Inventory Reservation Against Oversell

Design a reservation so two checkouts cannot buy the last item. Keep it in Postgres unless I said otherwise. RULES Stock is counted per: [SKU] Hold time: [MINUTES] Oversell policy: [NEVER] What decrements stock: [PAID ORDER, NOT ADD-TO-CART] DELIVER 1) The stock row with a constraint that available cannot go below zero. 2) The reservation: an update that subtracts only if enough remain, in one statement, returning whether it won. The losing request gets a clean "not available," not a 500. 3) Expiry: a scheduled release of unpaid reservations, idempotent, so it can run twice. 4) Payment success converts the reservation to a sale. Payment failure or expiry returns the units. Show both transitions and the single-row invariant. 5) The isolation or locking choice and the race you are closing. A test that runs two concurrent checkouts for one unit and asserts one success. Do not check stock in the application and then update later. Do not hold stock at add-to-cart unless I explicitly said to, because that is how inventory gets stuck.