18. Hinted Handoff

A write should not fail just because one replica is offline. Hinted handoff lets the system keep writing, and catches that replica up later.

Background

Depending on the consistency level, a distributed system can still serve write requests even when some nodes are down. Say the replication factor, the number of copies of the data, is three. A client writes at a quorum consistency level, meaning a majority of the replicas must acknowledge the write. If one node is down, the system can still write to the other two and satisfy the quorum, so the write succeeds