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From aaron morton <aa...@thelastpickle.com>
Subject Re: Facebook messaging and choice of HBase over Cassandra - what can we learn?
Date Mon, 22 Nov 2010 09:15:11 GMT
Cassandra can work in a consistent way, see some of this discussion and the Consistency section
here http://wiki.apache.org/cassandra/ArchitectureOverview

If you always read and write with CL.Quorum (or the other way discussed) you will have consistency.
Even if some of the replicas are temporarily inconsistent, or off line or whatever. Your reads
will be consistent, i.e. every client will get the same value or the read will not work. If
you want to work at a lower or higher consistency you can.

Eventually all replicas of a value will become consistent. 

There are a number of reasons why cassandra may not be a good fit, and I would guess something
else would be a problem before the consistency model. 
 
Hope that helps. 
Aaron

On 22 Nov 2010, at 21:57, David Boxenhorn wrote:

> It's true that Cassandra has "tunable consistency", but if eventual consistency is not
sufficient for most of your use cases, Cassandra becomes much less attractive. Am I wrong?
> 
> 
> 
> On Sun, Nov 21, 2010 at 7:56 PM, Eric Evans <eevans@rackspace.com> wrote:
> On Sun, 2010-11-21 at 11:32 -0500, Simon Reavely wrote:
> > As a cassandra user I think the key sentence for this community is:
> > "We found Cassandra's eventual consistency model to be a difficult
> > pattern to reconcile for our new Messages infrastructure."
> 
> In my experience, "we needed strong consistency", in conversations like
> these amounts to hand waving.  It's the fastest way to shut down that
> part of the discussion without having said anything at all.
> 
> > I think it would be useful to find out more about this statement from
> > Kannan and the facebook team. Does anyone have any contacts in the
> > Facebook team?
> 
> Good luck.  Facebook is notoriously tight-lipped about such things.
> 
> > My goal here is to understand usage patterns and whether or not the
> > Cassandra community can learn from this decision; maybe even
> > understand whether the Cassandra roadmap should be influenced by this
> > decision to address a target user base. Of course we might also
> > conclude that its just "not a Cassandra use-case"!
> 
> Understanding is a laudable goal, just try to avoid drawing conclusions
> (and call out others who are).
> 
> <rant>
> This is usually the point where a frenzy kicks in and folks assume that
> the Smart Guys at Facebook know something they don't, something that
> would invalidate their decision if they'd only known.
> 
> I seriously doubt they've uncovered some Truth that would fundamentally
> alter the reasoning behind *my* decision to use Cassandra, and so I plan
> to continue as I always have.  Following relevant research and
> development, collecting experience (my own and others), and applying it
> to the problems I face.
> </rant>
> 
> --
> Eric Evans
> eevans@rackspace.com
> 
> 


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