Car-Link
Rental fleet and partner booking system
A rental company was updating vehicle availability in Google Sheets while its website and partner bookings used the same information. I built a system to keep those records in sync without replacing the spreadsheet.
The problem
Rental staff managed pricing, availability, and deposits in Google Sheets. Manual re-entry into the application created inventory lag, double-booking risk during peak demand, and partner disputes when a referral could not be reconstructed.
What I built
An installable Apps Script trigger records sheet changes, increments a version, and places work in a sync queue. A signed request revalidates the public catalog, while server-side partner validation records the click and resolves the WhatsApp destination.
Engineering decisions
The script lock covers only the cell mutation and queue enqueue. External network calls happen after release. Idempotency keys, requested sheet versions, and server-side cookie validation keep retries and tampered input from changing the wrong record.
What shipped
The production app, technical documentation, and a standalone operator manual shipped together. The manual runs without a dependency install so desk staff can use it away from the development environment.