DistributedAdvanced
What does W + R > N give you?
“Explain quorum reads and writes.”
What this tests
- Quorum consistency
- Tuning C vs A per operation
Answers by level
Read the beginner answer first and notice what is missing.
With N replicas, W acknowledging a write and R contributing to a read, when W + R > N any read set overlaps any write set on at least one node, so a read always sees the latest write — strong consistency. N=3, W=2, R=2 is the common majority-both-ways setting.
W=1, R=1 is fast and eventually consistent; W=N gives no write availability if any node is down. The knobs let one system be CP or AP per operation.
Green flags · Red flags
Strong green flag · Explains the overlap guarantee, not just the inequality.
Green flags
- States the overlap rule
- Knows N=3/W=2/R=2
- Tunes per operation
Red flags
- "More replicas = better"
- Cannot explain why W + R > N works
Follow-up questions
F1
What consistency does W=1, R=1 give?
Scenario
A store runs N=3, W=1, R=1 for speed and users see stale reads. What quorum change fixes it and what does it cost?