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From "Jim Kellerman (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-810) Prevent temporary deadlocks when, during a scan with write operations, the region splits
Date Wed, 20 Aug 2008 02:10:44 GMT

    [ https://issues.apache.org/jira/browse/HBASE-810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623877#action_12623877
] 

Jim Kellerman commented on HBASE-810:
-------------------------------------

Looking at the code, I have realized that a lot has changed since HBASE-316.

I am currently working on three diagrams:
- how it works today
- how it should work
- what needs to be changed.

I have already found several cases where different objects are being used for synchronization
of the same operation. (Not sure how it got that way, but after looking at the code I'd be
surprised if there WERE NOT deadlocks).

I have three operations left to analyze: compaction, split and log rolling. I hope to finish
the analysis tomorrow.

> Prevent temporary deadlocks when, during a scan with write operations, the region splits
> ----------------------------------------------------------------------------------------
>
>                 Key: HBASE-810
>                 URL: https://issues.apache.org/jira/browse/HBASE-810
>             Project: Hadoop HBase
>          Issue Type: Bug
>    Affects Versions: 0.2.0
>            Reporter: Jean-Daniel Cryans
>            Priority: Blocker
>             Fix For: 0.2.1, 0.3.0
>
>
> HBASE-804 was not about the good problem, this one is. Anyone that iterates through the
results of a scanner and that rewrites data back into the row at each iteration will hit a
UnknownScannerException if a split occurs. See the stack in the referred jira. Timeline :
> Split occurs, acquires a write lock and waits for scanners to finish
> The scanner in the custom code iterates and writes data until the write is blocked by
the lock
> deadlock
> The scanner timeouts thus the region splits but the USE will be thrown when next() is
called
> Inside a Map, the task will simply be retried when the first one fails. Elsewhere, it
becomes more complicated.

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