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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-588) Still a 'hole' in scanners, even after HBASE-532
Date Thu, 17 Apr 2008 17:41:21 GMT
Still a 'hole' in scanners, even after HBASE-532
------------------------------------------------

                 Key: HBASE-588
                 URL: https://issues.apache.org/jira/browse/HBASE-588
             Project: Hadoop HBase
          Issue Type: Bug
            Reporter: stack
            Priority: Blocker


Before HBASE-532, as soon as a flush started, we called snapshot.  Snapshot used to copy current
live memcache into a 'snapshot' TreeMap inside in Memcache.  This snapshot TreeMap was an
accumulation of all snapshots since last flush.   Whenever we took out a scanner, we'd do
a copy of this snapshot into a new backing map carried by the scanner (Every outstanding Scanner
had complete copy).  Memcache snapshots were cleared when a flush started.   Flushing could
take near no time to up to tens of seconds during which an scanners taken out meantime would
not see the edits in the snapshot currently being flushed and gets or getFull would also return
incorrect answers because the content of the snapshot was not available to them.

HBASE-532 made it so the snapshot was available until flush was done -- until a file had made
it out to disk.  This fixed gets and getFull and any scanners taken out during flushing. 
But there is still a hole.  Any outstanding scanners will be going against the state of Store
Readers at time scanner was opened; they will not see the new flush file.

Chatting about this on IRC, Jim suggests that we pass either memcache or current snapshot
to each Scanner (Pass the snapshot if not empty).  The notion is that the Scanner would hold
on to the Scanner reference should it be cleared by flushing.  Upside is that scanner wouldn't
have to be concerned with the new flush that has been put out to disk.  Downsides are that
Scanner data could be way stale if for instance the memcache was near to flushing but we hadn't
done it yet.  And we wouldn't be clearing the snapshot promptly so would be some memory pressure.

Another suggestion is that flushing send an event.  Listeners such as outstanding scanners
would notice event and open the new Reader.  Would have to skip forward in the new Reader
to catch up with the current set but shouldn't be bad.  Same mechanism could be used to let
compactions be moved into place while scanners were outstanding closing down all existing
readers skipping to the current 'next' location in the new compacted store file.

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