One replica vs majority quorum
With one replica for reads and writes, a read can reach a follower that has not applied the latest write. A majority quorum is 2 of 3 for reads and for writes, so a read meets the latest write. One replica returns 178 stale reads. A majority quorum returns none, and p99 rises from 2.4 ms to 52 ms.
W 3 + R 3 of 5 · every read sees the last write
Quorum 2 of 3 · commit 9.0 ms
Case 1
Three replicas get 500 requests/s, and 80 percent are reads. Followers apply writes after 50 and 150 ms. No read may be stale, and p99 must stay below 200 ms.
| Measure | Goal | One replica: R 1, W 1 | Majority quorum: R 2, W 2 |
|---|---|---|---|
| Stale reads | none | 178 Fail | 0 Pass |
| Error rate | none | 0% Pass | 0% Pass |
| p99 latency | under 200 ms | 2.4 ms Pass | 52 ms Pass |
| CPU use | under 85% | 1.4% Pass | 1.9% Pass |
| Monthly cost | 12 USD/mo | 12 USD/mo |