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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: Cassandra 2 DC deployment
Date Tue, 12 Apr 2011 20:53:30 GMT
> When the down data center comes back up, the Quorum reads will result in a read-repair,
so you will get valid data.   Besides that, hinted handoff will take care of getting data
replicated to a previously down node.

*Eventually* though, but yes. I.e., there would be no expectation to
instantly go back to full consistency once it goes back up.

Also, I would argue that it's useful to consider this: If you're
implementing "automatic" fallback to ONE whenever QUORUM fails;
consider all cases where this might happen for reasons *other* than
there being a legitimate partition of the DC:s. For example, some
random networking issues causing fewer nodes to be up etc.

A valid question is: If you simply do automatic fallback whenever
QUORUM fails anyway, are you significantly increasing consistency with
respect to ONE anyway? In some cases yes, but just be sure you know
what you're doing... Keep in mind that when all nodes are up and all
is working well, CL.ONE doesn't mean that writes won't be replicated
to all nodes. It just means that only one is *required* - and same for
reads.

If you have some situation whereby you normally want the strict
requirement that a read subsequent to a write sees the written data,
that doesn't sound very compatible with automatically falling back to
CL.ONE...

Anyways, those are my off-the-cuff thoughts - maybe it doesn't apply
in the situation in question.
-- 
/ Peter Schuller

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