The WhatsApp Problem

The Scale

WhatsApp handles tens of billions of messages per day across 2 billion users. At peak, that's well over a million messages per second.

Unlike a social feed where content is pushed to many, each message targets a specific set of recipients — and must arrive in order, exactly once.

Three Core Challenges

This system must solve three problems at once:

1. Real-time delivery — Messages should arrive in under 500 milliseconds, feeling instant to both sender and receiver.

2. Offline support — Users go offline constantly. Messages sent while they're away must be waiting when they return.

3. Massive scale — 200 million+ concurrent connections. No single server can hold them all.

Why This Is Different

Most request-response systems handle stateless traffic. A user sends a request, gets a response, done.

Messaging is different. Each user holds a persistent connection to the server. The server must push messages to clients the instant they arrive — not wait for the client to ask.

What We'll Build

Over this course, you'll design a complete messaging platform:

• Foundation — Requirements, entities, WebSocket API, architecture overview
• Message delivery — Offline inbox, media handling, ordering
• Scaling — Redis Pub/Sub, multi-device support
• Reliability — Heartbeats, lost message recovery, presence
• Putting it together — The complete end-to-end picture

Each technology is introduced when you need it to solve a specific problem.