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https://issues.apache.org/jira/browse/HBASE-3136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923226#action_12923226
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Todd Lipcon commented on HBASE-3136:
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The "not work" will just be that the "update" phase fails, and we should then
retry, no? Or if the "verify data is in expected state" section fails, we
should sync() and reread?
> 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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