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From Eric Stevens <migh...@gmail.com>
Subject Re: Replication to second data center with different number of nodes
Date Sat, 28 Mar 2015 20:46:03 GMT
If you're curious about how Cassandra knows how to replicate data in the
remote DC, it's the same as in the local DC, replication is independent in
each, and you can even set a different replication strategy per keyspace
per datacenter.  Nodes in each DC take up num_tokens positions on a ring,
each partition key is mapped to a position on that ring, and whomever owns
that part of the ring is the primary for that data.  Then (oversimplified)
r-1 adjacent nodes become replicas for that same data.

On Fri, Mar 27, 2015 at 6:55 AM, Sibbald, Charles <Charles.Sibbald@bskyb.com
> wrote:

>
> http://www.datastax.com/documentation/cassandra/2.0/cassandra/configuration/configCassandra_yaml_r.html?scroll=reference_ds_qfg_n1r_1k__num_tokens
>
>  So go with a default 256, and leave initial token empty:
>
>  num_tokens: 256
>
> # initial_token:
>
>
>  Cassandra will always give each node the same number of tokens, the only
> time you might want to distribute this is if your instances are of
> different sizing/capability which is also a bad scenario.
>
>   From: Björn Hachmann <bjoern.hachmann@metrigo.de>
> Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
> Date: Friday, 27 March 2015 12:11
> To: user <user@cassandra.apache.org>
> Subject: Re: Replication to second data center with different number of
> nodes
>
>
> 2015-03-27 11:58 GMT+01:00 Sibbald, Charles <Charles.Sibbald@bskyb.com>:
>
>> Cassandra’s Vnodes config
>
>
> ​Thank you. Yes, we are using vnodes! The num_token parameter controls the
> number of vnodes assigned to a specific node.​
>
>  Might be I am seeing problems where are none.
>
>  Let me rephrase my question: How does Cassandra know it has to replicate
> 1/3 of all keys to each single node in the second DC? I can see two ways:
>  1. It has to be configured explicitly.
>  2. It is derived from the number of nodes available in the data center at
> the time `nodetool rebuild` is started.
>
>  Kind regards
> Björn
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