The Quorum Intersection
Replicated systems often store each value on N nodes. Rather than wait for all N on every operation, they use quorums. A write must be acknowledged by W nodes and a read must gather responses from R nodes.
The magic rule is the quorum intersection condition, that W plus R must be greater than N. When this holds, any read set and any write set must overlap in at least one node, because two sets that together exceed the total cannot be disjoint. That overlapping node has seen the latest write, so the read is guaranteed to encounter at least one up to date copy. By attaching version information the reader picks the freshest value.
You can tune the split. Setting W to N and R to one makes writes slow but reads fast and still consistent. Setting W and R both near N over half gives balance. The condition is what makes any of these safe.
There is a second condition, W greater than N over two, which prevents two writes from being accepted by disjoint sets at the same time, avoiding conflicting committed writes.
- W plus R greater than N guarantees read and write sets overlap.
- Tunable Shift the cost between reads and writes while staying consistent.
- Availability You tolerate up to N minus W or N minus R failures.
Key idea
When the write quorum plus the read quorum exceeds the replica count, every read set overlaps every write set in at least one node, guaranteeing the reader sees the most recent write.