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From "Jagadeesh Huliyar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CURATOR-3) LeaderLatch race condition causing extra nodes to be added in Zookeeper Edit
Date Mon, 05 Aug 2013 09:23:47 GMT

    [ https://issues.apache.org/jira/browse/CURATOR-3?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13729335#comment-13729335
] 

Jagadeesh Huliyar commented on CURATOR-3:
-----------------------------------------

Facing this same issue. A new node gets placed in Zookeeper for a latch path. After that latches
on that path are not able to get leadership. In my case "Process A" is releasing the leadership
by closing the leader latch. I am using latch for a number of batch jobs. Zookeeper and Curator
(Leader Latch) is being used to ensure that batch job on only one machine is executed (in
a HA environment). This is happening for only of the jobs. This job is run every minute. In
case of jobs that are running at less frequent intervals Leader latch is working fine.
                
> LeaderLatch race condition causing extra nodes to be added in Zookeeper Edit
> ----------------------------------------------------------------------------
>
>                 Key: CURATOR-3
>                 URL: https://issues.apache.org/jira/browse/CURATOR-3
>             Project: Apache Curator
>          Issue Type: Bug
>          Components: Recipes
>    Affects Versions: 2.0.0-incubating
>            Reporter: Jordan Zimmerman
>
> From https://github.com/Netflix/curator/issues/265
> Looks like there's a race condition in LeaderLatch. If LeaderLatch.close() is called
at the right time while the latch's watch handler is running, the latch will place another
node in Zookeeper after the latch is closed.
> Basically how it happens is this:
> 1) I have two processes contesting a LeaderLatch, ProcessA and ProcessB. ProcessA is
leader.
> 2) ProcessA loses leadership somehow (it releases, its connection goes down, etc.)
> 3) This causes ProcessB's watch to get called, check the state is still STARTED, and
if so the LeaderLatch will re-evaluate if it is leader.
> 4) While the watch handler is running, close() is called on the LeaderLatch on ProcessB.
This sets the LeaderLatch state to CLOSED, removes the znode from ZK and closes off the LeaderLatch.
> 5) The watch handler has already checked that the state is STARTED, so it does a getChildren()
on the latch path, and finds the latch's znode is missing. It goes ahead and calls reset(),
which places a new znode in Zookeeper.
> Result: The LeaderLatch is closed, but there is still a node in Zookeeper that isn't
associated with any LeaderLatch and won't go away until the session goes down. Subsequent
LeaderLatches at this path can never get leadership while that session is up.

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