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From Tatu Saloranta <tsalora...@gmail.com>
Subject Re: Memcached protocol?
Date Tue, 06 Apr 2010 06:54:10 GMT
On Mon, Apr 5, 2010 at 5:10 PM, Paul Prescod <paul@ayogo.com> wrote:
> On Mon, Apr 5, 2010 at 4:48 PM, Tatu Saloranta <tsaloranta@gmail.com> wrote:
>> ...
>>
>> I would think that there is also possibility of losing some
>> increments, or perhaps getting duplicate increments?
>
> I believe that with vector clocks in Cassandra 0.7 you won't lose
> anything. The conflict resolver will do the summation for you
> properly.
>
> If I'm wrong, I'd love to hear more, though.

I think the key is that this is not automatic -- there is no general
mechanism for aggregating distinct modifications. Point being that you
could choose one amongst right answers, but not what to do with
concurrent modifications. So what is done instead is have
application-specific resolution strategy which makes use of semantics
of operations, to know how to combine such concurrent modifications
into "correct" answer. I don't know if this is trivial for case of
counter increments: especially since two concurrent increments give
same new value; yet correct combined result would be one higher (both
used base, added one).

That is to say, my understanding was that vector clocks would be
required but not sufficient for reconciliation of concurrent value
updates.

I may be off here; apologies if I have misunderstood some crucial piece.

-+ Tatu +-

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