Consistency LineLesson
Quorums: N, W, R
Ledger keeps every balance on three copies. How many should a write wait for, and how many should a read ask — and what happens when copies go down?
15 minMediumStation: Quorums: N, W, RMembers lesson
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- Consistency models: strong to eventual
- CAP & PACELC
- Quorums: N, W, R
- Agree on the balance
What you'll be able to do
- 1Predict whether a read can miss the latest confirmed write from N, W and R, and how many copies can be down before writes or reads fail.
- 2Choose W and R for a Ledger table from its read / write mix and how much staleness and downtime it tolerates — fast reads, fast writes or balanced.
- 3Explain why W + R > N still isn't linearizable — sloppy quorums, concurrent writes settled by timestamps, a failed write left on some copies — and how hinted handoff, read repair and anti-entropy bring copies back together.
Inside the lesson
- steps
- 12
- questions
- 12
- simulations
- 2
You guess first, then run the system and see if you were right.
Step 1 · The setup
Two stops ago, Ben's page missed Ana's settle-up because a replica trailed the primary. Now Ledger's balances move to a different kind of store: it keeps three copies of every balance and lets each request choose how many copies to wait for. The team starts with the fastest setting: a write is done as soon as one copy has it, and a read asks one copy.
Steps 2–12 are for members