[jira] [Created] (KAFKA-1029) Zookeeper leader election stuck in ephemeral node retry loop

["Sam Meder (JIRA)" <ji...@apache.org>]

Sam Meder created KAFKA-1029:
--------------------------------

             Summary: Zookeeper leader election stuck in ephemeral node retry loop
                 Key: KAFKA-1029
                 URL: https://issues.apache.org/jira/browse/KAFKA-1029
             Project: Kafka
          Issue Type: Bug
          Components: controller
    Affects Versions: 0.8
            Reporter: Sam Meder
            Assignee: Neha Narkhede
            Priority: Blocker
             Fix For: 0.8


We're seeing the following log statements (over and over):

[2013-08-27 07:21:49,538] INFO conflict in /controller data: { "brokerid":3, "timestamp":"1377587945206", "version":1 } stored data: { "brokerid":2, "timestamp":"1377587460904", "version":1 } (kafka.utils.ZkUtils$)

[2013-08-27 07:21:49,559] INFO I wrote this conflicted ephemeral node [{ "brokerid":3, "timestamp":"1377587945206", "version":1 }] at /controller a while back in a different session, hence I will backoff for this node to be deleted by Zookeeper and retry (kafka.utils.ZkUtils$)

where the broker is essentially stuck in the loop that is trying to deal with left-over ephemeral nodes. The code looks a bit racy to me. In particular:

ZookeeperLeaderElector:

  def elect: Boolean = {
    controllerContext.zkClient.subscribeDataChanges(electionPath, leaderChangeListener)
    val timestamp = SystemTime.milliseconds.toString
    val electString = ...

    try {
      createEphemeralPathExpectConflictHandleZKBug(controllerContext.zkClient, electionPath, electString, leaderId,
        (controllerString : String, leaderId : Any) => KafkaController.parseControllerId(controllerString) == leaderId.asInstanceOf[Int],
        controllerContext.zkSessionTimeout)

leaderChangeListener is registered before the create call (by the way, it looks like a new registration will be added every elect call - shouldn't it register in startup()?) so can update leaderId to the current leader before the call to create. If that happens then we will continuously get node exists exceptions and the checker function will always return true, i.e. we will never get out of the while(true) loop.

I think the right fix here is to pass brokerId instead of leaderId when calling create, i.e.

createEphemeralPathExpectConflictHandleZKBug(controllerContext.zkClient, electionPath, electString, brokerId,
        (controllerString : String, leaderId : Any) => KafkaController.parseControllerId(controllerString) == leaderId.asInstanceOf[Int],
        controllerContext.zkSessionTimeout)

The loop dealing with the ephemeral node bug is now only triggered for the broker that owned the node previously, although I am still not 100% sure if that is sufficient.




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