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Web Server Middleware for Dart

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Shelf makes it easy to create and compose web servers and parts of web servers. How?

  • Expose a small set of simple types.
  • Map server logic into a simple function: a single argument for the request, the response is the return value.
  • Trivially mix and match synchronous and asynchronous processing.
  • Flexibility to return a simple string or a byte stream with the same model.


See example/example_server.dart

import 'package:shelf/shelf.dart' as shelf;
import 'package:shelf/shelf_io.dart' as io;

void main() {
  var handler = const shelf.Pipeline().addMiddleware(shelf.logRequests())

  io.serve(handler, 'localhost', 8080).then((server) {
    print('Serving at http://${}:${server.port}');

shelf.Response _echoRequest(shelf.Request request) {
  return new shelf.Response.ok('Request for "${request.url}"');

Handlers and Middleware

A handler is any function that handles a shelf.Request and returns a shelf.Response. It can either handle the request itself--for example, a static file server that looks up the requested URI on the filesystem--or it can do some processing and forward it to another handler--for example, a logger that prints information about requests and responses to the command line.

The latter kind of handler is called “middleware”, since it sits in the middle of the server stack. Middleware can be thought of as a function that takes a handler and wraps it in another handler to provide additional functionality. A Shelf application is usually composed of many layers of middleware with one or more handlers at the very center; the shelf.Pipeline class makes this sort of application easy to construct.

Some middleware can also take multiple handlers and call one or more of them for each request. For example, a routing middleware might choose which handler to call based on the request's URI or HTTP method, while a cascading middleware might call each one in sequence until one returns a successful response.

Middleware that routes requests between handlers should be sure to update each request's handlerPath and url. This allows inner handlers to know where they are in the application so they can do their own routing correctly. This can be easily accomplished using Request.change():

// In an imaginary routing middleware...
var component = request.url.pathComponents.first;
var handler = _handlers[component];
if (handler == null) return new Response.notFound(null);

// Create a new request just like this one but with whatever URL comes after
// [component] instead.
return handler(request.change(script: component));


An adapter is any code that creates shelf.Request objects, passes them to a handler, and deals with the resulting shelf.Response. For the most part, adapters forward requests from and responses to an underlying HTTP server; shelf_io.serve is this sort of adapter. An adapter might also synthesize HTTP requests within the browser using window.location and window.history, or it might pipe requests directly from an HTTP client to a Shelf handler.

API Requirements

An adapter must handle all errors from the handler, including the handler returning a null response. It should print each error to the console if possible, then act as though the handler returned a 500 response. The adapter may include body data for the 500 response, but this body data must not include information about the error that occurred. This ensures that unexpected errors don't result in exposing internal information in production by default; if the user wants to return detailed error descriptions, they should explicitly include middleware to do so.

An adapter should ensure that asynchronous errors thrown by the handler don‘t cause the application to crash, even if they aren’t reported by the future chain. Specifically, these errors shouldn‘t be passed to the root zone’s error handler; however, if the adapter is run within another error zone, it should allow these errors to be passed to that zone. The following function can be used to capture only errors that would otherwise be top-leveled:

/// Run [callback] and capture any errors that would otherwise be top-leveled.
/// If [this] is called in a non-root error zone, it will just run [callback]
/// and return the result. Otherwise, it will capture any errors using
/// [runZoned] and pass them to [onError].
catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) {
  if (Zone.current.inSameErrorZone(Zone.ROOT)) {
    return runZoned(callback, onError: onError);
  } else {
    return callback();

An adapter that knows its own URL should provide an implementation of the Server interface.

Request Requirements

When implementing an adapter, some rules must be followed. The adapter must not pass the url or handlerPath parameters to new shelf.Request; it should only pass requestedUri. If it passes the context parameter, all keys must begin with the adapter's package name followed by a period. If multiple headers with the same name are received, the adapter must collapse them into a single header separated by commas as per RFC 2616 section 4.2.

If the underlying request uses a chunked transfer coding, the adapter must decode the body before passing it to new shelf.Request and should remove the Transfer-Encoding header. This ensures that message bodies are chunked if and only if the headers declare that they are.

Response Requirements

An adapter must not add or modify any entity headers for a response.

If none of the following conditions are true, the adapter must apply chunked transfer coding to a response's body and set its Transfer-Encoding header to chunked:

  • The status code is less than 200, or equal to 204 or 304.
  • A Content-Length header is provided.
  • The Content-Type header indicates the MIME type multipart/byteranges.
  • The Transfer-Encoding header is set to anything other than identity.

Adapters may find the [addChunkedEncoding()][addChunkedEncoding] middleware useful for implementing this behavior, if the underlying server doesn't implement it manually.

When responding to a HEAD request, the adapter must not emit an entity body. Otherwise, it shouldn't modify the entity body in any way.

An adapter should include information about itself in the Server header of the response by default. If the handler returns a response with the Server header set, that must take precedence over the adapter's default header.

An adapter should include the Date header with the time the handler returns a response. If the handler returns a response with the Date header set, that must take precedence.