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From "Jeremiah Jordan" <>
Subject RE: Docs: Token Selection
Date Fri, 17 Jun 2011 18:32:08 GMT
	Run two clusters, one which has {DC1:2, DC2:1} and one which is
{DC1:1,DC2:2}.  You can't have both in the same cluster, otherwise it
isn't possible to tell where the data got written when you want to read
it.  For a given key "XYZ" you must be able to compute which nodes it is
stored on just using "XYZ", so a strategy where it is on nodes
DC1_1,DC1_2, and DC2_1 when a node in DC1 is the coordinator, and to
DC1_1, DC2_1 and DC2_2 when a node in DC2 is the coordinator won't work.
Given just "XYZ" I don't know where to look for the data.
	But, from the way you describe what you want to happen, clients
from DC1 aren't using data inserted by clients from DC2, so you should
just make two different Cassandra clusters.  Once for the DC1 guys which
is {DC1:2, DC2:1} and one for the DC2 guys which is {DC1:1,DC2:2}.

-----Original Message-----
From: AJ [] 
Sent: Friday, June 17, 2011 1:02 PM
Subject: Re: Docs: Token Selection

Hi Jeremiah, can you give more details?


On 6/17/2011 10:49 AM, Jeremiah Jordan wrote:
> Run two Cassandra clusters...
> -----Original Message-----
> From: Eric tamme []
> Sent: Friday, June 17, 2011 11:31 AM
> To:
> Subject: Re: Docs: Token Selection
>> What I don't like about NTS is I would have to have more replicas 
>> than I need.  {DC1=2, DC2=2}, RF=4 would be the minimum.  If I felt 
>> that 2 local replicas was insufficient, I'd have to move up to RF=6 
>> which seems like a waste... I'm predicting data in the TB range so 
>> I'm trying to keep replicas to a minimum.
>> My goal is to have 2-3 replicas in a local data center and 1 replica 
>> in another dc.  I think that would be enough barring a major 
>> catastrophe.  But, I'm not sure this is possible.  I define "local" 
>> as in the same data center as the client doing the insert/update.
> Yes, not being able to configure the replication factor differently
for each data center is a bit annoying.  Im assuming you basically want
DC1 to have a replication factor of {DC1:2, DC2:1} and DC2 to have
> I would very much like that feature as well, but I dont know the
feasibility of it.
> -Eric

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