consistency

Read/Write Quorum

N=5, W=3, R=3 • ✓ Strong consistency (W+R > N)

total nodes (N)5
write quorum (W)3
read quorum (R)3
W + R6 > N
N1
v0
version 0
N2
v0
version 0
N3
v0
version 0
N4
v0
version 0
N5
v0
version 0
Strong consistency: W + R = 6 > N = 5. Read quorum always overlaps with write quorum, guaranteeing latest value is read.

// quorum log

No operations yet

How It Works

  • Write succeeds when W nodes acknowledge
  • Read queries R nodes and returns latest version
  • If W + R > N: read/write sets overlap → strong consistency
  • Tune W and R for consistency vs availability tradeoffs

Use Cases

  • Cassandra, DynamoDB (tunable consistency)
  • Riak KV
  • Distributed key-value stores
  • CAP theorem tradeoffs (Dynamo paper)