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From Bryan Talbot <btal...@aeriagames.com>
Subject Re: In a multiple data center setup, do all the data centers have complete data irrespective of RF?
Date Mon, 20 May 2013 17:46:01 GMT
On Mon, May 20, 2013 at 10:01 AM, Pinak Pani <
nishant.has.a.question@gmail.com> wrote:

> Assume NetworkTopologyStrategy. So, I wanted to know whether a data-center
> will contain all the keys?
>
> This is the case:
>
> CREATE KEYSPACE appKS
>   WITH placement_strategy = 'NetworkTopologyStrategy'
>   AND strategy_options={DC1:3, DC2:3};
>
> Does DC1 and DC2 each contain complete database corpus? That is, if DC1
> blows, will I get all the data from DC2? Assume RF = 1.
>


Your config sample isn't RF=1 it's RF=3.  That's what the DC1:3 and DC2:3
mean -- set RF=3 for DC1 and RF=3 for DC2 for all rows of all CFs in this
keyspace.





>
> Sorry, for the very elementary question. This is the post that made me ask
> this question:
> http://www.onsip.com/blog/2011/07/15/intro-to-cassandra-and-networktopologystrategy
>
> It says,
>
> "NTS creates an iterator for EACH datacenter and places writes discretely
> for each. The result is that NTS basically breaks each datacenter into it's
> own logical ring when it places writes."
>

A lot of things change in fast moving projects in 2 years, so you'll have
to take anything written 2 years ago with a grain of salt and figure out if
it's still true with whatever version you're using.




>
> That seems to mean that each data-center behaves as an independent ring
> with initial_token. So, If I have 2 data centers and NTS, I am basically
> mirroring the database. Right?
>
>

Depending on how you've configured your placement strategy, but if you're
using DC1:3 and DC2:3 like you have above, then yes, you'd expect to have 3
copies of every row in both data centers for that keyspace.

-Bryan

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