Question library
Worked answers to real system-design interview questions.
Each page is the complete Staff+-level answer — built from the same question definition the interview simulator probes, and scored against the same rubric it grades with. The traps, the probes, the verdict ladder: nothing held back.
Design a payment system like Stripe.
Idempotency as a contract, the append-only double-entry ledger, saga failure modes, PCI scope — and the exact rubric the verdict comes from.
Design a digital wallet.
Double-spend prevention as a contract, the append-only double-entry ledger, atomic cross-shard transfers, holds and provisional credit — and the exact rubric the verdict comes from.
Design e-commerce checkout.
Oversell prevention as one conditional UPDATE, reservations that expire, auth-at-checkout capture-at-ship, the flash-sale hot SKU — and the exact rubric the verdict comes from.
Design hotel reservation.
Overbooking as deliberate policy, the atomic multi-night reserve, holds without money, no-shows feeding the ceiling — and the exact rubric the verdict comes from.
The library is growing — each new question ships with the full answer, the instruments, and its revision sheet.
Reading is the easy half.
The interview is 45 minutes of follow-ups against your design, live. The simulator asks these exact questions and scores you on the same rubric.