[jira] [Commented] (KAFKA-1451) Broker stuck due to leader election race

["Joe Stein (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-1451?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14092051#comment-14092051 ] 

Joe Stein commented on KAFKA-1451:
----------------------------------

Hi, two issues so far where found with leader election https://issues.apache.org/jira/browse/KAFKA-1387?focusedCommentId=14087063&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14087063 I don't know if the issues are related to each other or even to this just yet... the issues found were not happening on the 0.8.1 branch.... could be another 0.8.2 patch I supose but before I started trying to test on a 0.8.2 version without this patch (to isolate the root cause) I wanted to see if this type of scenario was tested or what thoughts were in general to this patch and how it might be affecting either of the two issues found in 0.8.2 trunk?  

> Broker stuck due to leader election race 
> -----------------------------------------
>
>                 Key: KAFKA-1451
>                 URL: https://issues.apache.org/jira/browse/KAFKA-1451
>             Project: Kafka
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 0.8.1.1
>            Reporter: Maciek Makowski
>            Assignee: Manikumar Reddy
>            Priority: Minor
>              Labels: newbie
>             Fix For: 0.8.2
>
>         Attachments: KAFKA-1451.patch, KAFKA-1451_2014-07-28_20:27:32.patch, KAFKA-1451_2014-07-29_10:13:23.patch
>
>
> h3. Symptoms
> The broker does not become available due to being stuck in an infinite loop while electing leader. This can be recognised by the following line being repeatedly written to server.log:
> {code}
> [2014-05-14 04:35:09,187] INFO I wrote this conflicted ephemeral node [{"version":1,"brokerid":1,"timestamp":"1400060079108"}] 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$)
> {code}
> h3. Steps to Reproduce
> In a single kafka 0.8.1.1 node, single zookeeper 3.4.6 (but will likely behave the same with the ZK version included in Kafka distribution) node setup:
> # start both zookeeper and kafka (in any order)
> # stop zookeeper
> # stop kafka
> # start kafka
> # start zookeeper
> h3. Likely Cause
> {{ZookeeperLeaderElector}} subscribes to data changes on startup, and then triggers an election. if the deletion of ephemeral {{/controller}} node associated with previous zookeeper session of the broker happens after subscription to changes in new session, election will be invoked twice, once from {{startup}} and once from {{handleDataDeleted}}:
> * {{startup}}: acquire {{controllerLock}}
> * {{startup}}: subscribe to data changes
> * zookeeper: delete {{/controller}} since the session that created it timed out
> * {{handleDataDeleted}}: {{/controller}} was deleted
> * {{handleDataDeleted}}: wait on {{controllerLock}}
> * {{startup}}: elect -- writes {{/controller}}
> * {{startup}}: release {{controllerLock}}
> * {{handleDataDeleted}}: acquire {{controllerLock}}
> * {{handleDataDeleted}}: elect -- attempts to write {{/controller}} and then gets into infinite loop as a result of conflict
> {{createEphemeralPathExpectConflictHandleZKBug}} assumes that the existing znode was written from different session, which is not true in this case; it was written from the same session. That adds to the confusion.
> h3. Suggested Fix
> In {{ZookeeperLeaderElector.startup}} first run {{elect}} and then subscribe to data changes.



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