Scaling Push Notifications to Billions

Issue #109: System Design Interview Roadmap • Section 4: Scalability

When 3 Billion Devices Expect Instant Delivery

WhatsApp delivers 100 billion messages daily to 2+ billion users. Instagram pushes likes and comments to 2 billion accounts in real-time. Behind these seamless experiences lies one of distributed systems' most challenging problems: reliably delivering billions of push notifications with sub-second latency while maintaining 99.99% uptime.

The moment your notification system grows beyond a few thousand users, traditional approaches collapse under their own weight. Connection management becomes a memory nightmare, message fan-out creates thundering herds, and delivery guarantees turn into distributed systems puzzles that even senior engineers struggle to solve.

What You'll Master Today

• Fan-out Strategies: Pull vs Push models and their breaking points at hyperscale

• Connection Management: How to maintain millions of WebSocket connections efficiently

• Delivery Guarantees: At-least-once, exactly-once, and the trade-offs that matter

• Enterprise Patterns: Real architectures from Meta, Apple, and Google's FCM

The Architecture That Scales to Billions