Quorum Reads & Writes

Table of Contents

🔴 P0 — the mathematical foundation for consistency in replicated systems

Problem #

With N replicas, how many must acknowledge a write (W) and how many must respond to a read (R) to guarantee the reader sees the latest write?

Mechanism #

The quorum condition: W + R > N

N = 3 replicas

Config 1: W=2, R=2 → overlap guaranteed → strong reads
Config 2: W=3, R=1 → writes slow (all must ack), reads fast (one suffices)
Config 3: W=1, R=3 → writes fast, reads slow (must check all)
Config 4: W=1, R=1 → NO overlap → eventual consistency only

Key Trade-offs #

Config	Write latency	Read latency	Consistency	Write availability
W=2, R=2	Medium	Medium	Strong	Tolerates 1 failure
W=N, R=1	Slow (all)	Fast	Strong	No write tolerance
W=1, R=N	Fast	Slow (all)	Strong	Highest write avail
W=1, R=1	Fastest	Fastest	Eventual only	Highest

Instinct #

W=2, R=2 with N=3 is the standard balanced configuration. Adjust based on read/write ratio: read-heavy → lower R, higher W. Write-heavy → lower W, higher R. But remember: quorums don’t guarantee linearisability by themselves — you also need mechanisms to handle concurrent writes (version vectors, last-write-wins).

References #

DDIA 2e Reference #

Chapter 5: Quorums for reading and writing
Chapter 9: Limitations of quorum consistency