Quorum Reads/Writes (R + W > N)
TechniqueDistributed SystemsMidSeniorUpdated
Tune per-operation consistency by requiring W replicas to ack writes and R replicas to answer reads. If R + W > N, every read overlaps the latest write.
When to Use Quorum Reads/Writes (R + W > N)
Leaderless Dynamo-style stores where you want to trade latency vs durability per call. N=3, W=2, R=2 is the canonical 'sloppy quorum' default.
In Practice
DynamoDB, Cassandra (consistency levels ONE/QUORUM/ALL), Riak. Cassandra's LOCAL_QUORUM picks W = ceil(N/2) + 1 within one datacenter to avoid cross-region latency.
How Quorum Reads/Writes (R + W > N) Comes Up in Interviews
Scenario
A Cassandra workload needs strong reads for billing queries but fast eventual reads for analytics, against the same N=3 keyspace.
Per-operation consistency level: billing uses QUORUM (R=2, W=2 so R+W>N), analytics uses ONE. Same data, tunable durability vs latency on each call.
Cross-DC, prefer LOCAL_QUORUM to avoid WAN latency. EACH_QUORUM forces ack in every DC and is reserved for critical writes.