Real-Time
real-time
System Design
WebSockets, Long Polling & SSE
Standard HTTP is a request-response protocol: the client asks, the server answers. But many modern applications need real-time, bidirectional communication - chat messages, live notifications, stock tickers, collaborative editing, and gaming. This lesson covers three techniques for real-time communication: long polling, Server-Sent Events (SSE), and WebSockets. You will learn how each works, their trade-offs, and when to use which in a system design interview.
Design a Chat System (WhatsApp)
Design a real-time chat system like WhatsApp serving 2B users sending 100B messages per day with sub-second delivery, presence indicators, and read receipts. The interview centerpiece is the persistent WebSocket connection layer: how many connections per server, how to route a message to a recipient who may be on a different server, and how to guarantee delivery when the recipient is offline. We cover the message delivery state machine (sent, delivered, read), the connection routing layer that maps user_id to a chat server, the message store for offline delivery, and presence/typing indicators that operate at a higher write rate than messages themselves.
Design Video Conferencing (Zoom)
Design a real-time video conferencing system like Zoom that supports 1-on-1 calls and meetings of up to 1000 participants with sub-200ms glass-to-glass latency, adapts to user bandwidth, and runs reliably across mobile networks. The interview centerpiece is the choice of media topology: peer-to-peer mesh (small calls), MCU mixing (centralized, expensive), or SFU forwarding (the modern standard). We cover the WebRTC stack (signaling vs media planes, ICE/STUN/TURN), simulcast and SVC for adaptive quality, recording pipelines, and how to keep latency low when participants span multiple continents.