Quorum (N/R/W)
Requiring a minimum number of replicas to agree on a read or write to tune consistency.
In a replicated store with N copies, a write needs W acknowledgements and a read needs R responses. When R + W > N the read and write sets overlap, guaranteeing strongly consistent reads; smaller quorums trade consistency for latency and availability.
Worked example: with N=3, choosing W=2, R=2 gives R+W=4 > 3, so every read set shares at least one replica with every write set and sees the newest value. Drop to W=1, R=1 and it is fast and highly available, but a read can land on a replica that missed the last write. Gotcha: R+W > N buys consistency, yet pushing W=N (all replicas must ack) means a single node down blocks every write — so tune R and W to the failure you actually fear, not to “strongest”.