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Booking System: Availability, Calendars, and Overbooking Rules
Designing reliable booking systems with correct time modeling, concurrency control, and real-world production constraints
Why Booking Systems Fail in Ways That Are Hard to Notice
Imagine you build a booking system. Every unit test passes. The staging environment looks correct. You deploy.
Three weeks later, a hotel guest arrives and finds their room already occupied. A meeting room is double-booked for the same hour. Two doctors are scheduled for the same operating theater at the same time.
Nothing crashed. No exceptions were logged. The system appeared to work — until it didn’t.
This is what makes booking systems genuinely hard. The failures are not loud. They are timing failures: two things happening at almost the same moment, both individually correct, together catastrophic. The code is right. The concurrency model is wrong.
This article covers the full picture: the mental model, the time overlap logic, the concurrency problem, and the PHP implementation that actually prevents overbooking — not just in unit tests, but under real production load.
