Designing a Notification System

A notification system delivers real-time and scheduled alerts to users via push, email, or SMS.

Requirements

• Support push, email, SMS
• 10M notifications/day
• Delivery guarantees (at least once)
• User preferences (opt-out, quiet hours)

Architecture

Event Producers (Orders, Payments)
        ↓
   Notification Service
        ↓
     Kafka Queue
        ↓
  ┌─────┬─────┬────┐
 Push  Email  SMS

Key Components

Event Ingestion

Services publish events to Kafka topics.

Notification Service

• Reads user preferences
• Templates messages
• Routes to appropriate channel

Channel Handlers

• Push: Apple APNs, Firebase FCM
• Email: SendGrid, SES
• SMS: Twilio

Retry Logic

Send → Success → Mark delivered
Send → Fail → Retry (exp backoff)
                 ↓
          Dead Letter Queue

User Preferences

Store in Redis for fast lookup:

• Allowed channels
• Quiet hours
• Notification types

Conclusion

A robust notification system uses message queues for decoupling, retry mechanisms for reliability, and preference management for user experience.