How WebSockets Power the Real-Time Web

Why does a chat message arrive instantly, but the web was never built for it? In this deep dive we unpack WebSockets: how engineers turned a request/response web into an open, two-way line, and what that costs at scale.You'll learn:- Why short polling, long polling and Server-Sent Events fall short (and the math behind 200,000 empty requests per second)- How the WebSocket handshake hides inside a normal HTTP request, and the "cryptographic high-five" that proves the server understood- Why clients mask every frame but servers never do, and the cache-poisoning attack it prevents- Half-open connections, idle timeouts and why heartbeats every 20–30 seconds keep sockets alive- Close code 1006, the "ghost code" every system design candidate should know- The real cost of state: 1 million connections × 20 KB = 20 GB of RAM before a single message- Gateways, pub/sub buses, the thundering herd, and exponential backoff with full jitter- Cross-site WebSocket hijacking, and why tokens never belong in the URL- When NOT to use WebSockets: SSE vs WebSockets vs gRPC- Head-of-line blocking and why QUIC and WebTransport may be nextPerfect for software engineers preparing for system design interviews.Chapters00:00 Intro: the web wasn't built for real time02:19 Short polling and the 200,000 requests/second problem03:54 Long polling04:42 Server-Sent Events05:31 WebSockets and full-duplex communication06:19 The handshake: disguised as HTTP07:55 Sec-WebSocket-Accept: the cryptographic high-five09:05 Why clients mask frames09:53 Cache poisoning explained11:50 Half-open connections12:38 Idle timeouts at every network hop13:47 Heartbeats: Ping and Pong14:36 Close code 100615:23 The cost of state16:57 Scaling with gateways and pub/sub18:34 The thundering herd19:46 Exponential backoff with full jitter20:58 Cross-site WebSocket hijacking22:32 Ticket-based authentication23:45 When not to use WebSockets24:55 The big trade-off: latency vs statefulness25:43 Head-of-line blocking26:55 What's next: QUIC and WebTransport#SystemDesign #WebSockets #SoftwareEngineering #InterviewPrep #BackendEngineeringInterview Prep Podcast