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Date: Thu, 1 Nov 2012 18:01:13 +0000 (UTC)
From: "Jun Rao (JIRA)" <jira@apache.org>
To: kafka-dev@incubator.apache.org
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Subject: [jira] [Commented] (KAFKA-532) Multiple controllers can co-exist
 during soft failures
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    [ https://issues.apache.org/jira/browse/KAFKA-532?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13488893#comment-13488893 ] 

Jun Rao commented on KAFKA-532:
-------------------------------

Thanks for patch v3. The overall approach seems to work. Some comments:

30. PartitionStateInfo and LeaderAndIsrRequest: When calculating size in sizeInBytes(), it's more readable if we put each number to be added in a separate line.

31. Partition.updateIsr(): I am thinking about what controllerEpoch the leader should use when updating the leaderAndIsr path. There is probably nothing wrong to use the controllerEpoch in replicaManager. However, it seems to make more sense to use the controllerEpoch in the leaderAndIsr path itself, since this update is actually not made by the controller.

32. ReplicaManager.controllerEpoch: Since this variable can be accessed from different threads, it needs to be a volatile. Also, we only need to update controllerEpoch if the one from the request is larger (but not equal). It probably should be initialized to 0 or -1?

33. LeaderElectionTest.testLeaderElectionWithStaleControllerEpoch(): I wonder if we really need to start a new broker. Can we just send a stale controller epoc using the controllerChannelManager in the current controller?

34. KafkaController: There seems to be a tricky issue with incrementing the controller epoc. We increment epoc in onControllerFailover() after the broker becomes a controller. What could happen is that broker 1 becomes the controller and goes to GC before we increment the epoc. Broker 2 becomes the new controller and increments the epoc. Broker 1 comes back from gc and increments epoc again. Now, broker 1's controller epoc is actually larger. Not sure what's the best way to address this. One thought is that immediately after controller epoc is incremented in onControllerFailover(), we check if this broker is still the controller (by reading the controller path in ZK). If not, we throw an exception. Also, epoc probably should be initialized to 0 if we want the first controller to have epoc 1.

35. We use int to represent both controller and leader epoc. There is the potential issue if the number wraps. We probably don't need to worry about it now.


> Multiple controllers can co-exist during soft failures
> ------------------------------------------------------
>
>                 Key: KAFKA-532
>                 URL: https://issues.apache.org/jira/browse/KAFKA-532
>             Project: Kafka
>          Issue Type: Bug
>    Affects Versions: 0.8
>            Reporter: Neha Narkhede
>            Assignee: Neha Narkhede
>            Priority: Blocker
>              Labels: bugs
>         Attachments: kafka-532-v1.patch, kafka-532-v2.patch, kafka-532-v3.patch
>
>   Original Estimate: 48h
>  Remaining Estimate: 48h
>
> If the current controller experiences an intermittent soft failure (GC pause) in the middle of leader election or partition reassignment, a new controller might get elected and start communicating new state change decisions to the brokers. After recovering from the soft failure, the old controller might continue sending some stale state change decisions to the brokers, resulting in unexpected failures. We need to introduce a controller generation id that increments with controller election. The brokers should reject any state change requests by a controller with an older generation id.

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