How to design a Notification System ?

Imagine you’re building a social platform. A user signs up. Someone likes a post. Someone comments. Each of these actions should trigger a notification. Sounds simple, right? But what happens when thousands of users trigger events at the same time? Let’s design it properly. Functional Requirements When an event is triggered → send a notification. If sending fails → retry. Support multiple channels (Email, Push, In-App). Non-functional Requirements High availability Notifications should not be lost (persistence) Scalable under traffic spikes Pluggable architecture (easy to add new channels) High-Level Architecture The basic architecture looks straightforward. But many people ask: Why use a message queue instead of directly sending the request to the notification service? Let’s say we want to send a welcome email when a new user signs up. Most email service providers impose rate limits. Assume the limit is 30 requests per second. Now imagine 100 users click the sign-up button within one