Amazon Locker is a self-service pickup system: instead of leaving a package at your door, a courier drops it into a secure locker compartment at a nearby pickup location, and you collect it yourself with a one-time code. When the courier deposits a package, the system picks a free compartment that fits the package size and issues a pickup code that is valid for a few days. You go to the locker, enter the code, the right compartment pops open, and you take your package. The code stops working after that, the compartment is free for the next package, and anything you never pick up eventually goes back to the courier.

The software model does not need to represent door latches, barcode scanners, SMS (Short Message Service) gateways, or payments. It needs to assign compartments, track occupancy, issue and redeem one-time codes exactly once, and avoid check-then-act bugs when a compartment is assigned or freed.