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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2283) row level atomicity
Date Thu, 04 Mar 2010 15:12:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2283?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12841321#action_12841321
] 

dhruba borthakur commented on HBASE-2283:
-----------------------------------------

If we say that we restart the region server when a write/sync to the region log fails, then
we can defer a fix for #2. If we do that, then we do not need any refactoring of the code
at all. We can solve #1 via putting a sync marker at the end of each transaction and making
the HLog.reader handle this marker correctly (details not yet worked out).

> row level atomicity 
> --------------------
>
>                 Key: HBASE-2283
>                 URL: https://issues.apache.org/jira/browse/HBASE-2283
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: Kannan Muthukkaruppan
>            Priority: Blocker
>             Fix For: 0.20.4, 0.21.0
>
>
> The flow during a HRegionServer.put() seems to be the following. [For now, let's just
consider single row Put containing edits to multiple column families/columns.]
> HRegionServer.put() does a:
>         HRegion.put();
>        syncWal()  (the HDFS sync call).  /* this is assuming we have HDFS-200 */
> HRegion.put() does a:
>   for each column family 
>   {
>       HLog.append(all edits to the colum family);
>       write all edits to Memstore;
>   }
> HLog.append() does a :
>   foreach edit in a single column family {
>     doWrite()
>   }
> doWrite() does a:
>    this.writer.append().
> There seems to be two related issues here that could result in inconsistencies.
> Issue #1: A put() does a bunch of HLog.append() calls. These in turn do a bunch of "write"
calls on the underlying DFS stream.  If we crash after having written out some append's to
DFS, recovery will run and apply a partial transaction to memstore.  
> Issue #2: The updates to memstore  should happen after the sync rather than before. Otherwise,
there is the danger that the write to DFS (sync) fails for some reason & we return an
error to the client, but we have already taken edits to the memstore. So subsequent reads
will serve uncommitted data.

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