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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: how does cassandra pick its replicant peers?
Date Fri, 25 Mar 2011 23:05:59 GMT
> Does anyone know how cassandra chooses the nodes for its other replicant copies?

This keeps coming up so I added a FAQ entry:

   http://wiki.apache.org/cassandra/FAQ#replicaplacement

I don't quite like the phrasing but I couldn't come up anything that
was sufficiently clear and complete right now.

> The first node gets the first copy because its token is assigned for that key.   But
what about the other copies of the data?
> Do the replicant nodes stay the same based on the token range?  Or are the other copies
send to any random node based on its load and availability?
> I think this is important in order to understand because it affects how to plan for situations
where a significant number of nodes are suddenly unavailable, such as the loss of a data center.

I hope the above is answered by the FAQ. If it's unclear please say so
and we can clarify.

> If the replicants are copied just based on random availability, then quorum writes could
survive on the remaining nodes.  But if the replicant nodes are somehow pre-determined, those
replicants may node be available and writes will fail.

I'm not really following this though. Why would you ever want data to
be placed based on "random availability"?

If you are writing at QUORUM, a quorum of nodes in the replicate set
must have ack:ed the write in order for the read to be considered
successful (similar for reads). If a sufficient amount of nodes are
up, you're fine. If not, then no - fundamentally that would violate
the requirement of quorum.

For example, if you're at RF=3, at least two nodes (in the replica set
for a given key) must be responding to your request in order for them
to succeed at QUORUM.

-- 
/ Peter Schuller

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