YOUR BRIEF
What to work through
Work through a payment system as a spoken system design interview. Start by clarifying the scope, sketch a working design and explain the trade-offs behind your choices.
- Pay-in: take money from a buyer by card for one or more sellers
- Pay-out: send money to sellers, for example monthly
- Record every money movement in a ledger
- Keep each seller’s balance in a wallet
- Show the status of a payment: pending, success or failed
BEFORE YOU START
Constraints to discuss
- Never charge a buyer twice for one order
- Correctness over throughput: about 10 transactions a second in Alex Xu’s framing
- No payment data lost when a component fails
- Mismatches with the payment provider found by a nightly reconciliation
Need a starting point?
Watch for these common gaps as you prepare:
- Treating money as a counter instead of a double-entry ledger
- Ignoring retries, so a timeout leads to a second charge
- Storing amounts as floating-point numbers
- No reconciliation with the provider
Why this question is in the collection
These sources connect the problem to company interview reports or preparation guides. They do not guarantee what a company will ask you.
- Stripe: On a Blind thread about Stripe’s final design round, one reply says the prompt was to design a payment system and another calls the round payments-focused.
- OpenAI: PracHub lists a reported OpenAI prompt dated 20 September 2026: a payment system with holds and captures, batch charges, reconciliation and idempotency.
- Airbnb: Design Gurus’ Airbnb guide says candidates report being asked to design payments.