以抛出异常！
if (connReq.getLastZxidSeen() > zkDb.dataTree.lastProcessedZxid) {
String msg = "Refusing session request for client "
+ cnxn.getRemoteSocketAddress()
+ " as it has seen zxid 0x"
+ Long.toHexString(connReq.getLastZxidSeen())
+ " our last zxid is 0x"
+ Long.toHexString(getZKDatabase().getDataTreeLastProcessedZxid())
+ " client must try another server";
LOG.info(msg);
throw new CloseRequestException(msg);
} 后来问同事是怎么做的迁移：先启动一套新的集群，然后关闭老的集群，同时在老集群的一个IP：2181起了一个haproxy代理新集群以为这样，可以做到透明迁移=。=，其实是触发了ZK的bug-832导致不停的重试连接，只有重启flume才可以解决
正确的迁移方式是，把新集群加入老集群，然后修改Flume配置等一段时间（flume自动reconfig）后再关闭老集群就不会触发这个问题了.
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