Design a movie ticket booking system

Prompt and scope

Customers browse shows at a cinema, pick specific seats, and pay. Two customers must never book the same seat for the same show. Unpaid selections are released after a few minutes.

Model and invariants

Cinema -> Screen -> Seat(row, number, type)
Show(id, screen_id, movie_id, starts_at)
ShowSeat(show_id, seat_id, state: AVAILABLE | HELD | BOOKED, hold_id, hold_expires_at)
Booking(id, show_id, seats, customer_id, status, payment_id)

The key invariant is on ShowSeat: a seat is held or booked by at most one customer at a time. Enforce it in the operation that changes state, not by checking availability first and updating later.

Holding seats atomically

UPDATE show_seats
SET state = 'HELD', hold_id = :hold, hold_expires_at = :expiry
WHERE show_id = :show AND seat_id IN (:seats)
  AND (state = 'AVAILABLE' OR (state = 'HELD' AND hold_expires_at < :now))

If the number of updated rows is smaller than the number of seats requested, roll back: someone else got one of them. In an in-memory interview version, one lock per show around the same check-and-set expresses the boundary.

Walkthrough

Two customers request seat C7 at the same moment. Both send the update; the database serializes them, the first matches the row, and the second updates zero rows and is told the seat is gone. The first customer pays; on success the seats move from HELD to BOOKED with the booking id. If payment fails or the hold expires, the seats return to AVAILABLE.

Tests and follow-up

Test concurrent holds on overlapping seat sets, expiry, payment success after expiry (reject and refund), and cancellation. Follow-ups: pricing by seat type and time, a waiting queue for sold-out shows, and seat maps cached for reads while writes go to the source of truth.