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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Cassandra vs HBase
Date Tue, 01 Sep 2009 21:42:59 GMT
> In exchange for always accepting writes, depending on what fails where
and when, reads in Cassandra may only retrieve stale data, or the
reads of some clients can
be inconsistent with reads of others.

Similar to what Amazon describes in the Dynamo paper, Cassandra lets
you specify how many replicas to block for on reads (R) and writes
(W).  If R + W > N (replication factor) then you will always have
strong consistency -- clients will always read the most recent value.
In particular you can get consistency while still having excellent
availability by blocking for a quorum for both R and W, so that is
probably the best default choice.  But different applications will
have different needs -- Facebook liked to insert with W=0 to minimize
latency on non-critical data, for instance.  So you can't have your
CAP cake and eat it too, but Cassandra gives you more flexibility in
picking your poison than most systems offer. :)


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