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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-3136) Stale reads from ZK can break the atomic CAS operations we have in ZKAssign
Date Wed, 20 Oct 2010 23:20:24 GMT
Stale reads from ZK can break the atomic CAS operations we have in ZKAssign
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                 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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