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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2834) Deferred deletes
Date Wed, 14 Jul 2010 08:50:53 GMT
Deferred deletes

                 Key: HBASE-2834
                 URL: https://issues.apache.org/jira/browse/HBASE-2834
             Project: HBase
          Issue Type: New Feature
            Reporter: Andrew Purtell

Tangentally mentioned in a blog post, James Hamilton talks about deferred deletes:
If you have an application error, administrative error, or database implementation bug that
losses data, then it is simply gone unless you have an offline copy. This, by the way, is
why I'm a big fan of deferred delete.  This is a technique where deleted items are marked
as deleted but not garbage collected until some days or preferably weeks later.  Deferred
delete is not full protection but it has saved my butt more than once and I'm a believer.
See On Designing and Deploying Internet-Scale Services (http://mvdirona.com/jrh/talksAndPapers/JamesRH_Lisa.pdf)
for more detail.
(See http://perspectives.mvdirona.com/2010/04/07/StonebrakerOnCAPTheoremAndDatabases.aspx)

Because deletes -- at least, after the initial write has been flushed from memstore -- are
tombstones, deferred delete in HBase could be supported if somehow tombstones could be invalidated,
an undelete operation in effect. This could be accomplished by adding support for tombstones
for deletes. Would complicate major compaction but otherwise not touch much. A typical use
case might be "resurrect any data deleted from _ts1_ to _ts2_ ", a period of 4 hours when
an application error was operative. In this case a new write would be issued to the table
that is a tombstone covering any deletes over that period of time. Users would defer major
compactions until safe checkpoint periods. 

Such guarantees could optionally be extended to the memstoe by using tombstones there as well.
But it would probably be sufficient to provide guidance that forcing a flush is  necessary
to insure edits are persisted in a way that allows for undeletion.

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