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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 Wed, 20 Oct 2010 23:30:23 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3136?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923230#action_12923230
] 

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

Yes.  There are two ways to make this work.  The easy way is to just sync() before we start.
 Or, as you describe, we retry after the update fails or if not in expected state we sync()
and re-read.

To get something done today, I'm going to just add sync() calls at the start of the two CAS
operations.  We can optimize later by being more optimistic and doing what you describe.

> 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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