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From Robin Verlangen <>
Subject Re: Expanding Cassandra on EC2 with consistency
Date Tue, 03 Jul 2012 13:22:35 GMT
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*

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