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If a write is made and a replica node for the key is down (and hinted_handoff_enabled == true), Cassandra will write a hint to:
versions prior to 1.0: a live replica node
version 1.0: the coordinator node
indicating that the write needs to be replayed to the unavailable node. If ConsistencyLevel.ANY was specified, the hint will count towards consistency if no replica nodes are alive for this key. Cassandra uses hinted handoff as a way to (1) reduce the time required for a temporarily failed node to become consistent again with live ones and (2) provide extreme write availability when consistency is not required.
A hinted write does NOT count towards ConsistencyLevel requirements of ONE, QUORUM, or ALL. Take the simple example of a cluster of two nodes, A and B, and a replication factor of 1 (each key is stored on one node). Suppose node A is down while we write key K to it with ConsistencyLevel.ONE. Then we must fail the write: recall from the API page that "if W + R > ReplicationFactor, where W is the number of nodes to block for on write, and R the number to block for on reads, you will have strongly consistent behavior; that is, readers will always see the most recent write."
Thus if we write a hint to B and call the write good because it is written "somewhere," there is no way to read the data at any ConsistencyLevel until A comes back up and B forwards the data to him. Historically, only the lowest ConsistencyLevel of ZERO would accept writes in this situation; for 0.6, we added ConsistencyLevel.ANY, meaning, "wait for a write to succeed anywhere, even a hinted write that isn't immediately readable."
For more details, see http://www.datastax.com/dev/blog/modern-hinted-handoff