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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: meaning of eventual consistency in Cassandra ?
Date Mon, 03 Jan 2011 19:31:39 GMT
> This means that nodes in cassandra cluster contain data that has been
> sharded onto serveral nodes as well as this sharded data may be
> replicated further across several nodes ? So cassandra storage
> utilizes both sharded as well as replication for load balancing? Is
> this correct ?

Yes, sort of (though depending on your definition of sharding it might
be slightly misleading). In short, replicas (copies of data) is placed
on a number (determined by the replication factor, or RF) of nodes
that participate in a ring where each node has a token associated with
it. The row key (see http://wiki.apache.org/cassandra/DataModel)
determines, along with the so-called 'replication strategy', which
nodes in the ring should have replicas of the data.

I just realized that I couldn't find a wiki page or section of the
Riptano docs that explained the DHT ring and its implications from a
top-down perspective. Am I missing something or is this something that
should be written, anyone?

Probably the best resource on this I found is
http://www.riptano.com/docs/0.6/operations/clustering

If you are interested in the reasoning behind it, I greatly recommend
the Amazon Dynamo paper:

   http://s3.amazonaws.com/AllThingsDistributed/sosp/amazon-dynamo-sosp2007.pdf

Cassandra does not exactly implement what is described, but it is
strongly inspired by it.

-- 
/ Peter Schuller

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