The quorum rule
In a replicated store with N copies of each key, a quorum controls how many replicas must respond. A write must reach W replicas before it is acknowledged, and a read must hear from R replicas before it returns.
The key relationship is that when R plus W is greater than N, every read overlaps at least one replica that saw the latest write. That overlap is what gives you a strong chance of reading current data.
Tuning the knobs
- High W makes writes durable but slower and less available.
- High R makes reads safer but slower.
- Lowering both improves availability but risks stale reads.
A common balanced choice sets W and R each to a majority of N, so any read and any write must overlap.
Because reads may touch multiple replicas with different versions, the client picks the newest using version metadata such as a vector clock.
Key idea
Choosing R and W so their sum exceeds N forces read and write sets to overlap, trading availability for fresher reads.