Design e-commerce checkout.

The opening move — minute one

“Checkout is a promise made before the money moves. At 10M orders a day that’s only ~120 a second — so this isn’t a throughput problem, it’s a promise problem: I reserve stock with a TTL, authorize — not capture — and confirm only when both hold. Oversell is the failure I design against; expiry is the safety net that cleans up everything else.”

The order state machine

PLACED → RESERVED → CONFIRMED → SHIPPED
PAYMENT_FAILED (from RESERVED: decline — keep the hold for a retry until the TTL) · EXPIRED (TTL passed — the sweeper returns the stock) · CANCELLED (pre-ship: void the auth — free, because capture waited)

Must-haves

1. Prevents oversell with an atomic conditional reserve — names the TOCTOU race in check-then-decrement and puts the check in the same statement as the write.
2. Reserves at place-order (not add-to-cart) with a TTL, a sweeper, and an explicit abandonment argument (~70% of carts walk).
3. Orchestrates payment as authorize-at-checkout, capture-at-ship — sequenced reserve → authorize → confirm so every pre-ship failure is a free void — with idempotent order placement on top.
4. Does the arithmetic (10M/day ≈ 120 TPS), concludes one ACID database over reflex-sharding, and splits browse (cached, stale-OK) from the reserve path (strongly consistent).

Red flags

• Read-then-write stock math anywhere on the reserve path — the oversell is not hypothetical; it’s two concurrent requests away.
• Capture at checkout, or any charge that can exist without a confirmed order (no orphan sweep).
• Reservations without expiry — every abandoned checkout leaks a unit until the storefront lies.

The 7 trap claims — each one is wrong

1. “reserve the stock when the item goes into the cart”
2. “check the stock, then decrement it”
3. “capture the payment at checkout”
4. “charge whatever price the cart shows”
5. “eventual consistency is fine — it’s just shopping”
6. “we’ll handle the flash sale by autoscaling”
7. “publish “order placed” to the bus, then write the order”