Door codes for a holiday rental company's smart locks

Your company manages 2,000 holiday homes. Each front door has a keypad lock that connects to your cloud over the home's Wi-Fi. A guest gets a 6-digit code when they book. It opens the door from 16:00 on the arrival day until 11:00 on the departure day. Cleaners and repair staff get their own codes for set times.

Today the lock sends each code typed on the keypad to your cloud, and opens if the cloud says yes. Last month your cloud was down for 3 hours on a Saturday afternoon, and 70 guests could not get in. Home Wi-Fi also drops, sometimes for a day or more.

Each lock can store 500 codes. It keeps time with its own clock, which it sets right whenever it is online. Guests book up to a year ahead, and a home has at most 80 bookings a year. When a booking is changed or cancelled, the lock must follow the change within 5 minutes if it is online.

What does a lock check when someone types a code, and how do new, changed and cancelled codes reach it?