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HTTP Server APIs

Deno currently has three HTTP Server APIs:

serve from std/http

A "Hello World" server

To start a HTTP server on a given port, you can use the serve function from std/http. This 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 handler function that returns a "Hello, World!" response for each request:

function handler(req: Request): Response {
  return new Response("Hello, World!");
}

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

To then listen on a port and handle requests you need to call the serve function from the https://deno.land/std@0.167.0/http/server.ts module, passing in the handler as the first argument:

import { serve } from "https://deno.land/std@0.167.0/http/server.ts";

serve(handler);

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

import { serve } from "https://deno.land/std@0.167.0/http/server.ts";

// To listen on port 4242.
serve(handler, { port: 4242 });

This same options bag can also be used to configure some other options, such as the hostname to listen on.

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:

async function handler(req: Request): Promise<Response> {
  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!");
}

⚠️ 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 a response

Most servers also 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:

function handler(req: Request): Response {
  const body = JSON.stringify({ message: "NOT FOUND" });
  return new Response(body, {
    status: 404,
    headers: {
      "content-type": "application/json; charset=utf-8",
    },
  });
}

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

function handler(req: Request): Response {
  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",
    },
  });
}

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

⚠️ Beware 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

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

To use HTTPS, use serveTls from the https://deno.land/std@0.167.0/http/server.ts module instead of serve. This takes two extra arguments in the options bag: certFile and keyFile. These are paths to the certificate and key files, respectively.

import { serveTls } from "https://deno.land/std@0.167.0/http/server.ts";

serveTls(handler, {
  port: 443,
  certFile: "./cert.pem",
  keyFile: "./key.pem",
});

HTTP/2 support

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

Deno.serveHttp

We generally recommnend that you use the serve API described above, as it handles all of the intricacies of parallel requests on a single connection, error handling, and so on. However, if you are interested creating your own robust and performant web servers in Deno, lower-level, native HTTP server APIs are available as of Deno 1.9 and later.

ℹ️ These APIs were stabilized in Deno 1.13 and no longer require --unstable flag.

⚠️ You should probably not be using this API, as it is not easy to get right. Use the serve API in the std/http library instead.

Listening for a connection

In order to accept requests, first you need to listen for a connection on a network port. To do this in Deno, you use Deno.listen():

const server = Deno.listen({ port: 8080 });

ℹ️ When supplying a port, Deno assumes you are going to listen on a TCP socket as well as bind to the localhost. You can specify transport: "tcp" to be more explicit as well as provide an IP address or hostname in the hostname property as well.

If there is an issue with opening the network port, Deno.listen() will throw, so often in a server sense, you will want to wrap it in the try ... catch block in order to handle exceptions, like the port already being in use.

You can also listen for a TLS connection (e.g. HTTPS) using Deno.listenTls():

const server = Deno.listenTls({
  port: 8443,
  certFile: "localhost.crt",
  keyFile: "localhost.key",
  alpnProtocols: ["h2", "http/1.1"],
});

The certFile and keyFile options are required and point to the appropriate certificate and key files for the server. They are relative to the CWD for Deno. The alpnProtocols property is optional, but if you want to be able to support HTTP/2 on the server, you add the protocols here, as the protocol negotiation happens during the TLS negotiation with the client and server.

ℹ️ Generating SSL certificates is outside of the scope of this documentation. There are many resources on the web which address this.

Handling connections

Once we are listening for a connection, we need to handle the connection. The return value of Deno.listen() or Deno.listenTls() is a Deno.Listener which is an async iterable which yields up Deno.Conn connections as well as provide a couple methods for handling connections.

To use it as an async iterable we would do something like this:

const server = Deno.listen({ port: 8080 });

for await (const conn of server) {
  // ...handle the connection...
}

Every connection made would yield up a Deno.Conn assigned to conn. Then further processing can be applied to the connection.

There is also the .accept() method on the listener which can be used:

const server = Deno.listen({ port: 8080 });

while (true) {
  try {
    const conn = await server.accept();
    // ... handle the connection ...
  } catch (err) {
    // The listener has closed
    break;
  }
}

Whether using the async iterator or the .accept() method, exceptions can be thrown and robust production code should handle these using try ... catch blocks. Especially when it comes to accepting TLS connections, there can be many conditions, like invalid or unknown certificates which can be surfaced on the listener and might need handling in the user code.

A listener also has a .close() method which can be used to close the listener.

Serving HTTP

Once a connection is accepted, you can use Deno.serveHttp() to handle HTTP requests and responses on the connection. Deno.serveHttp() returns a Deno.HttpConn. A Deno.HttpConn is like a Deno.Listener in that requests the connection receives from the client are asynchronously yielded up as a Deno.RequestEvent.

To deal with HTTP requests as async iterable it would look something like this:

const server = Deno.listen({ port: 8080 });

for await (const conn of server) {
  (async () => {
    const httpConn = Deno.serveHttp(conn);
    for await (const requestEvent of httpConn) {
      // ... handle requestEvent ...
    }
  })();
}

The Deno.HttpConn also has the method .nextRequest() which can be used to await the next request. It would look something like this:

const server = Deno.listen({ port: 8080 });

while (true) {
  try {
    const conn = await server.accept();
    (async () => {
      const httpConn = Deno.serveHttp(conn);
      while (true) {
        try {
          const requestEvent = await httpConn.nextRequest();
          // ... handle requestEvent ...
        } catch (err) {
          // the connection has finished
          break;
        }
      }
    })();
  } catch (err) {
    // The listener has closed
    break;
  }
}

Note that in both cases we are using an IIFE to create an inner function to deal with each connection. If we awaited the HTTP requests in the same function scope as the one we were receiving the connections, we would be blocking accepting additional connections, which would make it seem that our server was "frozen". In practice, it might make more sense to have a separate function all together:

async function handle(conn: Deno.Conn) {
  const httpConn = Deno.serveHttp(conn);
  for await (const requestEvent of httpConn) {
    // ... handle requestEvent
  }
}

const server = Deno.listen({ port: 8080 });

for await (const conn of server) {
  handle(conn);
}

In the examples from this point on, we will focus on what would occur within an example handle() function and remove the listening and connection "boilerplate".

HTTP Requests and Responses

HTTP requests and responses in Deno are essentially the inverse of web standard Fetch API. The Deno HTTP Server API and the Fetch API leverage the Request and Response object classes. So if you are familiar with the Fetch API you just need to flip them around in your mind and now it is a server API.

As mentioned above, a Deno.HttpConn asynchronously yields up Deno.RequestEvents. These request events contain a .request property and a .respondWith() method.

The .request property is an instance of the Request class with the information about the request. For example, if we wanted to know what URL path was being requested, we would do something like this:

async function handle(conn: Deno.Conn) {
  const httpConn = Deno.serveHttp(conn);
  for await (const requestEvent of httpConn) {
    const url = new URL(requestEvent.request.url);
    console.log(`path: ${url.pathname}`);
  }
}

The .respondWith() method is how we complete a request. The method takes either a Response object or a Promise which resolves with a Response object. Responding with a basic "hello world" would look like this:

async function handle(conn: Deno.Conn) {
  const httpConn = Deno.serveHttp(conn);
  for await (const requestEvent of httpConn) {
    await requestEvent.respondWith(
      new Response("hello world", {
        status: 200,
      }),
    );
  }
}

Note that we awaited the .respondWith() method. It isn't required, but in practice any errors in processing the response will cause the promise returned from the method to be rejected, like if the client disconnected before all the response could be sent. While there may not be anything your application needs to do, not handling the rejection will cause an "unhandled rejection" to occur which will terminate the Deno process, which isn't so good for a server. In addition, you might want to await the promise returned in order to determine when to do any cleanup from for the request/response cycle.

The web standard Response object is pretty powerful, allowing easy creation of complex and rich responses to a client, and Deno strives to provide a Response object that as closely matches the web standard as possible, so if you are wondering how to send a particular response, checkout the documentation for the web standard Response.

HTTP/2 Support

HTTP/2 support is effectively transparent within the Deno runtime. Typically HTTP/2 is negotiated between a client and a server during the TLS connection setup via ALPN. To enable this, you need to provide the protocols you want to support when you start listening via the alpnProtocols property. This will enable the negotiation to occur when the connection is made. For example:

const server = Deno.listenTls({
  port: 8443,
  certFile: "localhost.crt",
  keyFile: "localhost.key",
  alpnProtocols: ["h2", "http/1.1"],
});

The protocols are provided in order of preference. In practice, the only two protocols that are supported currently are HTTP/2 and HTTP/1.1 which are expressed as h2 and http/1.1.

Currently Deno does not support upgrading a plain-text HTTP/1.1 connection to an HTTP/2 cleartext connection via the Upgrade header (see: #10275), so therefore HTTP/2 support is only available via a TLS/HTTPS connection.

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 using the respondWith method. Only once respondWith is called with the returned response, the WebSocket is activated and can be used.

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.

Note: We are aware that this API can be challenging to use, and are planning to switch to WebSocketStream once it is stabilized and ready for use.

async function handle(conn: Deno.Conn) {
  const httpConn = Deno.serveHttp(conn);
  for await (const requestEvent of httpConn) {
    await requestEvent.respondWith(handleReq(requestEvent.request));
  }
}

function handleReq(req: Request): Response {
  const upgrade = req.headers.get("upgrade") || "";
  if (upgrade.toLowerCase() != "websocket") {
    return new Response("request isn't trying to upgrade to websocket.");
  }
  const { socket, response } = Deno.upgradeWebSocket(req);
  socket.onopen = () => console.log("socket opened");
  socket.onmessage = (e) => {
    console.log("socket message:", e.data);
    socket.send(new Date().toString());
  };
  socket.onerror = (e) => console.log("socket errored:", e);
  socket.onclose = () => console.log("socket closed");
  return response;
}

WebSockets are only supported on HTTP/1.1 for now. The connection the WebSocket was created on can not be used for HTTP traffic after a WebSocket upgrade has been performed.