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https://issues.apache.org/jira/browse/HBASE-4335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13125037#comment-13125037
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Jonathan Hsieh commented on HBASE-4335:
---------------------------------------
lgtm
> Splits can create temporary holes in .META. that confuse clients and
> regionservers
> ----------------------------------------------------------------------------------
>
> Key: HBASE-4335
> URL: https://issues.apache.org/jira/browse/HBASE-4335
> Project: HBase
> Issue Type: Bug
> Components: regionserver
> Affects Versions: 0.90.4
> Reporter: Joe Pallas
> Assignee: Lars Hofhansl
> Priority: Critical
> Fix For: 0.92.0
>
> Attachments: 4335-v2.txt, 4335-v3.txt, 4335-v4.txt, 4335-v5.txt,
> 4335.txt
>
>
> When a SplitTransaction is performed, three updates are done to .META.:
> 1. The parent region is marked as splitting (and hence offline)
> 2. The first daughter region is added (same start key as parent)
> 3. The second daughter region is added (split key is start key)
> (later, the original parent region is deleted, but that's not important to
> this discussion)
> Steps 2 and 3 are actually done concurrently by
> SplitTransaction.DaughterOpener threads. While the master is notified when a
> split is complete, the only visibility that clients have is whether the
> daughter regions have appeared in .META.
> If the second daughter is added to .META. first, then .META. will contain the
> (offline) parent region followed by the second daughter region. If the
> client looks up a key that is greater than (or equal to) the split, the
> client will find the second daughter region and use it. If the key is less
> than the split key, the client will find the parent region and see that it is
> offline, triggering a retry.
> If the first daughter is added to .META. before the second daughter, there is
> a window during which .META. has a hole: the first daughter effectively hides
> the parent region (same start key), but there is no entry for the second
> daughter. A region lookup will find the first daughter for all keys in the
> parent's range, but the first daughter does not include keys at or beyond the
> split key.
> See HBASE-4333 and HBASE-4334 for details on how this causes problems and
> suggestions for mitigating this in the client and regionserver.
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