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From Jonathan Ellis <>
Subject Re: RackAwareStrategy - add the third datacenter to live cluster with replication factor 3
Date Thu, 11 Feb 2010 21:34:19 GMT
On Thu, Feb 11, 2010 at 3:29 PM, B. Todd Burruss <> wrote:
> when using rack aware, RF=3, W=QUORUM ... does a write block until
> response from both data centers?

Quorum of 3 means 2, so it will block for first 2 replies, which
usually won't include the other DC.

BUT, remember it's asymmetrical.

Say you have replicas A C in DC1 and B in DC2.  Then clients using
QUORUM read/write talking to nodes in DC1 won't have to wait for a
reply from B, but clients using QUORUM in DC2 will have to wait for
either A or C since there is only one replica in DC2.

This is the motivation for the [unfinished] DatacenterShardStrategy
and the associated DCQUORUM consistency level -- the idea is, wait for
a quorum of the nodes _in the same data center that the client is
talking to_.


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