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From Tyler Hobbs <ty...@datastax.com>
Subject Re: How to come up with a predefined topology
Date Thu, 12 Jul 2012 05:54:10 GMT
I highly recommend specifying the same rack for all nodes (using
cassandra-topology.properties) unless you really have a good reason not too
(and you probably don't).  The way that replicas are chosen when multiple
racks are in play can be fairly confusing and lead to a data imbalance if
you don't catch it.

On Wed, Jul 11, 2012 at 10:53 PM, prasenjit mukherjee
<prasen.bea@gmail.com>wrote:

> > As far as I know there isn't any way to use the rack name in the
> strategy_options for a keyspace. You
> > might want to look at the code to dig into that, perhaps.
>
> Aha, I was wondering if I could do that as well ( specify rack options ) :)
>
> Thanks for the pointer, I will dig into the code.
>
> -Thanks,
> Prasenjit
>
> On Thu, Jul 12, 2012 at 5:33 AM, Richard Lowe <richard.lowe@arkivum.com>
> wrote:
> > If you then specify the parameters for the keyspace to use these, you
> can control exactly which set of nodes replicas end up on.
> >
> > For example, in cassandra-cli:
> >
> > create keyspace ks1 with placement_strategy =
> 'org.apache.cassandra.locator.NetworkTopologyStrategy' and strategy_options
> = { DC1_realtime: 2, DC1_analytics: 1, DC2_realtime: 1 };
> >
> > As far as I know there isn't any way to use the rack name in the
> strategy_options for a keyspace. You might want to look at the code to dig
> into that, perhaps.
> >
> > Whichever snitch you use, the nodes are sorted in order of proximity to
> the client node. How this is determined depends on the snitch that's used
> but most (the ones that ship with Cassandra) will use the default ordering
> of same-node < same-rack < same-datacenter < different-datacenter. Each
> snitch has methods to tell Cassandra which rack and DC a node is in, so it
> always knows which node is closest. Used with the Bloom filters this can
> tell us where the nearest replica is.
> >
> >
> >
> > -----Original Message-----
> > From: prasenjit mukherjee [mailto:prasen.bea@gmail.com]
> > Sent: 11 July 2012 06:33
> > To: user
> > Subject: How to come up with a predefined topology
> >
> > Quoting from
> http://www.datastax.com/docs/0.8/cluster_architecture/replication#networktopologystrategy
> > :
> >
> > "Asymmetrical replication groupings are also possible depending on your
> use case. For example, you may want to have three replicas per data center
> to serve real-time application requests, and then have a single replica in
> a separate data center designated to running analytics."
> >
> > Have 2 questions :
> > 1. Any example how to configure a topology with 3 replicas in one DC (
> with 2 in 1 rack + 1 in another rack ) and one replica in another DC ?
> >  The default networktopologystrategy with rackinferringsnitch will only
> give me equal distribution ( 2+2 )
> >
> > 2. I am assuming the reads can go to any of the replicas. Is there a
> client which will send query to a node ( in cassandra ring ) which is
> closest to the client ?
> >
> > -Thanks,
> > Prasenjit
> >
> >
>



-- 
Tyler Hobbs
DataStax <http://datastax.com/>

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