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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: row level atomicity
Date Wed, 03 Mar 2010 22:11:02 GMT
> (i)                  It looks like make several calls to this.write.append()
which in turn does a bunch of individual out.write (to the DFSOutputStream), as opposed to
just one interaction with the underlying DFS. If so, how do we guarantee that all the edits
either make it to HDFS or not atomically? Or is this just broken?

Yeah I thought about that too but I'm not sure how can we do a single
DFS operation of any number KVs.

> (ii)                The updates to memstore should happen after the sync rather
than before, correct? 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 could serve uncommitted data.

Indeed. The syncWal was taken back up in HRS as a way to optimize
batch Puts but the fact it's called after all the MemStore operations
is indeed a problem. I think we need to fix both (i) and (ii) by
ensuring we do only a single append for whatever we have to put and
then syncWAL once before processing the MemStore. But, the other
problem here is that the row locks have to be taken out on all rows
before everything else in the case of a Put[] else we aren't atomic.
And then I think some checks are ran under HRegion that we would need
to run before everything else.

Quite a big change but I think it's needed.


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