Server-Sent Events (SSE) for One-Way Real-Time Push

Developers reach for WebSockets when they only need server-to-client push (notifications, live feeds, progress), adding bidirectional complexity they don't need.

SSE uses plain HTTP, auto-reconnects, and works through proxies. The server sends text/event-stream and the client uses the EventSource API.

// Server (Node.js/Express)
app.get('/events', (req, res) => {
  res.setHeader('Content-Type', 'text/event-stream');
  res.setHeader('Cache-Control', 'no-cache');
  res.setHeader('Connection', 'keep-alive');

  const send = (data) => res.write(`data: ${JSON.stringify(data)}\n\n`);
  const interval = setInterval(() => send({ time: Date.now() }), 1000);

  req.on('close', () => clearInterval(interval));
});

// Client
const es = new EventSource('/events');
es.onmessage = (e) => console.log(JSON.parse(e.data));
es.onerror = (e) => console.error('SSE error', e);

SSE is simpler than WebSockets for push-only use cases: native reconnection, standard HTTP (works with load balancers and auth headers), and browser support goes back to IE polyfills.