---
title: "Writing an HTTP Server"
description: "A guide to creating HTTP servers in Deno. Learn about the Deno.serve API, request handling, WebSocket support, response streaming, and how to build production-ready HTTP/HTTPS servers with automatic compression."
oldUrl:
  - /runtime/manual/runtime/http_server_apis/
  - /runtime/manual/examples/http_server/
  - /runtime/tutorials/http_server/
---

HTTP servers are the backbone of the web, allowing you to access websites,
download files, and interact with web services. They listen for incoming
requests from clients (like web browsers) and send back responses.

When you build your own HTTP server, you have complete control over its behavior
and can tailor it to your specific needs. You may be using it for local
development, to serve your HTML, CSS, and JS files, or building a REST API -
having your own server lets you define endpoints, handle requests and manage
data.

## Deno's built-in HTTP server

Deno has a built in HTTP server API that allows you to write HTTP servers. The
[`Deno.serve`](https://docs.deno.com/api/deno/~/Deno.serve) API supports
HTTP/1.1 and HTTP/2.

### A "Hello World" server

The `Deno.serve` function takes a handler function that will be called for each
incoming request, and is expected to return a response (or a promise resolving
to a response).

Here is an example of a server that returns a "Hello, World!" response for each
request:

```ts title="server.ts"
Deno.serve((_req) => {
  return new Response("Hello, World!");
});
```

The handler can also return a `Promise<Response>`, which means it can be an
`async` function.

To run this server, you can use the `deno run` command:

```sh
deno run --allow-net server.ts
```

There are many more examples of using `Deno.serve` in the
[Examples collection](/examples/#network).

### Listening on a specific port

By default `Deno.serve` will listen on port `8000`, but this can be changed by
passing in a port number in options bag as the first or second argument:

```js title="server.ts"
// To listen on port 4242.
Deno.serve({ port: 4242 }, handler);

// To listen on port 4242 and bind to 0.0.0.0.
Deno.serve({ port: 4242, hostname: "0.0.0.0" }, handler);
```

### Inspecting the incoming request

Most servers will not answer with the same response for every request. Instead
they will change their answer depending on various aspects of the request: the
HTTP method, the headers, the path, or the body contents.

The request is passed in as the first argument to the handler function. Here is
an example showing how to extract various parts of the request:

```ts
Deno.serve(async (req) => {
  console.log("Method:", req.method);

  const url = new URL(req.url);
  console.log("Path:", url.pathname);
  console.log("Query parameters:", url.searchParams);

  console.log("Headers:", req.headers);

  if (req.body) {
    const body = await req.text();
    console.log("Body:", body);
  }

  return new Response("Hello, World!");
});
```

:::caution

Be aware that the `req.text()` call can fail if the user hangs up the connection
before the body is fully received. Make sure to handle this case. Do note this
can happen in all methods that read from the request body, such as `req.json()`,
`req.formData()`, `req.arrayBuffer()`, `req.body.getReader().read()`,
`req.body.pipeTo()`, etc.

:::

### Responding with real data

Most servers do not respond with "Hello, World!" to every request. Instead they
might respond with different headers, status codes, and body contents (even body
streams).

Here is an example of returning a response with a 404 status code, a JSON body,
and a custom header:

```ts title="server.ts"
Deno.serve((req) => {
  const body = JSON.stringify({ message: "NOT FOUND" });
  return new Response(body, {
    status: 404,
    headers: {
      "content-type": "application/json; charset=utf-8",
    },
  });
});
```

### Responding with a stream

Response bodies can also be streams. Here is an example of a response that
returns a stream of "Hello, World!" repeated every second:

```ts title="server.ts"
Deno.serve((req) => {
  let timer: number;
  const body = new ReadableStream({
    async start(controller) {
      timer = setInterval(() => {
        controller.enqueue("Hello, World!\n");
      }, 1000);
    },
    cancel() {
      clearInterval(timer);
    },
  });
  return new Response(body.pipeThrough(new TextEncoderStream()), {
    headers: {
      "content-type": "text/plain; charset=utf-8",
    },
  });
});
```

:::note

Note the `cancel` function above. This is called when the client hangs up the
connection. It is important to make sure that you handle this case, otherwise
the server will keep queuing up messages forever, and eventually run out of
memory.

:::

Be aware that the response body stream is "cancelled" when the client hangs up
the connection. Make sure to handle this case. This can surface itself as an
error in a `write()` call on a `WritableStream` object that is attached to the
response body `ReadableStream` object (for example through a `TransformStream`).

### HTTPS support

To use HTTPS, pass two extra arguments in the options: `cert` and `key`. These
are contents of the certificate and key files, respectively.

```js
Deno.serve({
  port: 443,
  cert: Deno.readTextFileSync("./cert.pem"),
  key: Deno.readTextFileSync("./key.pem"),
}, handler);
```

:::note

To use HTTPS, you will need a valid TLS certificate and a private key for your
server.

:::

### HTTP/2 support

HTTP/2 support is "automatic" when using the HTTP server APIs with Deno. You
just need to create your server, and it will handle HTTP/1 or HTTP/2 requests
seamlessly.

HTTP/2 is also supported over cleartext with prior knowledge.

### Automatic body compression

The HTTP server has built in automatic compression of response bodies. When a
response is sent to a client, Deno determines if the response body can be safely
compressed. This compression happens within the internals of Deno, so it is fast
and efficient.

Currently Deno supports gzip and brotli compression. A body is automatically
compressed if the following conditions are true:

- The request has an
  [`Accept-Encoding`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding)
  header which indicates the requester supports `br` for Brotli or `gzip`. Deno
  will respect the preference of the
  [quality value](https://developer.mozilla.org/en-US/docs/Glossary/Quality_values)
  in the header.
- The response includes a
  [`Content-Type`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type)
  which is considered compressible. (The list is derived from
  [`jshttp/mime-db`](https://github.com/jshttp/mime-db/blob/master/db.json) with
  the actual list
  [in the code](https://github.com/denoland/deno/blob/v1.21.0/ext/http/compressible.rs).)
- The response body is greater than 64 bytes.

When the response body is compressed, Deno will set the `Content-Encoding`
header to reflect the encoding, as well as ensure the `Vary` header is adjusted
or added to indicate which request headers affected the response.

In addition to the logic above, there are a few reasons why a response **won’t**
be compressed automatically:

- The response contains a `Content-Encoding` header. This indicates your server
  has done some form of encoding already.
- The response contains a
  [`Content-Range`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Range)
  header. This indicates that your server is responding to a range request,
  where the bytes and ranges are negotiated outside of the control of the
  internals to Deno.
- The response has a
  [`Cache-Control`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control)
  header which contains a
  [`no-transform`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control#other)
  value. This indicates that your server doesn’t want Deno or any downstream
  proxies to modify the response.

### Serving WebSockets

Deno can upgrade incoming HTTP requests to a WebSocket. This allows you to
handle WebSocket endpoints on your HTTP servers.

To upgrade an incoming `Request` to a WebSocket you use the
`Deno.upgradeWebSocket` function. This returns an object consisting of a
`Response` and a web standard `WebSocket` object. The returned response should
be used to respond to the incoming request.

Because the WebSocket protocol is symmetrical, the `WebSocket` object is
identical to the one that can be used for client side communication.
Documentation for it can be found
[on MDN](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket).

```ts title="server.ts"
Deno.serve((req) => {
  if (req.headers.get("upgrade") != "websocket") {
    return new Response(null, { status: 426 });
  }

  const { socket, response } = Deno.upgradeWebSocket(req);
  socket.addEventListener("open", () => {
    console.log("a client connected!");
  });

  socket.addEventListener("message", (event) => {
    if (event.data === "ping") {
      socket.send("pong");
    }
  });

  return response;
});
```

The connection the WebSocket was created on can not be used for HTTP traffic
after a WebSocket upgrade has been performed.

:::note

Note that WebSockets are only supported on HTTP/1.1 for now.

:::

## Default fetch export

Another way to create an HTTP server in Deno is by exporting a default `fetch`
function. [The fetch API](/api/web/~/fetch) initiates an HTTP request to
retrieve data from across a network and is built into the Deno runtime.

```ts title="server.ts"
export default {
  fetch(request) {
    const userAgent = request.headers.get("user-agent") || "Unknown";
    return new Response(`User Agent: ${userAgent}`);
  },
} satisfies Deno.ServeDefaultExport;
```

You can run this file with the `deno serve` command:

```sh
deno serve server.ts
```

The server will start and display a message in the console. Open your browser
and navigate to [http://localhost:8000/](http://localhost:8000/) to see the
user-agent information.

The
[`Deno.ServeDefaultExport`](https://docs.deno.com/api/deno/~/Deno.ServeDefaultExport)
interface defines the structure for default exports that can be used with the
`deno serve` command. To ensure your code is type-checked properly, make sure to
add `satisfies Deno.ServeDefaultExport` to the `export default { ... }`.

## Building on these examples

You will likely want to expand on these examples to create more complex servers.
Deno recommends using [Oak](https://jsr.io/@oak/oak) for building web servers.
Oak is a middleware framework for Deno's HTTP server, designed to be expressive
and easy to use. It provides a simple way to create web servers with middleware
support. Check out the [Oak documentation](https://oakserver.github.io/oak/) for
examples of how to define routes.