Design Ticket Booking System
High Priorityhard20 min readUpdated June 5, 2026
Question
Design a thread-safe ticket booking system that prevents double-booking under high concurrency.
At first glance, the requirement sounds simple: never sell the same seat twice. But once thousands of users compete for a limited set of seats, the problem becomes a real concurrency challenge.
This scenario shows up in many real-world systems: movie and event ticketing, train and flight seat reservations, stadium bookings, and even limited-inventory flash sales where “items” behave like seats.
Core Requirements
- Temporary holds: A user should be able to hold seats while entering payment details.
- Hold expiration: If payment is not completed within a time window, the hold must expire and the seats should become available again.
- Group booking: Multiple users may want to reserve adjacent seats together as a single atomic action, either the whole block is held or nothing is.
- Race conditions at the millisecond level: If two users click “Book Now” for the same seat at the same moment, exactly one booking must succeed. The other request must fail cleanly, not produce a second confirmation email or leave the system in an inconsistent state.
The system must guarantee correctness first, even under extreme concurrency, while still providing a smooth user experience for holds, expiry, and group reservations.