1. System Definition

A notification system is a backend service that delivers timely information to users across various channels (email, SMS, push, in-app). Its purpose is to inform or alert users about events relevant to them, such as social updates, order statuses, reminders, etc. For example, after an e-commerce purchase, the system might send an order confirmation email, an SMS for payment confirmation, and a push notification when the package ships. In a social network, it would notify you (via in-app alert or push) when someone likes your post