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Date: Sat, 27 Oct 2012 20:55:12 +0000 (UTC)
From: "Lars Hofhansl (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
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Subject: [jira] [Commented] (HBASE-7051) Read/Updates
 (increment,checkAndPut) should properly read MVCC
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    [ https://issues.apache.org/jira/browse/HBASE-7051?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13485497#comment-13485497 ] 

Lars Hofhansl commented on HBASE-7051:
--------------------------------------

Thinking about this a bit more. Maybe the "nuclear" option isn't that nuclear after all.
The internalPut or internalDelete of checkAndMutate will wait for all prior transactions to finish anyway. Now we just wait in the beginning, so we get only slightly less concurrency but correctness.

I think the same will actually work for Increment and Append and we can get rid of the setting-the-memstoreTS-to-0 nonsense.

                
> Read/Updates (increment,checkAndPut) should properly read MVCC
> --------------------------------------------------------------
>
>                 Key: HBASE-7051
>                 URL: https://issues.apache.org/jira/browse/HBASE-7051
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.96.0
>            Reporter: Gregory Chanan
>
> See, for example:
> {code}
> // TODO: Use MVCC to make this set of increments atomic to reads
> {code}
> Here's an example of what I can happen (would probably be good to write up a test case for each read/update):
> Concurrent update via increment and put.
> The put grabs the row lock first and updates the memstore, but releases the row lock before the MVCC is advanced.  Then, the increment grabs the row lock and reads right away, reading the old value and incrementing based on that.
> There are a few options here:
> 1) Waiting for the MVCC to advance for read/updates: the downside is that you have to wait for updates on other rows.
> 2) Have an MVCC per-row (table configuration): this avoids the unnecessary contention of 1)
> 3) Transform the read/updates to write-only with rollup on read..  E.g. an increment  would just have the number of values to increment.

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