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From "Jordan Zimmerman (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CURATOR-3) LeaderLatch race condition causing extra nodes to be added in Zookeeper Edit
Date Tue, 26 Mar 2013 22:29:15 GMT
Jordan Zimmerman created CURATOR-3:
--------------------------------------

             Summary: 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
            Reporter: Jordan Zimmerman
            Assignee: 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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