`Bun.serve()` supports server-side WebSockets, with on-the-fly compression, TLS support, and a Bun-native publish-subscribe API.

{% callout %}

**⚡️ 7x more throughput** — Bun's WebSockets are fast. For a [simple chatroom](https://github.com/oven-sh/bun/tree/main/bench/websocket-server/README.md) on Linux x64, Bun can handle 7x more requests per second than Node.js + [`"ws"`](https://github.com/websockets/ws).

| Messages sent per second | Runtime                        | Clients |
| ------------------------ | ------------------------------ | ------- |
| ~700,000                 | (`Bun.serve`) Bun v0.2.1 (x64) | 16      |
| ~100,000                 | (`ws`) Node v18.10.0 (x64)     | 16      |

Internally Bun's WebSocket implementation is built on [uWebSockets](https://github.com/uNetworking/uWebSockets).
{% /callout %}

## Start a WebSocket server

Below is a simple WebSocket server built with `Bun.serve`, in which all incoming requests are [upgraded](https://developer.mozilla.org/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism) to WebSocket connections in the `fetch` handler. The socket handlers are declared in the `websocket` parameter.

```ts
Bun.serve({
  fetch(req, server) {
    // upgrade the request to a WebSocket
    if (server.upgrade(req)) {
      return; // do not return a Response
    }
    return new Response("Upgrade failed :(", { status: 500 });
  },
  websocket: {}, // handlers
});
```

The following WebSocket event handlers are supported:

```ts
Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    message(ws, message) {}, // a message is received
    open(ws) {}, // a socket is opened
    close(ws, code, message) {}, // a socket is closed
    drain(ws) {}, // the socket is ready to receive more data
  },
});
```

{% details summary="An API designed for speed" %}

In Bun, handlers are declared once per server, instead of per socket.

`ServerWebSocket` expects you to pass a `WebSocketHandler` object to the `Bun.serve()` method which has methods for `open`, `message`, `close`, `drain`, and `error`. This is different than the client-side `WebSocket` class which extends `EventTarget` (onmessage, onopen, onclose),

Clients tend to not have many socket connections open so an event-based API makes sense.

But servers tend to have **many** socket connections open, which means:

- Time spent adding/removing event listeners for each connection adds up
- Extra memory spent on storing references to callbacks function for each connection
- Usually, people create new functions for each connection, which also means more memory

So, instead of using an event-based API, `ServerWebSocket` expects you to pass a single object with methods for each event in `Bun.serve()` and it is reused for each connection.

This leads to less memory usage and less time spent adding/removing event listeners.
{% /details %}

The first argument to each handler is the instance of `ServerWebSocket` handling the event. The `ServerWebSocket` class is a fast, Bun-native implementation of [`WebSocket`](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket) with some additional features.

```ts
Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    message(ws, message) {
      ws.send(message); // echo back the message
    },
  },
});
```

### Sending messages

Each `ServerWebSocket` instance has a `.send()` method for sending messages to the client. It supports a range of input types.

```ts
ws.send("Hello world"); // string
ws.send(response.arrayBuffer()); // ArrayBuffer
ws.send(new Uint8Array([1, 2, 3])); // TypedArray | DataView
```

### Headers

Once the upgrade succeeds, Bun will send a `101 Switching Protocols` response per the [spec](https://developer.mozilla.org/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism). Additional `headers` can be attched to this `Response` in the call to `server.upgrade()`.

```ts
Bun.serve({
  fetch(req, server) {
    const sessionId = await generateSessionId();
    server.upgrade(req, {
      headers: {
        "Set-Cookie": `SessionId=${sessionId}`,
      },
    });
  },
  websocket: {}, // handlers
});
```

### Contextual data

Contextual `data` can be attached to a new WebSocket in the `.upgrade()` call. This data is made available on the `ws.data` property inside the WebSocket handlers.

```ts
type WebSocketData = {
  createdAt: number;
  channelId: string;
  authToken: string;
};

// TypeScript: specify the type of `data`
Bun.serve<WebSocketData>({
  fetch(req, server) {
    // use a library to parse cookies
    const cookies = parseCookies(req.headers.get("Cookie"));
    server.upgrade(req, {
      // this object must conform to WebSocketData
      data: {
        createdAt: Date.now(),
        channelId: new URL(req.url).searchParams.get("channelId"),
        authToken: cookies["X-Token"],
      },
    });

    return undefined;
  },
  websocket: {
    // handler called when a message is received
    async message(ws, message) {
      const user = getUserFromToken(ws.data.authToken);

      await saveMessageToDatabase({
        channel: ws.data.channelId,
        message: String(message),
        userId: user.id,
      });
    },
  },
});
```

To connect to this server from the browser, create a new `WebSocket`.

```ts#browser.js
const socket = new WebSocket("ws://localhost:3000/chat");

socket.addEventListener("message", event => {
  console.log(event.data);
})
```

{% callout %}
**Identifying users** — The cookies that are currently set on the page will be sent with the WebSocket upgrade request and available on `req.headers` in the `fetch` handler. Parse these cookies to determine the identity of the connecting user and set the value of `data` accordingly.
{% /callout %}

### Pub/Sub

Bun's `ServerWebSocket` implementation implements a native publish-subscribe API for topic-based broadcasting. Individual sockets can `.subscribe()` to a topic (specified with a string identifier) and `.publish()` messages to all other subscribers to that topic. This topic-based broadcast API is similar to [MQTT](https://en.wikipedia.org/wiki/MQTT) and [Redis Pub/Sub](https://redis.io/topics/pubsub).

```ts
const server = Bun.serve<{ username: string }>({
  fetch(req, server) {
    const url = new URL(req.url);
    if (url.pathname === "/chat") {
      console.log(`upgrade!`);
      const username = getUsernameFromReq(req);
      const success = server.upgrade(req, { data: { username } });
      return success
        ? undefined
        : new Response("WebSocket upgrade error", { status: 400 });
    }

    return new Response("Hello world");
  },
  websocket: {
    open(ws) {
      const msg = `${ws.data.username} has entered the chat`;
      ws.subscribe("the-group-chat");
      ws.publish("the-group-chat", msg);
    },
    message(ws, message) {
      // this is a group chat
      // so the server re-broadcasts incoming message to everyone
      ws.publish("the-group-chat", `${ws.data.username}: ${message}`);
    },
    close(ws) {
      const msg = `${ws.data.username} has left the chat`;
      ws.unsubscribe("the-group-chat");
      ws.publish("the-group-chat", msg);
    },
  },
});

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

Calling `.publish(data)` will send the message to all subscribers of a topic _except_ the socket that called `.publish()`.

### Compression

Per-message [compression](https://websockets.readthedocs.io/en/stable/topics/compression.html) can be enabled with the `perMessageDeflate` parameter.

```ts
Bun.serve({
  fetch(req, server) {}, // upgrade logic
  websocket: {
    // enable compression and decompression
    perMessageDeflate: true,
  },
});
```

Compression can be enabled for individual messages by passing a `boolean` as the second argument to `.send()`.

```ts
ws.send("Hello world", true);
```

For fine-grained control over compression characteristics, refer to the [Reference](#reference).

### Backpressure

The `.send(message)` method of `ServerWebSocket` returns a `number` indicating the result of the operation.

- `-1` — The message was enqueued but there is backpressure
- `0` — The message was dropped due to a connection issue
- `1+` — The number of bytes sent

This gives you better control over backpressure in your server.

## Connect to a `Websocket` server

To connect to an external socket server, either from a browser or from Bun, create an instance of `WebSocket` with the constructor.

```ts
const socket = new WebSocket("ws://localhost:3000");
```

In browsers, the cookies that are currently set on the page will be sent with the WebSocket upgrade request. This is a standard feature of the `WebSocket` API.

For convenience, Bun lets you setting custom headers directly in the constructor. This is a Bun-specific extension of the `WebSocket` standard. _This will not work in browsers._

```ts
const socket = new WebSocket("ws://localhost:3000", {
  headers: {
    // custom headers
  },
});
```

To add event listeners to the socket:

```ts
// message is received
socket.addEventListener("message", event => {});

// socket opened
socket.addEventListener("open", event => {});

// socket closed
socket.addEventListener("close", event => {});

// error handler
socket.addEventListener("error", event => {});
```

## Reference

```ts
namespace Bun {
  export function serve(params: {
    fetch: (req: Request, server: Server) => Response | Promise<Response>;
    websocket?: {
      message: (
        ws: ServerWebSocket,
        message: string | ArrayBuffer | Uint8Array,
      ) => void;
      open?: (ws: ServerWebSocket) => void;
      close?: (ws: ServerWebSocket) => void;
      error?: (ws: ServerWebSocket, error: Error) => void;
      drain?: (ws: ServerWebSocket) => void;
      perMessageDeflate?:
        | boolean
        | {
            compress?: boolean | Compressor;
            decompress?: boolean | Compressor;
          };
    };
  }): Server;
}

type Compressor =
  | `"disable"`
  | `"shared"`
  | `"dedicated"`
  | `"3KB"`
  | `"4KB"`
  | `"8KB"`
  | `"16KB"`
  | `"32KB"`
  | `"64KB"`
  | `"128KB"`
  | `"256KB"`;

interface Server {
  pendingWebsockets: number;
  publish(
    topic: string,
    data: string | ArrayBufferView | ArrayBuffer,
    compress?: boolean,
  ): number;
  upgrade(
    req: Request,
    options?: {
      headers?: HeadersInit;
      data?: any;
    },
  ): boolean;
}

interface ServerWebSocket {
  readonly data: any;
  readonly readyState: number;
  readonly remoteAddress: string;
  send(message: string | ArrayBuffer | Uint8Array, compress?: boolean): number;
  close(code?: number, reason?: string): void;
  subscribe(topic: string): void;
  unsubscribe(topic: string): void;
  publish(topic: string, message: string | ArrayBuffer | Uint8Array): void;
  isSubscribed(topic: string): boolean;
  cork(cb: (ws: ServerWebSocket) => void): void;
}
```

<!--
### `Bun.serve(params)`

{% param name="params" %}
Configuration object for WebSocket server
{% /param %}

{% param name=" fetch" %}
`(req: Request, server: Server) => Response | Promise<Response>`

Call `server.upgrade(req)` to upgrade the request to a WebSocket connection. This method returns `true` if the upgrade succeeds, or `false` if the upgrade fails.
{% /param %}

{% param name=" websocket" %}
Configuration object for WebSocket server
{% /param %}

{% param name="  message" %}
`(ws: ServerWebSocket, message: string | ArrayBuffer | Uint8Array) => void`

This handler is called when a `WebSocket` receives a message.
{% /param %}

{% param name="  open" %}
`(ws: ServerWebSocket) => void`

This handler is called when a `WebSocket` is opened.
{% /param %}

{% param name="  close" %}
`(ws: ServerWebSocket, code: number, message: string) => void`

This handler is called when a `WebSocket` is closed.
{% /param %}

{% param name="  drain" %}
`(ws: ServerWebSocket) => void`

This handler is called when a `WebSocket` is ready to receive more data.
{% /param %}

{% param name="  perMessageDeflate" %}
`boolean | {\n  compress?: boolean | Compressor;\n  decompress?: boolean | Compressor \n}`

Enable per-message compression and decompression. This is a boolean value or an object with `compress` and `decompress` properties. Each property can be a boolean value or one of the following `Compressor` types:

- `"disable"`
- `"shared"`
- `"dedicated"`
- `"3KB"`
- `"4KB"`
- `"8KB"`
- `"16KB"`
- `"32KB"`
- `"64KB"`
- `"128KB"`
- `"256KB"`

{% /param %}

### `ServerWebSocket`

{% param name="readyState" %}
`number`

The current state of the `WebSocket` connection. This is one of the following values:

- `0` `CONNECTING`
- `1` `OPEN`
- `2` `CLOSING`
- `3` `CLOSED`

{% /param %}

{% param name="remoteAddress" %}

`string`

The remote address of the `WebSocket` connection
{% /param %}

{% param name="data" %}
The data associated with the `WebSocket` connection. This is set in the `server.upgrade()` call.
{% /param %}

{% param name=".send()" %}
`send(message: string | ArrayBuffer | Uint8Array, compress?: boolean): number`

Send a message to the client. Returns a `number` indicating the result of the operation.

- `-1`: the message was enqueued but there is backpressure
- `0`: the message was dropped due to a connection issue
- `1+`: the number of bytes sent

The `compress` argument will enable compression for this message, even if the `perMessageDeflate` option is disabled.
{% /param %}

{% param name=".subscribe()" %}
`subscribe(topic: string): void`

Subscribe to a topic
{% /param %}

{% param name=".unsubscribe()" %}
`unsubscribe(topic: string): void`

Unsubscribe from a topic
{% /param %}

{% param name=".publish()" %}
`publish(topic: string, data: string | ArrayBufferView | ArrayBuffer, compress?: boolean): number;`

Send a message to all subscribers of a topic
{% /param %}

{% param name=".isSubscribed()" %}
`isSubscribed(topic: string): boolean`

Check if the `WebSocket` is subscribed to a topic
{% /param %}
{% param name=".cork()" %}
`cork(cb: (ws: ServerWebSocket) => void): void;`

Batch a set of operations on a `WebSocket` connection. The `message`, `open`, and `drain` callbacks are automatically corked, so
you only need to call this if you are sending messages outside of those
callbacks or in async functions.

```ts
ws.cork((ws) => {
  ws.send("first");
  ws.send("second");
  ws.send("third");
});
```

{% /param %}

{% param name=".close()" %}
`close(code?: number, message?: string): void`

Close the `WebSocket` connection
{% /param %}

### `Server`

{% param name="pendingWebsockets" %}
Number of in-flight `WebSocket` messages
{% /param %}

{% param name=".publish()" %}
`publish(topic: string, data: string | ArrayBufferView | ArrayBuffer, compress?: boolean): number;`

Send a message to all subscribers of a topic
{% /param %}

{% param name=".upgrade()" %}
`upgrade(req: Request): boolean`

Upgrade a request to a `WebSocket` connection. Returns `true` if the upgrade succeeds, or `false` if the upgrade fails.
{% /param %} -->