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Replication in two data center configuration

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Started 1 year ago by
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Anonymous
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Hi all, thanks in advance for your help! :)

Our team has a two data center configuration. We have a 6 node cluster with 3 nodes on each site. We want to have a replication factor of 3, replicating once to the local site and once cross site. Currently, we are using strategy_options={DC1:2,DC2:1}; for our keyspace, but notice that DC1 will always have two replicas while DC2 will always only have one replica, regardless of node.

What is the suggested way to specify our strategy so that each node will replicate once locally and once cross site?

Thank you!!
Alvin

Posted 1 year ago #
Anonymous
Unregistered

Any ideas?

Posted 1 year ago #
jackson@datastax.com
Moderator

Hello Alvin,

When setting up strategy option as "{DC1:2,DC2:1}", Cassandra will place 2 replicas in DC1 and 1 replica in DC2, as you have observed. This is the expected behavior.

All replicas are the same in Cassandra. There is no concept of replicating an extra copy within the "local datacenter", and replicate once in the "remote". You could do "{DC1:2, DC2:2}" such that each datacenter has 2 replicas for every write it receives.

Regards,
Jackson

Posted 1 year ago #
jbellis
Moderator

"Two copies local, one copy remote" is not supported, because then the replicas would have to change depending on which DC you're querying from.

OldNetworkTopologyStrategy would allow 3 total replicas, where sometimes the 3rd is in DC1, and other times in DC2 (deterministically). Or you could just leave it as just two replicas, or four as Jackson suggested.

Posted 1 year ago #
Anonymous
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Appreciate the responses. We'll probably end up going with a 2 and 2 setup after we get a few more nodes up, thanks!!

Posted 1 year ago #

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