How can we understand the concept of replication factor in cassandra? -

what replication factor in cassandra , how affect single dc or multiple dc nodes ?

cassandra stores replicas on multiple nodes ensure reliability , fault tolerance. the total number of replicas across cluster referred replication factor. replication factor of 1 means there 1 copy of each row on 1 node. replication factor of 2 means 2 copies of each row, each copy on different node. replicas equally important; there no primary or master replica

when creating keyspace, need specify replication factor on each dc.

example single dc simplestrategy:

create keyspace excelsior replication = { 'class' : 'simplestrategy', 'replication_factor' : 3 }; 

• here specify replication_factor 3 means, each row placed on 3 different node.

example multi dc :

create keyspace excalibur replication = {'class' : 'networktopologystrategy', 'dc1' : 3, 'dc2' : 2}; 

• this example sets 3 replicas data center named dc1 , 2 replicas data center named dc2

source : https://docs.datastax.com/en/cassandra/2.1/cassandra/architecture/architecturedatadistributereplication_c.html