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Subject: Re: Quorum, Hector, and datacenter preference 
From: Jonathan Colby <jonathan.colby@gmail.com>
In-Reply-To: <52BE6DCD-68AC-45D1-B6CD-7F5CA19339B9@gmail.com>
Date: Thu, 24 Mar 2011 14:46:36 +0100
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References: <52BE6DCD-68AC-45D1-B6CD-7F5CA19339B9@gmail.com>
To: user@cassandra.apache.org

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 -
>=20
> 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=3D3.
>=20
> 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?
>=20
> 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).
>=20
> 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.
>=20
> Is anyone doing this?  Is there a flaw in my logic?
>=20
>=20