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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3136) Stale reads from ZK can break the atomic CAS operations we have in ZKAssign
Date Thu, 21 Oct 2010 00:33:23 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923260#action_12923260
] 

Jonathan Gray commented on HBASE-3136:
--------------------------------------

Opened HBASE-3137 to optimize this.

> 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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