Movie ticket booking system

Establish the contract

Ask whether seats are reserved, whether users choose exact seats, hold duration, payment provider, expected peak traffic, and whether venues operate across time zones. Model immutable show metadata separately from per-show seat inventory. A booking flow should create a short-lived seat hold, collect payment, then atomically confirm the booking; unsuccessful or expired holds release inventory.

The critical invariant is that a seat for one show has at most one active hold or confirmed booking. Enforce it with a unique (showId, seatId) constraint plus a transaction or compare-and-set update, not only an application-side availability check. Make confirmation idempotent using a client request key, publish notifications through an outbox after commit, and run an expiry worker for holds. Discuss a read cache for seat maps but make the authoritative write path transactional. Test double clicks, payment callbacks arriving twice, expiry during payment, and partial failures. Complexity is dominated by the number of selected seats, typically O(k) transactional writes for k seats.