> ## Documentation Index
> Fetch the complete documentation index at: https://bun.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Build a publish-subscribe WebSocket server

Bun's server-side `WebSocket` API provides a native pub-sub API. Sockets can be subscribed to a set of named channels using `socket.subscribe(<name>)`; messages can be published to a channel using `socket.publish(<name>, <message>)`.

This code snippet implements a single-channel chat server.

```ts server.ts icon="https://mintcdn.com/bun-1dd33a4e/JUhaF6Mf68z_zHyy/icons/typescript.svg?fit=max&auto=format&n=JUhaF6Mf68z_zHyy&q=85&s=7ac549adaea8d5487d8fbd58cc3ea35b" theme={"theme":{"light":"github-light","dark":"dracula"}}
const server = Bun.serve({
  fetch(req, server) {
    const cookies = req.headers.get("cookie");
    const username = getUsernameFromCookies(cookies);
    const success = server.upgrade(req, { data: { username } });
    if (success) return undefined;

    return new Response("Hello world");
  },
  websocket: {
    // TypeScript: specify the type of ws.data like this
    data: {} as { username: string },

    open(ws) {
      const msg = `${ws.data.username} has entered the chat`;
      ws.subscribe("the-group-chat");
      server.publish("the-group-chat", msg);
    },
    message(ws, message) {
      // the server re-broadcasts incoming messages to everyone
      server.publish("the-group-chat", `${ws.data.username}: ${message}`);
    },
    close(ws) {
      const msg = `${ws.data.username} has left the chat`;
      server.publish("the-group-chat", msg);
      ws.unsubscribe("the-group-chat");
    },
  },
});

console.log(`Listening on ${server.hostname}:${server.port}`);
```