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https://issues.apache.org/jira/browse/HADOOP-8217?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13242797#comment-13242797
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Eli Collins commented on HADOOP-8217:
-------------------------------------
I think the sequence number approach makes sense. This is effectively two
transactions (one to make NN1 active, one to make NN2 active), each with it's
own zxid, and each time we execute a new transaction we need to fence previous
ones.
> 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: auto-failover, ha
> Affects Versions: 0.24.0
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Attachments: hadoop-8217-testcase.txt
>
>
> 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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