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From Michael Theroux <>
Subject Re: Expanding Cassandra on EC2 with consistency
Date Tue, 03 Jul 2012 14:07:59 GMT
Yes, I saw LOCAL_QUORUM.  The definition I saw was:

Ensure that the write has been written to <ReplicationFactor> / 2 + 1 nodes, within
the local datacenter (requiresNetworkTopologyStrategy)

This will allow a quorum  within a datacenter.  However, I think this means that if availability
zones are racks, that the quorum would still be across availability zones.


On Jul 3, 2012, at 9:22 AM, Robin Verlangen wrote:

> Hi Mike,
> I'm not sure about all your questions, however you should take a look at LOCAL_QUORUM
for your question about consistency level reads/writes.
> 2012/7/3 Michael Theroux <>
> Hello,
> We are currently running a web application utilizing Cassandra on EC2.  Given the recent
outages experienced with Amazon, we want to consider expanding Cassandra across availability
zones sooner rather than later.
> We are trying to determine the optimal way to deploy Cassandra in this deployment.  We
are researching the NetworkTopologyStrategy, and the EC2Snitch.  We are also interested in
providing a high level of read or write consistency,
> My understanding is that the EC2Snitch recognizes availability zones as racks, and regions
as data-centers.  This seems to be a common configuration.  However, if we were to want to
utilize queries with a READ or WRITE consistency of QUORUM, would there be a high possibility
that the communication necessary to establish a quorum, across availability zones?
> My understanding is that the NetworkTopologyStrategy attempts to prefer replicas be stored
on other racks within the datacenter, which would equate to other availability zones in EC2.
 This implies to me that in order to have the quorum of nodes necessary to achieve consistency,
that Cassandra will communicate with nodes across availability zones.
> First, is my understanding correct?  Second, given the high latency that can sometimes
exists between availability zones, is this a problem, and instead we should treat availability
zones as data centers?
> Ideally, we would be able to setup a situation where we could store replicas across availability
zones in case of failure, but establish a high level of read or write consistency within a
single availability zone.
> I appreciate your responses,
> Thanks,
> -Mike
> -- 
> With kind regards,
> Robin Verlangen
> Software engineer
> W
> E
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