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From "Jiafu Jiang (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (ZOOKEEPER-2930) Leader cannot be elected due to network timeout of some members.
Date Fri, 03 Nov 2017 13:59:00 GMT

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-2930?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jiafu Jiang updated ZOOKEEPER-2930:
-----------------------------------
    Description: 
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_zk3 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 will send 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 by 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_zk3) because
the ofs_zk3 has 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_zk3 are attached.

  was:
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_zk3 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 will send 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 by 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_zk3 are attached.


> Leader cannot be elected due to network timeout of some members.
> ----------------------------------------------------------------
>
>                 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
>         Attachments: zoo.cfg, zookeeper1.log, zookeeper2.log
>
>
> 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_zk3 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 will send 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 by 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_zk3)
because the ofs_zk3 has 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_zk3 are attached.



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