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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: UnavailableException with 1 node down and RF=2?
Date Fri, 28 Oct 2011 08:58:53 GMT
> If you want to survive node failures, use an RF above 1. And then make
> sure to use an appropriate consistency level.

To elaborate a bit: RF, or replication factor, is the *total* number
of copies of any piece of data in the cluster. So with only one copy,
the data will not be available when a single node is down.

Consistency levels control how many nodes are required to respond to
requests before it is considered successful, and this has implications
on availability. For example, if you want to survive a single node
going down and you use RF=2, you must use ConsistencyLevel.ONE. If you
used QUORUM or ALL, any read or write would fail (QUORUM of 2 is 2).

Probably a common setup is to use RF=3 because it allows you to
survive a node going down, while also allowing you to use QUORUM. But,
whether that matters will be up to your use-case.

-- 
/ Peter Schuller (@scode, http://worldmodscode.wordpress.com)

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