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Subject: Re: Cassandra 2 DC deployment
From: Jonathan Colby <jonathan.colby@gmail.com>
In-Reply-To: <BANLkTimVzKYmJQLcREcCx1XqjeT1H00=uA@mail.gmail.com>
Date: Tue, 12 Apr 2011 18:13:48 +0200
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References: <BANLkTimVzKYmJQLcREcCx1XqjeT1H00=uA@mail.gmail.com>
To: user@cassandra.apache.org

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.

You're example is a little unrealistic because you could theoretically =
have a DC with only one node.  So CL.ONE would work every time.   But if =
you have more than 1 node, you have to decide if your application can =
tolerate getting "NULL"  for a read if the write hasn't propagated from =
the responsible node to the replica.

disclaimer:  I'm a cassandra novice.

On Apr 12, 2011, at 5:12 PM, Raj N wrote:

> Hi experts,
>      We are planning to deploy Cassandra in 2 datacenters. Let assume =
there are 3 nodes, RF=3D3, 2 nodes in 1 DC and 1 node in 2nd DC. Under =
normal operations, we would read and write at QUORUM. What we want to do =
though is if we lose a datacenter which has 2 nodes, DC1 in this case, =
we want to downgrade our consistency to ONE. Basically I am saying that =
whenever there is a partition, then prefer availability over =
consistency. In order to do this we plan to catch UnavailableException =
and take corrective action. So try QUORUM under normal circumstances, if =
unavailable try ONE. My questions -
> Do you guys see any flaws with this approach?=20
> What happens when DC1 comes back up and we start reading/writing at =
QUORUM again? Will we read stale data in this case?
>=20
> Thanks
> -Raj