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From "Todd Lipcon (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-8217) Edge case split-brain race in ZK-based auto-failover
Date Mon, 26 Mar 2012 22:24:26 GMT
Edge case split-brain race in ZK-based auto-failover
----------------------------------------------------

                 Key: HADOOP-8217
                 URL: https://issues.apache.org/jira/browse/HADOOP-8217
             Project: Hadoop Common
          Issue Type: Bug
          Components: ha
    Affects Versions: 0.24.0
            Reporter: Todd Lipcon
            Assignee: Todd Lipcon


As discussed in HADOOP-8206, the current design for automatic failover has the following race:
- ZKFC1 gets active lock
- ZKFC1 is about to send transitionToActive() and machine freezes (eg GC pause + swapping)
- ZKFC1 loses its ZK lock, ZKFC2 gets ZK lock
- ZKFC2 calls transitionToStandby on NN1, and transitions NN2 to active
- ZKFC1 wakes up from pause, calls transitionToActive(), now we have a bad situation

This is rare, since it requires ZKFC1 to freeze longer than its ZK session timeout, but worth
fixing, since the results can be disastrous.

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