The Write Path

High-level write path 

Being a peer-to-peer system, any node in the Cassandra cluster can handle a write request. The node receiving the write request is called the Coordinator node. Based on the RF defined for the table, the coordinator node forwards the request to all the nodes holding the token range. The coordinator then waits for acknowledgment from the number of nodes defined in consistency level (CL). Once the consistency level is achieved for the write operation, the coordinator responds back to the client. 

In case a replica node is down, the coordinator node stores a copy of the data, called a hint, to replay the data when the replica returns to the ring.

For multi-datacenter deployments, the coordinator contacts a node in each of the other datacenters, to act as a remote coordinator and propagate the write to all replicas in their respective datacenters.