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From Jonathan Colby <>
Subject Re: Quorum, Hector, and datacenter preference
Date Thu, 24 Mar 2011 13:46:36 GMT
Indeed I found the big flaw in my own logic.   Even writing to the "local" cassandra nodes
does not guarantee where the replicas will end up.   The decision where to write the first
replica is based on the token ring, which is spread out on all nodes regardless of datacenter.
  right ?

On Mar 24, 2011, at 2:02 PM, Jonathan Colby wrote:

> Hi -
> Our cluster is spread between 2 datacenters.   We have a straight-forward IP assignment
so that OldNetworkTopology (rackinferring snitch) works well.    We have cassandra clients
written in Hector in each of those data centers.   The Hector clients all have a list of all
cassandra nodes across both data centers.  RF=3.
> Is there an order as to which data center gets the first write?    In other words, would
(or can) the Hector client do its first write to the cassandra nodes in its own data center?
> It would be ideal it Hector chose the "local" cassandra nodes.  That way, if one data
center is unreachable, the Quorum of replicas in cassandra is still reached (because it was
written to the working data center first).
> Otherwise, if the cassandra writes are really random from the Hector client point-of-view,
a data center outage would result in a read failure for any data that has 2 replicas in the
lost data center.
> Is anyone doing this?  Is there a flaw in my logic?

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