Understanding Hinted Handoff (in Cassandra 0.8)

Hinted Handoff is an optional feature in Cassandra that enhances write availability when consistency is not required. It helps reduce the time needed for a failed node to become consistent again with live ones, especially during false-positive failures caused by flakey networks. When a replica node for a row is known to be down, Cassandra writes a hint to a live replica indicating that the write needs to be replayed to the unavailable node. Hinted Handoff does not count towards ConsistencyLevel requirements and cannot replace repair mechanisms. It minimizes extra load on the cluster to avoid cascading failure but can increase effective load in certain situations, such as writing at ConsistencyLevel.ANY when all replicas are down. When removing a node from the cluster, Cassandra automatically removes hints targeting that node. Hinted Handoff is an optimization for consistency and reduces the occurrence of stale reads but does not eliminate them entirely.