Seat holds and reservation timeouts in a flash sale: an interactive simulator

Selling a limited number of seats, rooms or items means deciding when a buyer gets one. Taking it only at payment lets many buyers race for the same seat and most of them lose after typing in a card. A hold reserves the seat when it is picked, with a time to live so abandoned checkouts give it back. The length of the hold, and whether it is renewed while the buyer is still there, decides how many seats sit idle and how many genuine buyers lose theirs.

The playground runs a flash sale of 500 seats to 3,000 buyers for half an hour under four policies. Change how many buyers abandon checkout.

What each policy costs

• No holds sells out fastest, in 3.6 minutes here, but only because buyers race each other at payment: 848 of them reach the end of checkout and are told the seat has gone.
• A long hold, 15 minutes, means nobody who reaches payment loses their seat. But about a third of holders walk away, and each of their seats stays blocked until the hold lapses: 2,931 seat-minutes held for people who had left. Buyers see a sell-out that is not real, give up, and 59 seats are still unsold after half an hour.
• A short hold, 4 minutes, gives abandoned seats back sooner but runs out under slow buyers, 305 of whom lose their seat at payment.
• A short hold renewed while the buyer is active gets both: the checkout page extends the hold every few seconds, so genuine buyers keep it as long as they need, and a seat comes back 2 minutes after its buyer leaves. Nobody fails at payment and only 1 seat is unsold.

Holds in SQL

-- take a hold: only if the seat is free, or its hold has lapsed
UPDATE seats
   SET held_by = :buyer, held_until = now() + interval '2 minutes'
 WHERE seat_id = :seat
   AND status = 'available'
   AND (held_until IS NULL OR held_until < now());
-- 1 row updated: the seat is yours; 0 rows: someone else has it

-- the checkout page renews while the buyer is active
UPDATE seats SET held_until = now() + interval '2 minutes'
 WHERE seat_id = :seat AND held_by = :buyer AND held_until > now();

-- payment succeeded: sell only if the hold is still ours
UPDATE seats SET status = 'sold'
 WHERE seat_id = :seat AND held_by = :buyer AND held_until > now();

Each step is one conditional update, so two buyers can never hold or buy the same seat. Expired holds need no cleanup job: the condition on held_until treats them as free. For very hot sales, holds often live in Redis with a key per seat and an expiry, and a virtual waiting room admits buyers in batches so the hold store is not hit by everyone at once.

In a system design interview

For ticketing, hotel or flash-sale designs, explain the reservation lifecycle: “Picking a seat takes a hold with a short expiry using a conditional update, so only one buyer can hold it. The checkout page renews the hold while the buyer is active, and payment only succeeds if the hold is still theirs. Expired holds are free by definition, so there is no cleanup race. In front of it, a waiting room limits how many buyers are picking seats at once.” Expect the follow-ups: what happens when payment succeeds after the hold expired, how to show availability without a hot read path, and how to stop bots from holding everything.

Read the guide to bookings and reservations or inventory and flash sales, see optimistic vs pessimistic locking, or watch a strong candidate design a ticketing system.

Questions people ask