Design a Notification System

Design a system that sends push, email, and SMS notifications at scale. The hard problems are fanout to millions of recipients, at-least-once delivery, deduplication, and user preference enforcement.

What you will learn

• Design a multi-channel notification pipeline (push, email, SMS, in-app)
• Guarantee at-least-once delivery without sending duplicates to users
• Handle fanout to millions of recipients without blocking the triggering event
• Enforce user notification preferences and quiet hours

Every app sends notifications. A like on Instagram, a payment confirmation from your bank, a promotional offer from an e-commerce platform. Each event may trigger a push notification, an email, an SMS, or an in-app alert — or all four, depending on user preferences. The system that handles this is harder to build correctly than it appears: fanout, delivery guarantees, deduplication, throttling, and quiet hours all have to work together.

Functional requirements:

• Send notifications via push (iOS/Android), email, SMS, and in-app
• Triggered by internal events (new message, payment) or scheduled campaigns
• Respect user notification preferences (per-channel opt-out, quiet hours)
• Track delivery status: sent, delivered, failed
• Support batched marketing campaigns (millions of recipients)

Non-functional requirements:

• At-least-once delivery — never silently drop a notification
• No duplicates — same event should not notify the user twice
• Latency for transactional notifications (payment confirmed): < 10 seconds
• Latency for marketing campaigns: eventual (within minutes/hours is acceptable)
• High availability — notification service outage = users miss critical alerts

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