Real-Time Systems

WebSockets vs SSE. Scaling stateful connections. When HTTP isn't enough.

When HTTP Isn't Enough

HTTP is pull-based: the client asks, the server responds. But some features require push:
• Live chat — new messages must appear instantly
• Collaborative editing — see others' cursors in real time
• Live notifications — order status updates, likes
• Live dashboards — real-time metrics
• Multiplayer games — game state sync
• Live sports scores — instant score updates

HTTP polling (client asks every second) is wasteful and laggy. Real-time protocols solve this properly.

WebSockets

WebSocket: a persistent, bidirectional, full-duplex TCP connection between client and server.

1. Client sends HTTP Upgrade request
2. Server agrees → connection upgrades to WebSocket
3. Now both sides can send messages anytime
4. Connection stays open until explicitly closed

// Server (Node.js with 'ws' library)
const wss = new WebSocketServer({ port: 8080 });
wss.on("connection", (ws) => {
  ws.on("message", (data) => {
    console.log("received:", data);
    ws.send("pong"); // send back
  });
  ws.send("Welcome!"); // push to client without being asked
});

Use cases: chat, collaborative tools, games, live dashboards.

Scaling challenge: WebSockets are stateful. A user connected to Server A can't receive messages from Server B. Solution: Pub/Sub with Redis — all servers subscribe to a channel, publish to all servers.

Server-Sent Events (SSE)

SSE: server pushes events to client over a regular HTTP connection. One-way (server → client only). Simpler than WebSockets.

// Server
res.setHeader("Content-Type", "text/event-stream");
res.setHeader("Cache-Control", "no-cache");
setInterval(() => {
  res.write(`data: {"price": ${getStockPrice()}}\n\n`);
}, 1000);

// Client
const es = new EventSource("/stream");
es.onmessage = (e) => console.log(JSON.parse(e.data));

Benefits: automatic reconnection built in, works over HTTP/1.1, passes through proxies easily.

Use when: live notifications, activity feeds, live search results, stock tickers.

Use WebSockets when you need bidirectional communication. Use SSE when server → client only.