How BookMyShow auto-deallocate seats for incomplete transactions(System Design)

There has been a serious interest in terms of how movie ticketing system like BookMyShow maintains auto-deallocation of seats after 15 minutes for incomplete transaction like payment not completed.

  1. When two seats are selected and clicked Pay by a Guest user and then do nothing.
When two seats are selected and clicked Pay.

2. When another use visits the same show and sees the those two seats are not available for booking.

Another user view of the same show.

Thus, BMS manages to hold those two seats for a specific duration of time and de-allocate automatically after certain threshold, 15 minutes(say).

How to implement ?

As you would be already having a table storing some mapping between the show and the seats and maintain another table for active reservations for any booking that has been requested whether complete or incomplete.

Whenever a booking request comes, follow the sequence.

  1. Whenever a new request comes, and payment button is clicked, insert into Reservation table and also updates the ShowSeatMapper table with the allocated seats so if a new user comes and again fetches the list of seats for a particular show, it would see them as allocated already.

There is an alternate theory/design in terms of choosing a distributed cache which has inherent property of TTL and events would expire automatically triggering a callback of updating the DB. If we take example of Redis which supports https://redis.io/topics/notification but there is one caveat. Basically expired events are generated when the Redis server deletes the key and not when the time to live theoretically reaches the value of zero

3. Thinking in terms of performance overhead, we can try to keep the Reservation table as small as possible. With reference to (4) above, if the job sees the payment is successful, and whether threshold has expired or not, it would remove the row and move to the Booking table.

4. All the notifications and post processing would be consumed from the Booking table.