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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: meaning of eventual consistency in Cassandra ?
Date Mon, 03 Jan 2011 17:58:49 GMT
> What is the meaning of eventual consistency in Cassandra when nodes in
> a single cluster do not mantain the copies of same data but rather
> data is distributed among nodes. Since a single peice of data is
> recorded at a single place(node),Why wouldn't Cassandra return the
> recent value from that single place of record? How do multiple copies
> arise in this situation ? Where are the replicas in Cassandra cluster
> ?

There is normally not just a single copy. If you run with RF
(replication factor) = 1, you have a single copy. But this is only
useful if you don't care about redundancy at all.

With multiple replicas, the consistency depends on what you're doing.
For example the choice of consistency level (see the levels listed on
http://wiki.apache.org/cassandra/API).

However note that even with RF=1, there are some things that are still
only "eventual". For example if you submit a batch mutation,
concurrent readers may see a partially applied batch mutation for a
given row even though it is only being written to a single node.

-- 
/ Peter Schuller

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