WebSockets vs Server-Sent Events vs Polling

HTTP was designed for request-response, but many applications need the server to push updates to the browser: chat messages, stock prices, notifications. There are four approaches, each with different tradeoffs.

1. Polling (Short Polling)

// Client asks every N seconds: "Any new data?"
setInterval(async () => {
  const res = await fetch("/api/messages");
  const messages = await res.json();
  updateUI(messages);
}, 5000); // every 5 seconds

Pros: Dead simple. Works everywhere. Cons: High latency (up to 5 seconds), wasted bandwidth (many empty responses).

2. Long Polling

async function waitForMessages() {
  const res = await fetch("/api/messages");  // server holds the request open
  const messages = await res.json();
  if (messages.length > 0) updateUI(messages);
  waitForMessages(); // immediately reconnect
}

Pros: Lower latency than polling, simpler than WebSockets. Cons: Still creates a new connection per request, hard to scale, no multiplexing.

3. Server-Sent Events (SSE)

const source = new EventSource("/api/messages/stream");
source.onmessage = (event) => {
  const data = JSON.parse(event.data);
  updateUI(data);
};

Pros: Single HTTP connection, automatic reconnection, built into browsers, text-based (human readable). Cons: Server-to-client only (no client-to-server), one event stream per connection, not supported in IE.

4. WebSockets

const ws = new WebSocket("wss://example.com/ws");
ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  updateUI(data);
};
ws.onopen = () => ws.send(JSON.stringify({ type: "subscribe", channel: "chat" }));

Pros: Full-duplex (both directions), low latency, no HTTP overhead. Cons: More complex, no automatic reconnect, harder to proxy/load balance, binary protocol.

Comparison Table

When to Use Each

• Polling: When updates are infrequent and latency doesn't matter.
• Long polling: When you need low latency but can't use WebSockets (legacy browsers, restrictive proxies).
• SSE: When the server mainly pushes data (notifications, live feeds) and you want simple HTTP semantics.
• WebSockets: When you need real-time bidirectional communication (chat, multiplayer games, collaboration).