WebSockets vs Server-Sent Events vs Long Polling: Choosing Real-Time for Your Stack

Real-time features define modern apps — live dashboards, chat, notifications. Three approaches exist, each with different tradeoffs. Here's when to use each.

Long Polling

Client sends request, server holds it open until data arrives:

// Server
app.get('/poll', async (req, res) => {
  const timeout = setTimeout(() => res.json({ data: null }), 30000);
  const unsubscribe = eventEmitter.once('update', (data) => {
    clearTimeout(timeout);
    res.json({ data });
  });
});

// Client
async function longPoll() {
  while (true) {
    const { data } = await fetch('/poll').then(r => r.json());
    if (data) handleUpdate(data);
  }
}

Use when: Simplest implementation matters, infrequent updates, no WebSocket support.

Avoid when: High-frequency updates — each request creates overhead.

Server-Sent Events (SSE)

One-way stream from server to client over a persistent HTTP connection:

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

  const sendEvent = (data: object) => {
    res.write(`data: ${JSON.stringify(data)}\n\n`);
  };

  const interval = setInterval(() => {
    sendEvent({ timestamp: Date.now(), value: Math.random() });
  }, 1000);

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

// Client
const source = new EventSource('/events');
source.onmessage = (event) => {
  const data = JSON.parse(event.data);
  updateDashboard(data);
};

Use when: Server pushes data only (dashboards, notifications, progress bars), need automatic reconnection.

Avoid when: You need client-to-server messages — SSE is one-directional.

WebSockets

Full-duplex channel — both sides can send at any time:

import { WebSocketServer } from 'ws';

const wss = new WebSocketServer({ port: 8080 });

wss.on('connection', (ws) => {
  ws.on('message', (message) => {
    const data = JSON.parse(message.toString());

    // Broadcast to all clients
    wss.clients.forEach(client => {
      if (client.readyState === WebSocket.OPEN) {
        client.send(JSON.stringify({ type: 'broadcast', data }));
      }
    });
  });
});

// Client
const ws = new WebSocket('ws://localhost:8080');
ws.onopen = () => ws.send(JSON.stringify({ type: 'join', room: 'general' }));
ws.onmessage = (event) => handleMessage(JSON.parse(event.data));

Use when: Chat, collaborative editing, gaming, anything bidirectional.

Avoid when: You only need server-to-client push — SSE is simpler.

Decision Matrix

Scenario	Best Choice
Analytics dashboard	SSE
Chat application	WebSocket
Progress notifications	SSE
Multiplayer game	WebSocket
Simple notifications	Long polling
Collaborative editor	WebSocket

Scaling WebSockets

WebSockets require sticky sessions or a pub/sub layer:

import { createClient } from 'redis';
const redis = createClient();

// When message received on WS, publish to Redis
ws.on('message', async (msg) => {
  await redis.publish('chat', msg.toString());
});

// Subscribe to Redis, broadcast to local WS clients
const sub = redis.duplicate();
await sub.subscribe('chat', (message) => {
  wss.clients.forEach(client => {
    if (client.readyState === WebSocket.OPEN) client.send(message);
  });
});

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