Jiafu Jiang created ZOOKEEPER-2930:
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Summary: Leader cannot be elected due to network timeout of some
member.
Key: ZOOKEEPER-2930
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2930
Project: ZooKeeper
Issue Type: Bug
Components: leaderElection
Affects Versions: 3.4.10
Environment: Java 8
ZooKeeper 3.4.11(from github)
Centos6.5
Reporter: Jiafu Jiang
Priority: Major
I deploy a cluster of ZooKeeper with three nodes:
ofs_zk1:20.10.11.101, 30.10.11.101
ofs_zk2:20.10.11.102, 30.10.11.102
ofs_zk3:20.10.11.103, 30.10.11.103
I shutdown the network interfaces of ofs_zk2 using "ifdown eth0 eth1" command.
It is supposed that the new Leader should be elected in some seconds, but the
fact is, ofs_zk1 and ofs_zk2 just keep electing again and again, but none of
them can become the new Leader.
I change the log level to DEBUG (the default is INFO), and restart zookeeper
servers on ofs_zk1 and ofs_zk2 again, but it can not fix the problem.
I read the log and the ZooKeeper source code, and I think I find the reason.
When the potential leader(says ofs_zk3) begins the
election(FastLeaderElection.lookForLeader()), it sends notifications to all the
servers.
When it fails to receive any notification during a timeout, it will resend the
notifications, and double the timeout. This process will repeat until any
notification is received or the timeout reaches a max value.
The FastLeaderElection.sendNotifications() just put the notification message
into a queue and return. The WorkerSender is responsable to send the
notifications.
The WorkerSender just process the notifications one bye one by passing the
notifications to QuorumCnxManager. Here comes the problem, the
QuorumCnxManager.toSend() blocks for a long time when the notification is send
to ofs_zk2(whose network is down) and some notifications (which belongs to
ofs_zk1) will thus be blocked for a long time. The repeated notifications by
FastLeaderElection.sendNotifications() just make things worse.
Here is the related source code:
{code:java}
public void toSend(Long sid, ByteBuffer b) {
/*
* If sending message to myself, then simply enqueue it (loopback).
*/
if (this.mySid == sid) {
b.position(0);
addToRecvQueue(new Message(b.duplicate(), sid));
/*
* Otherwise send to the corresponding thread to send.
*/
} else {
/*
* Start a new connection if doesn't have one already.
*/
ArrayBlockingQueue<ByteBuffer> bq = new
ArrayBlockingQueue<ByteBuffer>(SEND_CAPACITY);
ArrayBlockingQueue<ByteBuffer> bqExisting =
queueSendMap.putIfAbsent(sid, bq);
if (bqExisting != null) {
addToSendQueue(bqExisting, b);
} else {
addToSendQueue(bq, b);
}
// This may block!!!
connectOne(sid);
}
}
{code}
Therefore, when ofs_zk3 believes that it is the leader, it begins to wait the
epoch ack, but in fact the ofs_zk1 does not receive the notification(which says
the leader is ofs_zk1) because the ofs_zk3 have not send the notification(the
notification may still in the sendqueue of WorkerSender). At last, the
potential leader ofs_zk3 fails to receive the epoch ack in timeout, so it quit
the leader and begins a new election.
The log files of ofs_zk1 and ofs_zk2 are attached.
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