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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-1806) Scanners do not respect row locks; scanner view could return a skewed view on row if ongoing update
Date Tue, 01 Sep 2009 16:42:32 GMT

    [ https://issues.apache.org/jira/browse/HBASE-1806?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12749983#action_12749983
] 

Andrew Purtell commented on HBASE-1806:
---------------------------------------

I've always accepted this fact about scanners. It's debatable if we should take the performance
hit to consider transactional semantics of any kind for scanners. Could be enough to just
document that TableMappers should test for inconsistent state from the application's perspective
and ignore the record as appropriate. Can get another shot at it during the next job. Of course
master scanners are a special case. I wonder why this did not occur to me before... Moving
META into ZK is the appropriate strategy to use for this and other reasons IMO. 

> Scanners do not respect row locks; scanner view could return a skewed view on row if
ongoing update
> ---------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-1806
>                 URL: https://issues.apache.org/jira/browse/HBASE-1806
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: stack
>
> What I'm seeing is that BaseScanner misses updates made by an update milliseconds before
-- even hundreds of milliseconds before.  See hbase-1784 where I'm seeing double-assignment
of regions.
> Scanners do not respect row locks.  They should else could return a row with partial
updates committed.  What if a .META. region has tens of storefiles and a scan does a get full
row which takes a long time.  Say an update comes in during this read.  First it will go in
because no row lock is outstanding.  Second, we'll miss the edit given we look at things in
order -- memstore, then each storefile down to the oldest.  What if the update is followed
by an update of server state; e.g. region is moved out of intransition state?  And inside
in same server, say the master, it makes decisions dependent on what it sees when it does
a scanner#next; e.g. BaseScanner checking for assignment?

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