---
title: Server
description: Use `Bun.serve` to start a high-performance HTTP server in Bun
---

## Basic Setup

```ts title="index.ts" icon="/icons/typescript.svg"
const server = Bun.serve({
  // `routes` requires Bun v1.2.3+
  routes: {
    // Static routes
    "/api/status": new Response("OK"),

    // Dynamic routes
    "/users/:id": req => {
      return new Response(`Hello User ${req.params.id}!`);
    },

    // Per-HTTP method handlers
    "/api/posts": {
      GET: () => new Response("List posts"),
      POST: async req => {
        const body = await req.json();
        return Response.json({ created: true, ...body });
      },
    },

    // Wildcard route for all routes that start with "/api/" and aren't otherwise matched
    "/api/*": Response.json({ message: "Not found" }, { status: 404 }),

    // Redirect from /blog/hello to /blog/hello/world
    "/blog/hello": Response.redirect("/blog/hello/world"),

    // Serve a file by lazily loading it into memory
    "/favicon.ico": Bun.file("./favicon.ico"),
  },

  // (optional) fallback for unmatched routes:
  // Required if Bun's version < 1.2.3
  fetch(req) {
    return new Response("Not Found", { status: 404 });
  },
});

console.log(`Server running at ${server.url}`);
```

---

## HTML imports

Bun supports importing HTML files directly into your server code, enabling full-stack applications with both server-side and client-side code. HTML imports work in two modes:

**Development (`bun --hot`):** Assets are bundled on-demand at runtime, enabling hot module replacement (HMR) for a fast, iterative development experience. When you change your frontend code, the browser automatically updates without a full page reload.

**Production (`bun build`):** When building with `bun build --target=bun`, the `import index from "./index.html"` statement resolves to a pre-built manifest object containing all bundled client assets. `Bun.serve` consumes this manifest to serve optimized assets with zero runtime bundling overhead. This is ideal for deploying to production.

```ts
import myReactSinglePageApp from "./index.html";

Bun.serve({
  routes: {
    "/": myReactSinglePageApp,
  },
});
```

HTML imports don't just serve HTML — it's a full-featured frontend bundler, transpiler, and toolkit built using Bun's [bundler](/docs/bundler), JavaScript transpiler and CSS parser. You can use this to build full-featured frontends with React, TypeScript, Tailwind CSS, and more.

For a complete guide on building full-stack applications with HTML imports, including detailed examples and best practices, see [/docs/bundler/fullstack](/docs/bundler/fullstack).

---

## Configuration

### Changing the `port` and `hostname`

To configure which port and hostname the server will listen on, set `port` and `hostname` in the options object.

```ts
Bun.serve({
  port: 8080, // defaults to $BUN_PORT, $PORT, $NODE_PORT otherwise 3000 // [!code ++]
  hostname: "mydomain.com", // defaults to "0.0.0.0" // [!code ++]
  fetch(req) {
    return new Response("404!");
  },
});
```

To randomly select an available port, set `port` to `0`.

```ts
const server = Bun.serve({
  port: 0, // random port // [!code ++]
  fetch(req) {
    return new Response("404!");
  },
});

// server.port is the randomly selected port
console.log(server.port);
```

You can view the chosen port by accessing the `port` property on the server object, or by accessing the `url` property.

```ts
console.log(server.port); // 3000
console.log(server.url); // http://localhost:3000
```

### Configuring a default port

Bun supports several options and environment variables to configure the default port. The default port is used when the `port` option is not set.

- `--port` CLI flag

```sh
bun --port=4002 server.ts
```

- `BUN_PORT` environment variable

```sh
BUN_PORT=4002 bun server.ts
```

- `PORT` environment variable

```sh terminal icon="terminal"
PORT=4002 bun server.ts
```

- `NODE_PORT` environment variable

```sh terminal icon="terminal"
NODE_PORT=4002 bun server.ts
```

---

## Unix domain sockets

To listen on a [unix domain socket](https://en.wikipedia.org/wiki/Unix_domain_socket), pass the `unix` option with the path to the socket.

```ts
Bun.serve({
  unix: "/tmp/my-socket.sock", // path to socket
  fetch(req) {
    return new Response(`404!`);
  },
});
```

### Abstract namespace sockets

Bun supports Linux abstract namespace sockets. To use an abstract namespace socket, prefix the `unix` path with a null byte.

```ts
Bun.serve({
  unix: "\0my-abstract-socket", // abstract namespace socket
  fetch(req) {
    return new Response(`404!`);
  },
});
```

Unlike unix domain sockets, abstract namespace sockets are not bound to the filesystem and are automatically removed when the last reference to the socket is closed.

---

## idleTimeout

To configure the idle timeout, set the `idleTimeout` field in Bun.serve.

```ts
Bun.serve({
  // 10 seconds:
  idleTimeout: 10,

  fetch(req) {
    return new Response("Bun!");
  },
});
```

This is the maximum amount of time a connection is allowed to be idle before the server closes it. A connection is idling if there is no data sent or received.

---

## export default syntax

Thus far, the examples on this page have used the explicit `Bun.serve` API. Bun also supports an alternate syntax.

```ts server.ts
import type { Serve } from "bun";

export default {
  fetch(req) {
    return new Response("Bun!");
  },
} satisfies Serve.Options<undefined>;
```

The type parameter `<undefined>` represents WebSocket data — if you add a `websocket` handler with custom data attached via `server.upgrade(req, { data: ... })`, replace `undefined` with your data type.

Instead of passing the server options into `Bun.serve`, `export default` it. This file can be executed as-is; when Bun sees a file with a `default` export containing a `fetch` handler, it passes it into `Bun.serve` under the hood.

---

## Hot Route Reloading

Update routes without server restarts using `server.reload()`:

```ts
const server = Bun.serve({
  routes: {
    "/api/version": () => Response.json({ version: "1.0.0" }),
  },
});

// Deploy new routes without downtime
server.reload({
  routes: {
    "/api/version": () => Response.json({ version: "2.0.0" }),
  },
});
```

---

## Server Lifecycle Methods

### `server.stop()`

To stop the server from accepting new connections:

```ts
const server = Bun.serve({
  fetch(req) {
    return new Response("Hello!");
  },
});

// Gracefully stop the server (waits for in-flight requests)
await server.stop();

// Force stop and close all active connections
await server.stop(true);
```

By default, `stop()` allows in-flight requests and WebSocket connections to complete. Pass `true` to immediately terminate all connections.

### `server.ref()` and `server.unref()`

Control whether the server keeps the Bun process alive:

```ts
// Don't keep process alive if server is the only thing running
server.unref();

// Restore default behavior - keep process alive
server.ref();
```

### `server.reload()`

Update the server's handlers without restarting:

```ts
const server = Bun.serve({
  routes: {
    "/api/version": Response.json({ version: "v1" }),
  },
  fetch(req) {
    return new Response("v1");
  },
});

// Update to new handler
server.reload({
  routes: {
    "/api/version": Response.json({ version: "v2" }),
  },
  fetch(req) {
    return new Response("v2");
  },
});
```

This is useful for development and hot reloading. Only `fetch`, `error`, and `routes` can be updated.

---

## Per-Request Controls

### `server.timeout(Request, seconds)`

Set a custom idle timeout for individual requests:

```ts
const server = Bun.serve({
  async fetch(req, server) {
    // Set 60 second timeout for this request
    server.timeout(req, 60);

    // If they take longer than 60 seconds to send the body, the request will be aborted
    await req.text();

    return new Response("Done!");
  },
});
```

Pass `0` to disable the timeout for a request.

### `server.requestIP(Request)`

Get client IP and port information:

```ts
const server = Bun.serve({
  fetch(req, server) {
    const address = server.requestIP(req);
    if (address) {
      return new Response(`Client IP: ${address.address}, Port: ${address.port}`);
    }
    return new Response("Unknown client");
  },
});
```

Returns `null` for closed requests or Unix domain sockets.

---

## Server Metrics

### `server.pendingRequests` and `server.pendingWebSockets`

Monitor server activity with built-in counters:

```ts
const server = Bun.serve({
  fetch(req, server) {
    return new Response(
      `Active requests: ${server.pendingRequests}\n` + `Active WebSockets: ${server.pendingWebSockets}`,
    );
  },
});
```

### `server.subscriberCount(topic)`

Get count of subscribers for a WebSocket topic:

```ts
const server = Bun.serve({
  fetch(req, server) {
    const chatUsers = server.subscriberCount("chat");
    return new Response(`${chatUsers} users in chat`);
  },
  websocket: {
    message(ws) {
      ws.subscribe("chat");
    },
  },
});
```

---

## Benchmarks

Below are Bun and Node.js implementations of a simple HTTP server that responds `Bun!` to each incoming `Request`.

```ts Bun
Bun.serve({
  fetch(req: Request) {
    return new Response("Bun!");
  },
  port: 3000,
});
```

```ts
require("http")
  .createServer((req, res) => res.end("Bun!"))
  .listen(8080);
```

The `Bun.serve` server can handle roughly 2.5x more requests per second than Node.js on Linux.

| Runtime | Requests per second |
| ------- | ------------------- |
| Node 16 | ~64,000             |
| Bun     | ~160,000            |

<Frame>
  ![image](https://user-images.githubusercontent.com/709451/162389032-fc302444-9d03-46be-ba87-c12bd8ce89a0.png)
</Frame>

---

## Practical example: REST API

Here's a basic database-backed REST API using Bun's router with zero dependencies:

<CodeGroup>

```ts server.ts expandable icon="file-code"
import type { Post } from "./types.ts";
import { Database } from "bun:sqlite";

const db = new Database("posts.db");
db.exec(`
  CREATE TABLE IF NOT EXISTS posts (
    id TEXT PRIMARY KEY,
    title TEXT NOT NULL,
    content TEXT NOT NULL,
    created_at TEXT NOT NULL
  )
`);

Bun.serve({
  routes: {
    // List posts
    "/api/posts": {
      GET: () => {
        const posts = db.query("SELECT * FROM posts").all();
        return Response.json(posts);
      },

      // Create post
      POST: async req => {
        const post: Omit<Post, "id" | "created_at"> = await req.json();
        const id = crypto.randomUUID();

        db.query(
          `INSERT INTO posts (id, title, content, created_at)
           VALUES (?, ?, ?, ?)`,
        ).run(id, post.title, post.content, new Date().toISOString());

        return Response.json({ id, ...post }, { status: 201 });
      },
    },

    // Get post by ID
    "/api/posts/:id": req => {
      const post = db.query("SELECT * FROM posts WHERE id = ?").get(req.params.id);

      if (!post) {
        return new Response("Not Found", { status: 404 });
      }

      return Response.json(post);
    },
  },

  error(error) {
    console.error(error);
    return new Response("Internal Server Error", { status: 500 });
  },
});
```

```ts types.ts icon="/icons/typescript.svg"
export interface Post {
  id: string;
  title: string;
  content: string;
  created_at: string;
}
```

</CodeGroup>

---

## Reference

```ts expandable See TypeScript Definitions
interface Server extends Disposable {
  /**
   * Stop the server from accepting new connections.
   * @param closeActiveConnections If true, immediately terminates all connections
   * @returns Promise that resolves when the server has stopped
   */
  stop(closeActiveConnections?: boolean): Promise<void>;

  /**
   * Update handlers without restarting the server.
   * Only fetch and error handlers can be updated.
   */
  reload(options: Serve): void;

  /**
   * Make a request to the running server.
   * Useful for testing or internal routing.
   */
  fetch(request: Request | string): Response | Promise<Response>;

  /**
   * Upgrade an HTTP request to a WebSocket connection.
   * @returns true if upgrade successful, false if failed
   */
  upgrade<T = undefined>(
    request: Request,
    options?: {
      headers?: Bun.HeadersInit;
      data?: T;
    },
  ): boolean;

  /**
   * Publish a message to all WebSocket clients subscribed to a topic.
   * @returns Bytes sent, 0 if dropped, -1 if backpressure applied
   */
  publish(
    topic: string,
    data: string | ArrayBufferView | ArrayBuffer | SharedArrayBuffer,
    compress?: boolean,
  ): ServerWebSocketSendStatus;

  /**
   * Get count of WebSocket clients subscribed to a topic.
   */
  subscriberCount(topic: string): number;

  /**
   * Get client IP address and port.
   * @returns null for closed requests or Unix sockets
   */
  requestIP(request: Request): SocketAddress | null;

  /**
   * Set custom idle timeout for a request.
   * @param seconds Timeout in seconds, 0 to disable
   */
  timeout(request: Request, seconds: number): void;

  /**
   * Keep process alive while server is running.
   */
  ref(): void;

  /**
   * Allow process to exit if server is only thing running.
   */
  unref(): void;

  /** Number of in-flight HTTP requests */
  readonly pendingRequests: number;

  /** Number of active WebSocket connections */
  readonly pendingWebSockets: number;

  /** Server URL including protocol, hostname and port */
  readonly url: URL;

  /** Port server is listening on */
  readonly port: number;

  /** Hostname server is bound to */
  readonly hostname: string;

  /** Whether server is in development mode */
  readonly development: boolean;

  /** Server instance identifier */
  readonly id: string;
}

interface WebSocketHandler<T = undefined> {
  /** Maximum WebSocket message size in bytes */
  maxPayloadLength?: number;

  /** Bytes of queued messages before applying backpressure */
  backpressureLimit?: number;

  /** Whether to close connection when backpressure limit hit */
  closeOnBackpressureLimit?: boolean;

  /** Called when backpressure is relieved */
  drain?(ws: ServerWebSocket<T>): void | Promise<void>;

  /** Seconds before idle timeout */
  idleTimeout?: number;

  /** Enable per-message deflate compression */
  perMessageDeflate?:
    | boolean
    | {
        compress?: WebSocketCompressor | boolean;
        decompress?: WebSocketCompressor | boolean;
      };

  /** Send ping frames to keep connection alive */
  sendPings?: boolean;

  /** Whether server receives its own published messages */
  publishToSelf?: boolean;

  /** Called when connection opened */
  open?(ws: ServerWebSocket<T>): void | Promise<void>;

  /** Called when message received */
  message(ws: ServerWebSocket<T>, message: string | Buffer): void | Promise<void>;

  /** Called when connection closed */
  close?(ws: ServerWebSocket<T>, code: number, reason: string): void | Promise<void>;

  /** Called when ping frame received */
  ping?(ws: ServerWebSocket<T>, data: Buffer): void | Promise<void>;

  /** Called when pong frame received */
  pong?(ws: ServerWebSocket<T>, data: Buffer): void | Promise<void>;
}

interface TLSOptions {
  /** Certificate authority chain */
  ca?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Server certificate */
  cert?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Path to DH parameters file */
  dhParamsFile?: string;

  /** Private key */
  key?: string | Buffer | BunFile | Array<string | Buffer | BunFile>;

  /** Reduce TLS memory usage */
  lowMemoryMode?: boolean;

  /** Private key passphrase */
  passphrase?: string;

  /** OpenSSL options flags */
  secureOptions?: number;

  /** Server name for SNI */
  serverName?: string;
}
```
