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From aaron morton <>
Subject Re: question on setup for writes into 2 datacenters
Date Sun, 24 Jul 2011 22:40:30 GMT
Quick reminder, with RF == 2 the QUORUM is 2 as well. So when using LOCAL_QUORUM with RF 2+2
you will effectively be using LOCAL_ALL which may not be what you want. As De La Soul sang,
3 is the magic number for minimum fault tolerance (QUORUM is then 2). 

Aaron Morton
Freelance Cassandra Developer

On 23 Jul 2011, at 10:04, Sameer Farooqui wrote:

> It sounds like what you're looking for is write consistency of local_quorum:
> local_quorum would mean the write has to be successful on a majority of nodes in DC1
(so 2) before it is considered successful.
> If you use just quorum write, it'll have to be committed to 3 replicas out of the 4 before
it's considered successful.
> On Fri, Jul 22, 2011 at 1:57 PM, Dean Hiller <> wrote:
> Ideally, we would want to have a replication factor of 4, and a minimum write consistency
of 2 (which looking at the default in cassandra.yaml is to memory first with asynch to disk...perfect
so far!!!)
> Now, obviously, I can get the partitioner setup to make sure I get 2 replicas in each
data center.  The next thing I would want to guarantee however is that if a write came into
datacenter 1, it would write to the two nodes in datacenter 1 and asynchronously replicate
to datacenter 2.  Is this possible?  Does cassandra already handle that or is there something
I could do to get cassandra to do that?
> In this mode, I believe I can have both datacenters be live as well as be backup for
the other not wasting resources.
> thanks,
> Dean

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