Distributed Systems
Quorums: free-recall review
Retrieval beats re-reading. For each prompt, say or write a full answer from memory — including the R, W, N arithmetic — before you open the model answer. The effort of reconstruction is what makes the overlap invariant stick.
Reconstruct the unit’s spine — why R + W > N forces overlap, why QUORUM beats ALL, what tunable consistency buys, and exactly what sloppy quorum forfeits — without looking back at the lesson.
- 01State the R + W > N invariant and prove, in one sentence, why it guarantees a read sees the latest write.
- 02Why is QUORUM (W=2, R=2 at N=3) the production default rather than write-ALL or read-ALL?
- 03What does 'tunable consistency' mean operationally, and what does choosing R and W trade off?
- 04Describe sloppy quorum plus hinted handoff, and state precisely what guarantee it suspends and when.
- 05Name the three silent quorum failure modes from the unit and say why each errors nowhere.
- 06How do read repair and anti-entropy repair converge replicas, and what is their relationship to the overlap guarantee?
If you could reconstruct each answer from memory, you hold the unit’s spine: R + W > N forces overlap by pigeonhole and must be strict; QUORUM beats ALL because it tolerates one node down on both paths while keeping overlap; tunable consistency means setting R and W per operation along the consistency/availability/latency triangle; sloppy quorum plus hinted handoff buys write availability during partitions by suspending overlap while a designated node is down; the three silent failure modes (W=1 lost write, sloppy stale read, sub-overlap drift) all error nowhere because they are correct-by-configuration; and read repair plus Merkle-tree anti-entropy converge the gaps eventually but never replace the read-time guarantee.