How write requests work.
The coordinator sends a write request to all replicas that own the row being written. As long as all replica nodes are up and available, they will get the write regardless of the consistency level specified by the client. The write consistency level determines how many replica nodes must respond with a success acknowledgment in order for the write to be considered successful. Success means that the data was written to the commit log and the memtable as described in the write path.
For example, in a single datacenter 10 node cluster with a replication factor of 3, an
incoming write will go to all 3 nodes that own the requested row. If the write consistency level
specified by the client is
ONE, the first node to complete the write responds
back to the coordinator, which then proxies the success message back to the client. A consistency
ONE means that it is possible that 2 of the 3 replicas can miss the
write if they happen to be down at the time the request is made. If a replica misses a write, the
row is made consistent later using one of its built-in repair
mechanisms: hinted handoff, read repair, or anti-entropy node repair.