Glosarium
Contents

Backend Development › Product Building Blocks

Inventory and Reservations

Temporarily holding stock or seats so two buyers can't get the last one.

Backend Engineer · Senior

Also known as: inventory and reservations, inventory reservation, stock reservation

Inventory and reservations is the problem of tracking limited stock and holding an item for a customer while they check out, without overselling. A single product with one unit left must go to exactly one buyer, even if two people click “buy” at the same instant — and the hold should expire if the buyer abandons checkout.

checkout start → reserve 1 unit (expires in 15 min) → pay → confirm (reservation → sale)
                                          └─ abandon → reservation expires, unit returns

The core is a reservation: a time-limited claim on stock, created atomically, that prevents others from taking it. On successful payment the reservation becomes a sale; on expiry or failure, the stock is released.

The classic mistakes:

  • Check-then-decrement without atomicity. “Is stock > 0? yes → decrement” in two steps lets two buyers both pass the check and oversell. Use an atomic conditional update (SET stock = stock - 1 WHERE stock >= 1) or a reservation with a constraint (see atomic update).
  • No reservation expiry. A shopper abandons checkout and the units are held forever, so sellable stock disappears. Reservations must expire and release.
  • Expiry races. A reservation expiring at the same moment a payment confirms must not both release and sell the same unit; the transition must be atomic and idempotent.
  • Overselling by counting reservations wrong. Available stock = physical stock − active reservations; getting this formula or its updates wrong oversells or undersells. Keep it consistent.
  • Hot-row contention. A popular item’s stock row serialises all checkouts on it; under flash-sale load, that becomes a bottleneck. Consider sharded counters or queueing.
  • Non-idempotent retries. Retried “confirm” or “reserve” calls can reserve twice or sell twice; make the operations idempotent (see idempotence).
  • Ignoring the financial side. Stock movements are financial events; recording them in a ledger keeps inventory and accounting consistent.
  • Negative stock from returns/corrections. Manual adjustments, refunds and restocks must respect the same rules; guard against driving stock negative.

How to design it: model stock and reservations with atomic, conditional updates; expire reservations and release atomically; make reserve/confirm/cancel idempotent; and compute availability as stock minus active holds. Handle contention for hot items. It’s a classic concurrency problem where correctness is measured in money — get the atomicity right and use transactions to keep the transitions consistent.