9 docs tagged with "Tier 4 — Production system design"

Rule: technically valid is not the same as trustworthy, and blocking too much costs more than

Rule: money moved and the order updated are two facts in two places, and no transaction spans

Rule: payment status says whether a transaction succeeded; only a ledger says where the money

Rule: a provider response and a webhook are external signals, not truth. The only source of

Rule: a payment is a state machine with an idempotency key, never a boolean. paid = true

Rule: reconciliation is a legitimate source of change, not a silent patch. Every repair it

Rule: a refund is a new transaction, not an undo. It has its own state machine, its own

Rule: "payment succeeded" is an intention; settlement is when the money actually arrives.

Open-ended by construction: "design Twitter" has no correct answer, and the interviewer is not