# Hands-on extension > An optional practical exercise of 45 to 60 minutes that follows a PhotoRobot Academy module. Category: Training and certification. A hands-on extension is an optional practical exercise attached to a training module. It is scheduled separately from the core module. ## How PhotoRobot uses it For example, the hands-on extension of module I02, Webhook Architecture, builds a working webhook receiver with signature verification, idempotency and asynchronous processing. ## Related terms - [Academy module](https://www.photorobot.com/glossary/academy-module): The basic unit of the PhotoRobot Academy curriculum: one topic in 30 to 90 minutes. - [Webhook](https://www.photorobot.com/glossary/webhook): An HTTP request one system sends to another automatically when an event happens. - [Idempotency](https://www.photorobot.com/glossary/idempotency): Processing the same event twice without changing the result.