Quorum.

also: quorums · read quorum · write quorum · quorum reads · quorum writes · W + R > N

The number of replicas that must answer: with N replicas, write quorum W and read quorum R, W + R > N makes reads overlap writes.

the two-minute refresher

N = replicas, W = acks needed for a write, R = replicas asked on a read.
W + R > N means the read set and write set share at least one node.
N=3: W=2, R=2 overlaps; W=1, R=1 is faster but only eventually consistent.
Overlap alone is not full linearizability: sloppy quorums and concurrent writes still cause anomalies.

also: quorums · read quorum · write quorum · quorum reads · quorum writes · W + R > N

The number of replicas that must answer: with N replicas, write quorum W and read quorum R, W + R > N makes reads overlap writes.

N = replicas, W = acks needed for a write, R = replicas asked on a read.
W + R > N means the read set and write set share at least one node.
N=3: W=2, R=2 overlaps; W=1, R=1 is faster but only eventually consistent.
Overlap alone is not full linearizability: sloppy quorums and concurrent writes still cause anomalies.