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.sibb...@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.hachm...@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.sibb...@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 > Information in this email including any attachments may be privileged, > confidential and is intended exclusively for the addressee. The views > expressed may not be official policy, but the personal views of the > originator. If you have received it in error, please notify the sender by > return e-mail and delete it from your system. You should not reproduce, > distribute, store, retransmit, use or disclose its contents to anyone. > Please note we reserve the right to monitor all e-mail communication > through our internal and external networks. SKY and the SKY marks are > trademarks of Sky plc and Sky International AG and are used under licence. > Sky UK Limited (Registration No. 2906991), Sky-In-Home Service Limited > (Registration No. 2067075) and Sky Subscribers Services Limited > (Registration No. 2340150) are direct or indirect subsidiaries of Sky plc > (Registration No. 2247735). All of the companies mentioned in this > paragraph are incorporated in England and Wales and share the same > registered office at Grant Way, Isleworth, Middlesex TW7 5QD. >
