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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