14. Sell a limited drop without overselling

The brief

Design checkout for a drop of 2,000 individually sellable items. A product manager wants a countdown reservation, a trustworthy order confirmation and a waitlist when inventory is exhausted.

• 50,000 customers may arrive within a minute, but only 2,000 items exist.
• The payment provider can send duplicated or out-of-order webhooks; a timed-out charge can later succeed.
• A user can retry checkout from two devices. Product policy permits one item per customer.

Constraints

Reservation lifetime≤ 300 seconds

Declare the reservation window offered to a customer; it must not exceed five minutes.

Inventory invariant

Confirmed allocations must never exceed available inventory; distinguish an authorization from a captured payment.

One item per customer

Enforce the policy across concurrent requests and retries.

Late payment outcome

Define the customer outcome and compensation when a payment succeeds after a reservation expires.

What to cover

1. 01

   Customer and API contract

   Specify reserve, pay, confirm and status behavior, including idempotency keys and honest pending states.

2. 02

   Data and state machine

   Show reservation/order/payment states, inventory ownership and the atomic transition that prevents oversell.

3. 03

   Race walkthrough

   Trace expiry racing with a delayed successful payment and two-device retries.

4. 04

   Load and admission

   Estimate burst load, protect the invariant under overload and explain fair-enough admission or its limitations.

5. 05

   Product tradeoff

   Compare holding inventory before payment with allocating after payment, including the customer experience.

Worked designs

No worked design has been published for this brief yet. You can start an attempt and share your approach in the discussion.

Review rubric

AI feedback uses these criteria. Scores are practice feedback.