WebSockets vs Server-Sent Events for a notification feature

Question

I want to push notifications to users in real time (new answer, new message). Should I use WebSockets or SSE?

Accepted Answer

For **server → client** notifications, **SSE** is usually the simpler and better choice:

- It's plain HTTP: it works through proxies and CDNs, and cookie auth works as-is.
- The browser's `EventSource` **reconnects automatically**, with `Last-Event-ID` resume.
- It's one-way, which is exactly what notifications are. The client sends actions through normal `POST` requests.

```js
app.get('/events', (req, res) => {
  res.set({ 'Content-Type': 'text/event-stream', 'Cache-Control': 'no-cache', Connection: 'keep-alive' });
  const send = (evt) => res.write(`id: ${evt.id}
data: ${JSON.stringify(evt)}

`);
  const unsub = bus.subscribe(req.user.id, send);
  req.on('close', unsub);
});
```

Pick **WebSockets** when you need frequent **bidirectional**, low-latency traffic: multiplayer games, collaborative editing, live cursors.

With either one and multiple server instances, use a pub/sub layer (Redis, Postgres `LISTEN/NOTIFY`) so an event reaches the instance holding the user's connection.

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