Multiple datacenter write requests

How write requests work when using multiple datacenters.

In multiple datacenter deployments, DataStax Distribution of Apache Cassandra™ (DDAC) 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 write consistency levels is LOCAL_ONE or LOCAL_QUORUM, only the nodes in the same datacenter as the coordinator node must respond to the client request for the request to succeed. Use either LOCAL_ONE or LOCAL_QUORUM to reduce geographical latency and lessen the impact on response times of client write requests.

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