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From Todd Lipcon <>
Subject Re: cassandra vs hbase summary (was facebook messaging)
Date Mon, 22 Nov 2010 22:14:18 GMT
On Mon, Nov 22, 2010 at 1:58 PM, David Jeske <> wrote:

> On Mon, Nov 22, 2010 at 11:52 AM, Todd Lipcon <> wrote:
>> Not quite. The replica synchronization code is pretty messy, but basically
>> it will take the longest replica that may have been synced, not a quorum.
>> i.e the guarantee is that "if you successfully sync() data, it will be
>> present after replica synchronization". Unsynced data *may* be present after
>> replica synchronization.
>> But keep in mind that recovery is blocking in most cases - ie if the RS is
>> writing to a pipeline and waiting on acks, and one of the nodes in the
>> pipeline dies, then it will recover the pipeline (without the dead node) and
>> continue syncing to the remaining two nodes. The client is still blocked at
>> this point.
> I see. So it sounds like my statement #1 was wrong. Will the RS ever
> timeout the write and fail in the face of not being able to push it to HDFS?
> Is it correct to say:
> Once a write is issued to HBase, it will either catistrophicly fail (i.e.
> disconnect), in which case the write with either have failed or succeeded,
> and if it succeeded, future reads will always show that write? As opposed to
> Cassandra, which in all configurations where reads allow a subset of all
> nodes, can "fail" a write while having the write show a temporary period of
> inconsistency (depending on who you talk to) followed by the write either
> applying or not applying depending on whether or not it actually wrote a
> single node during the "failure to meet the write consistency request"?

Yes, this seems accurate to me.

> Does Cassandra have any return result which distinguishes between these two
> states:
> 1 - your data was not written to any nodes (true failure)
> 2 - your data was written to at least 1 node, but not enough to meet your
> write-consistency count
> ?

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