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Posted to issues@nifi.apache.org by markap14 <gi...@git.apache.org> on 2018/04/19 13:09:58 UTC
[GitHub] nifi pull request #2646: NIFI-5096: Periodically poll ZooKeeper to determine...
GitHub user markap14 opened a pull request:
https://github.com/apache/nifi/pull/2646
NIFI-5096: Periodically poll ZooKeeper to determine the leader for ea…
…ch registered role in Leader Election. This avoids a condition whereby a node may occasionally fail to receive notification that it is no longer the elected leader.
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You can merge this pull request into a Git repository by running:
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Alternatively you can review and apply these changes as the patch at:
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To close this pull request, make a commit to your master/trunk branch
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This closes #2646
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commit 757c4e2445d052b593fbea8f0d9a36bac001f44f
Author: Mark Payne <ma...@...>
Date: 2018-04-19T13:05:32Z
NIFI-5096: Periodically poll ZooKeeper to determine the leader for each registered role in Leader Election. This avoids a condition whereby a node may occasionally fail to receive notification that it is no longer the elected leader.
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by mcgilman <gi...@git.apache.org>.
Github user mcgilman commented on the issue:
https://github.com/apache/nifi/pull/2646
@markap14 I see. It appears then that the underlying issue is that either (1) the stateChange method is not being invoked or (2) the leader thread interruption is not happening/working. We could include your proposed changes and update our implementation of stateChanged to set `leader` to false when the `newState` is LOST or SUSPENDED before invoking super. Additionally, in `takeLeadership` we should loop while not stopped and is leader. This should help if the underlying issue was (2) while the polling could act as additional insurance.
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by mcgilman <gi...@git.apache.org>.
Github user mcgilman commented on the issue:
https://github.com/apache/nifi/pull/2646
Thanks @markap14! This has been merged to master.
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[GitHub] nifi pull request #2646: NIFI-5096: Periodically poll ZooKeeper to determine...
Posted by asfgit <gi...@git.apache.org>.
Github user asfgit closed the pull request at:
https://github.com/apache/nifi/pull/2646
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by mcgilman <gi...@git.apache.org>.
Github user mcgilman commented on the issue:
https://github.com/apache/nifi/pull/2646
@markap14 These changes look like they will reduce the time when there are multiple primaries (leaders) in the event the existing primary isn't notified of the change. That's definitely an improvement, however, I was curious if you tried incorporating a `ConnectionStateListener` for the node to be more proactively notified when the connection is SUSPENDED or LOST. Here's an SO post [1] where it is discussed.
[1] https://stackoverflow.com/questions/41042798/how-to-handle-apache-curator-distributed-lock-loss-of-connection
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by markap14 <gi...@git.apache.org>.
Github user markap14 commented on the issue:
https://github.com/apache/nifi/pull/2646
@mcgilman I agree. I have pushed a new commit that does just that.
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by markap14 <gi...@git.apache.org>.
Github user markap14 commented on the issue:
https://github.com/apache/nifi/pull/2646
@mcgilman we do indeed implement the ConnectionStateListener, but we do so only to log the fact and then call super.stateChanged(). When we call super.stateChanged(), that will throw CancelLeadershipException, which in turn is supposed to interrupt our listener. We followed the "Error Handling" guidance provided by Apache Curator: https://curator.apache.org/curator-recipes/leader-election.html
So we are handling the SUSPENDED and LOST scenarios as is recommended. And this works 99% of the time. Unfortunately, we do occasionally see scenarios where it does not interrupt the thread and as such the node believes that it retains the lock. It's not clear, when this happens, if the thread just wasn't interrupted for some reason, or if the notification of SUSPENDED/LOST never was received, or what exactly is occurring that prevents our ElectionListener from being interrupted.
That's why I went with the solution of periodically polling ZooKeeper, to check the state. That way, whatever the cause of the thread not being interrupted, we still will break out. If you think it makes sense, though, we can detect the LOST state specifically and have that trigger us to leave the election, in addition to polling?
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[GitHub] nifi issue #2646: NIFI-5096: Periodically poll ZooKeeper to determine the le...
Posted by mcgilman <gi...@git.apache.org>.
Github user mcgilman commented on the issue:
https://github.com/apache/nifi/pull/2646
@markap14 Looks like our `LeaderSelectionListener` does implement `ConnectionStateListener`. Do we need to update code on our side to explicitly give up leadership in the SUSPENDED or LOST scenarios?
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