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From Shu Zhang <szh...@mediosystems.com>
Subject RE: Basic Cassandra Architecture questions
Date Fri, 11 Feb 2011 18:06:32 GMT
"So if Key A is supposed to go to Node, 1,2,3 then the commit log for Key A will be on each
of these nodes?"

There isn't a commit log per key, just one for each node tracking what's been written to that
node. If a node1 determines node2 or node3 should handle a request it received, it'll route
it to one of them and not append anything to its own commit log. 
________________________________________
From: Ryan King [ryan@twitter.com]
Sent: Friday, February 11, 2011 9:46 AM
To: user@cassandra.apache.org
Cc: cassandra-user@incubator.apache.org
Subject: Re: Basic Cassandra Architecture questions

On Fri, Feb 11, 2011 at 9:37 AM, mcasandra <mohitanchlia@gmail.com> wrote:
>
> Is commit log file maintained on every node that's responsible to keep key
> ranges? So if Key A is supposed to go to Node, 1,2,3 then the commit log for
> Key A will be on each of these nodes? Is this commit log like redo log of
> oracle, which is used in case of failure to roll forward/back the writes?

Sorta, the commitlog is used to rebuild the memtables when a machine restarts.

> I am trying to think why R + W > N is said to be consistent and not R + W =
> N?

You get consistency when you have a guarantee of overlap between the
read and write sets. R + W > N is just another way of saying "there
are some hosts that are in both the successful read and write sets".

-ryan

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