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What is replication factor in cassandra and how does it affect single DC or multiple DC nodes ?

iNeedToAskThis
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  • See https://stackoverflow.com/questions/24587869/understand-cassandra-replication-factor-versus-consistency-level – Don Branson Sep 11 '17 at 02:55

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Cassandra stores replicas on multiple nodes to ensure reliability and fault tolerance. The total number of replicas across the cluster is referred to as the replication factor. A replication factor of 1 means that there is only one copy of each row on one node. A replication factor of 2 means two copies of each row, where each copy is on a different node. All replicas are equally important; there is no primary or master replica

When creating keyspace, you need to specify the replication factor on each DC.

Example Single DC with SimpleStrategy: 

CREATE KEYSPACE Excelsior WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 3 };
  • Here we specify replication_factor 3 means, Each row will be placed on three different node.

Example Multi DC :

CREATE KEYSPACE Excalibur WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'dc1' : 3, 'dc2' : 2};
  • This example sets three replicas for a data center named dc1 and two replicas for a data center named dc2

Source : https://docs.datastax.com/en/cassandra/2.1/cassandra/architecture/architectureDataDistributeReplication_c.html

Ashraful Islam
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  • I would rephrase this part: "Here we specify replication_factor 3 means three copies of each row will be placed on different node." It makes it sound as if a copy from the responsible node will be placed 3 times on another node. – Simon Fontana Oscarsson Sep 11 '17 at 08:46