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Date: Wed, 9 Nov 2011 14:25:57 -0800
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 <CAO5xsd0TDHb5AxddbiANyMWRf2CuxytszJaWhyeto7Kk402y5g@mail.gmail.com>
Subject: Re: Will writes with < ALL consistency eventually propagate?
From: Peter Schuller <peter.schuller@infidyne.com>
To: user@cassandra.apache.org
Content-Type: text/plain; charset=UTF-8

> handful of nodes that I write to with a CL of QUORUM (or there abouts).

If your goal is to service reads w/o waiting for remote servers, you
probably would want to use LOCAL_QUORUM (quorum within a data center)
or ONE for reads. That however assumes an RF of >= 3 in each data
center (which means many copies in total if you have many data
centers).

Carefully plan how many copies you want in each DC, keeping in mind
that 3+ is required for LOCAL_QUORUM to be useful, and keeping in mind
that if you have only one copy per DC the latency to data will be
vastly increased whenever a node is down even if temporarily.

And consider whether you require that written data is guaranteed to be
visible globally upon subsequent read or not.

I don't know the situation, but I suspect some application logic on
top of Cassandra is useful for a CDN like situation. At least if you
have a lot of data and you care about cost efficiency. If it's okay to
just say RF=3 per DC (so e.g. 9 copies for 3 DC:s) and e.g. use
LOCAL_QUOROM for reads, and use LOCAL_QUORUM against each DC for
writes (or else just having less strict consistency guarantees), then
that's easy. It just won't be cost-effective if you start having to
scale to lots and lots of data (or writes).


-- 
/ Peter Schuller (@scode, http://worldmodscode.wordpress.com)