Database / Apache Cassandra Intermediate and Advanced interview questions
What is hinted handoff in Cassandra?
Hinted handoff is Cassandra's way of tolerating a temporarily unavailable replica without failing the write or immediately falling out of sync.
- The coordinator sends the write to all replicas as usual.
- If a replica is down or unreachable, the coordinator (or another live node) stores a hint — a record of the mutation that replica missed.
- Once the failed replica comes back and gossip detects it as alive, the hint is replayed to it, catching it back up without needing a full repair.
Hints are only stored for a limited window, controlled by max_hint_window_in_ms (typically 3 hours by default). If a replica is down longer than that, hints stop accumulating for it, and it must instead be caught up via anti-entropy repair once it returns.
Hinted handoff is a best-effort mechanism, not a guarantee: hints can be lost if the coordinator itself crashes before replaying them, which is exactly why periodic repair is still necessary even with hinted handoff enabled.
What does a coordinator do when a replica is unreachable during a write?
What happens if a replica stays down longer than the hint window?
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