How to Build a Booking App

Model availability as rules, not slots

The naive approach generates a row per bookable slot. It works until someone changes their working hours, adds a holiday, or you need to show a year ahead — then you're regenerating enormous tables.

Model availability as rules plus exceptions: recurring windows ("Tuesdays 9-5"), overrides ("closed 24 December"), and existing bookings subtracted at query time. Compute free slots on read for the window being displayed.

This makes rule changes instant and keeps the data small.

Double-booking is a concurrency problem

Two users loading the same slot and both clicking book is the normal case at any volume, not an edge case. Checking availability then inserting is a race — the check passes for both.

Fix it in the database, not in application logic:

A unique constraint on (resource, time range), which Postgres exclusion constraints express directly
Or a transaction with row-level locking on the resource while booking

Application-level checks will let doubles through under load, and the resulting customer conversations are the worst kind.

Timezones will hurt you

The rules that prevent most bugs:

Store UTC, always. Convert at display time only.
Store the provider's timezone as a property, because "9am" means their 9am.
Recurring events store the local time and zone, not a UTC offset — otherwise daylight-saving shifts move every future occurrence by an hour.
Show both timezones when participants differ. Ambiguity here causes no-shows.

Scope for a booking MVP

Availability rules with exceptions
Booking with a database-level concurrency guarantee
Confirmation and reminder notifications — reminders reduce no-shows more than anything else you can build
Cancellation and reschedule with a policy window
Calendar sync if your users live in Google Calendar or Outlook, which most do

Cancellation policy is product, not settings

Decide before building: how late can someone cancel, is there a fee, what happens to a payment, and can the provider cancel? These rules touch payments, notifications, and availability simultaneously, so retrofitting them means touching all three.

Timeline

Two to three weeks for a scoped booking MVP with rule-based availability, safe concurrent booking, notifications, and calendar sync.

Specific builds

Deeper breakdowns of products in this category:

FAQ

How do you prevent double-bookings?

Enforce it in the database, not in application code. A unique or exclusion constraint on the resource and time range, or a transaction with row-level locking on the resource, makes concurrent booking safe. Check-then-insert in application logic is a race condition that will let doubles through under load.

How should availability be stored?

As rules plus exceptions — recurring windows, overrides for holidays, and existing bookings subtracted at query time — rather than as pre-generated slot rows. Rule changes then apply instantly and the data stays small, where slot generation forces expensive regeneration whenever hours change.

What's the most common timezone bug in booking apps?

Storing recurring events as a UTC offset instead of a local time plus zone. When daylight saving shifts, every future occurrence moves by an hour. Store UTC for one-off instants, store local time and timezone for recurrence, and display both parties' zones when they differ.