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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-11667) Comment ClientScanner logic for NSREs.
Date Wed, 06 Aug 2014 02:44:12 GMT

    [ https://issues.apache.org/jira/browse/HBASE-11667?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14087161#comment-14087161
] 

Lars Hofhansl commented on HBASE-11667:
---------------------------------------

bq. Keeping last thing we delivered – state – would purge all that is grey in this area.
In a sense we do, and then use only when encountering a NRSE exception. If we just remember
what we delivered (and don't require a real key) we'd always have to rescan the entire region
upon a NSRE (not just the last rows worth of scanner caching) and then ignore everything we
delivered so far. So definitely less efficient, and might lead to other weird issues if scans
do not return the exact same row keys upon repeated execution (for example when a filter or
region observer messes with things).

I'll commit the comment update to all branches for now; but I'll keep thinking about this.
This is ugly right now, especially in trunk with the region replicas.


> Comment 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
>            Priority: Minor
>             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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