zookeeper-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Jiafu Jiang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-2930) Leader cannot be elected due to network timeout of some member.
Date Fri, 03 Nov 2017 08:17:00 GMT
Jiafu Jiang created ZOOKEEPER-2930:

             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)
            Reporter: Jiafu Jiang
            Priority: Major

I deploy a cluster of ZooKeeper with three nodes:


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:

    public void toSend(Long sid, ByteBuffer b) {
         * If sending message to myself, then simply enqueue it (loopback).
        if (this.mySid == sid) {
             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,
             if (bqExisting != null) {
                 addToSendQueue(bqExisting, b);
             } else {
                 addToSendQueue(bq, b);
             // This may block!!!

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.

This message was sent by Atlassian JIRA

View raw message