I would replicate your different keyspaces to different DCs and scale those appropriately 
So, for example, HighLoad KS replicates to really-huge-dc, which would have, 10 nodes, LowerLoad KS replicates to smaller-dc with 5 nodes.
The idea is , you do not mix your different keyspaces in the same datacenter ( this is possible with NetworkTopology ) or for redundancy/HA purposes you place a single replica in the other keyspace's DC but you direct your applications to the "primary" DC of the keyspace, with LOCAL_QUORUM or ONE reads.

Regards,
Andras

From: Matthias Zeilinger <Matthias.Zeilinger@bwinparty.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Friday 12 April 2013 07:57
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: RE: multiple Datacenter values in PropertyFileSnitch

I΄m using for each application it΄s own keyspace.

What I want is to split up for different load patterns.

So that 2 apps with same and very high load pattern are not clashing.

 

For other load patterns I want to use another splitting.

 

Is there any best practice or should I scale out, so that the complete load can be distributed to on all nodes?

 

Br,

Matthias Zeilinger

Production Operation – Shared Services

 

P: +43 (0) 50 858-31185

M: +43 (0) 664 85-34459

E: matthias.zeilinger@bwinparty.com

 

bwin.party services (Austria) GmbH

Marxergasse 1B

A-1030 Vienna

 

www.bwinparty.com

 

From: aaron morton [mailto:aaron@thelastpickle.com]
Sent: Donnerstag, 11. April 2013 20:48
To: user@cassandra.apache.org
Subject: Re: multiple Datacenter values in PropertyFileSnitch

 

A node can only exist in one DC and one rack. 

 

Use different keyspaces as suggested. 

 

Cheers

 

-----------------

Aaron Morton

Freelance Cassandra Consultant

New Zealand

 

@aaronmorton

 

On 12/04/2013, at 1:47 AM, Jabbar Azam <ajazam@gmail.com> wrote:



Hello,

I'm not an expert but I don't think you can do what you want. The way to separate data for applications on the same cluster is to use different tables for different applications or use multiple keyspaces, a keyspace per application. The replication factor you specify for each keyspace specifies how many copies of the data are stored in each datacenter.

You can't specify that data for a particular application is stored on a specific node, unless that node is in its own cluster.

I think of a cassandra cluster as a shared resource where all the applications have access to all the nodes in the cluster.

 


Thanks

Jabbar Azam

 

On 11 April 2013 14:13, Matthias Zeilinger <Matthias.Zeilinger@bwinparty.com> wrote:

Hi,

 

I would like to create big cluster for many applications.

Within this cluster I would like to separate the data for each application, which can be easily done via different virtual datacenters and the correct replication strategy.

What I would like to know, if I can specify for 1 node multiple values in the PropertyFileSnitch configuration, so that I can use 1 node for more applications?

For example:

6 nodes:

3 for App A

3 for App B

4 for App C

 

I want to have such a configuration:

Node 1 – DC-A& DC-C

Node 2 – DC-B & DC-C

Node 3 – DC-A & DC-C

Node 4 – DC-B & DC-C

Node 5 – DC-A

Node 6 – DC-B

 

Is this possible or does anyone have another solution for this?

 

 

Thx & br matthias