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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-11667) Simplify ClientScanner logic for NSREs.
Date Tue, 05 Aug 2014 06:48:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-11667?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14085881#comment-14085881
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Lars Hofhansl commented on HBASE-11667:
---------------------------------------

I also wonder now how this would behave with a filter that filters most (or all) KVs for a
region in the case when we have a stale cache due to splits... Since the ClientScanner has
nothing to go by it would redo the scan of the entire prior region.


> Simplify ClientScanner logic for NSREs.
> ---------------------------------------
>
>                 Key: HBASE-11667
>                 URL: https://issues.apache.org/jira/browse/HBASE-11667
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Lars Hofhansl
>            Assignee: Lars Hofhansl
>             Fix For: 0.99.0, 2.0.0, 0.94.23, 0.98.6
>
>         Attachments: 11667-0.94.txt, 11667-doc-0.94.txt, 11667-trunk.txt, HBASE-11667-0.98.patch,
IntegrationTestBigLinkedListWithRegionMovement.patch
>
>
> We ran into an issue with Phoenix where a RegionObserver coprocessor intercepts a scan
and returns an aggregate (in this case a count) with a fake row key. It turns out this does
not work when the {{ClientScanner}} encounters NSREs, as it uses the last key it saw to reset
the scanner to try again (which in this case would be the fake key).
> While this is arguably a rare case and one could also argue that a region observer just
shouldn't do this... While looking at {{ClientScanner}}'s code I found this logic not necessary.
> A NSRE occurred because we contacted a region server with a key that it no longer hosts.
This is the start key, so it is always correct to retry with this same key. That simplifies
the ClientScanner logic and also make this sort of coprocessors possible,



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