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From Michael Theroux <>
Subject Expanding Cassandra on EC2 with consistency
Date Tue, 03 Jul 2012 13:16:47 GMT

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,

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