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From "HBase Review Board (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3136) Stale reads from ZK can break the atomic CAS operations we have in ZKAssign
Date Fri, 22 Oct 2010 19:06:16 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923974#action_12923974
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HBase Review Board commented on HBASE-3136:
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Message from: stack@duboce.net

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Ship it!


+1 on getting it in as is.  I think its going to kill my bulk assign improvements but yesterday
discussing w/ Jon and Karthik, there are plenty of options to explore yet... We'll do in another
issue.

- stack





> Stale reads from ZK can break the atomic CAS operations we have in ZKAssign
> ---------------------------------------------------------------------------
>
>                 Key: HBASE-3136
>                 URL: https://issues.apache.org/jira/browse/HBASE-3136
>             Project: HBase
>          Issue Type: Bug
>          Components: zookeeper
>    Affects Versions: 0.89.20100621, 0.89.20100924, 0.90.0
>            Reporter: Jonathan Gray
>            Assignee: Jonathan Gray
>            Priority: Blocker
>             Fix For: 0.90.0
>
>
> With ZK based region transitions, we rely on atomic state changes of regions in transition.
 For example, an RS needs to atomically switch a node from OFFLINE to OPENING, or the master
needs to delete nodes that are in OPENED state, etc...
> The way we implement this is by:
> - Read existing data (returns byte[] and version in Stat)
> - Verify data is in expected state
> - Update to the new state, passing the expected version previously read
> This doesn't always work as expected because that initial read of the existing data could
be a stale read (in ZK, writes are quorum writes but reads are not so you can get stale data).
> Can provide a more explicit example if anyone is interested, but a fix is coming.

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