How a Single Postgres Exclusion Constraint Prevents Double Bookings in a Parking App

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Developer building ParkEase, a peer-to-peer parking marketplace in India, faced a classic race condition where two users could simultaneously book the same parking slot. Instead of using application-level locks, Redis mutexes, or serializable transaction retry loops, the developer solved the problem with a single PostgreSQL exclusion constraint on the booking_slots table. The constraint uses a GiST index to ensure no two confirmed or active bookings can share the same space, vehicle type, slot index, and overlapping time period. Cancelled or released bookings are excluded from the constraint via a WHERE predicate, preserving historical records while immediately freeing slots. The approach centralises the no-overlap rule entirely within the database, making it impossible for any code path to accidentally bypass it.

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