nosql - how Cassandra chooses the coordinator node and the replication nodes?

Question

How the Cassandra client chooses the coordinator node? is the coordinator node stores the data sent by the client before replicating?

Answer 1

The coordinator is selected by the driver based on the policy you have set. Common policies are DCAwareRoundRobinPolicy and TokenAware Policy.

For DCAwareRoundRobinPolicy, the driver selects the coordinator node based on its round robin policy. See more here: http://docs.datastax.com/en/drivers/java/2.1/com/datastax/driver/core/policies/DCAwareRoundRobinPolicy.html

For TokenAwarePolicy, it selects a coordinator node that has the data being queried - to reduce "hops" and latency. More info: http://docs.datastax.com/en/drivers/java/2.1/com/datastax/driver/core/policies/TokenAwarePolicy.html

It is a best practice to wrap policies so there is a primary and secondary policy should there be an issue. More information available at the links above.