Multiple datacenter write requests

How write requests work when using multiple datacenters.

In multiple datacenter deployments, Cassandra optimizes write performance by choosing one coordinator node. The coordinator node contacted by the client application forwards the write request to one replica in each of the other datacenters, with a special tag to forward the write to the other local replicas.

If the write consistency level is LOCAL_ONE or LOCAL_QUORUM, only the nodes in the same datacenter as the coordinator node must respond to the client request in order for the request to succeed. Use either LOCAL_ONE or LOCAL_QUORUM to reduce geographical latency lessen the impact on client write request response times.

Figure 1. Multiple datacenter cluster with 3 replica nodes and consistency set to QUORUM