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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Does Cassandra use vector clocks
Date Wed, 23 Feb 2011 04:39:05 GMT
On Tue, Feb 22, 2011 at 7:37 PM,  <Jeremy.Truelove@barclayscapital.com> wrote:
> Vector clocks was more of a Dynamo thing, I read a write up somewhere on some of reasons
why Cassandra puts this issue on the user but I can't locate it currently unfortunately. Hope
this helps.

Basically: vector clocks tell you there was a conflict, but not how to
resolve it (that is, you simply don't have enough information to
resolve it even if you push that back to the client a la Dynamo).
What dynamo-like systems mostly VC for is the trivial case of "client
X updated field 1, client Y updated field 2, so I can resolve that
into a merged value containing both updates."  But Cassandra already
handles that by splitting the row up into column-per-field, so VC
doesn't add anything for us.

Some more in depth explanation of the information-to-resolve-conflict
problem: http://pl.atyp.us/wordpress/?p=2601

-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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