Quorum
The number of replicas that must take part in a read or a write. With N replicas, read quorum R, and write quorum W, R + W > N means every read asks at least one replica that acknowledged the latest write. That is not linearizability: concurrent writes, failed writes, and sloppy quorums can still return old values.
Data
In Quorum#
A Wide-column Store keeps each key on N replicas (1 to 5); you set R, the replicas a read asks, and W, the replicas a write waits for. With R + W > N reads never miss the latest write; a weaker quorum costs fewer replica operations and waits for an earlier reply, but some reads come back stale. A quorum that needs every replica fails while one replica is lost, and even a strong quorum gets 2% stale and 1% duplicate checkouts.
Read next#
Raft, Eventual consistency, Leaderless replication, Network partition