| Index: mojo/public/dart/third_party/shelf/README.md
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| +## Web Server Middleware for Dart
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| +
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| +[](https://travis-ci.org/dart-lang/shelf)
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| +[](https://coveralls.io/r/dart-lang/shelf)
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| +
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| +## Introduction
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| +
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| +**Shelf** makes it easy to create and compose **web servers** and **parts of web
|
| +servers**. How?
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| +
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| +* Expose a small set of simple types.
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| +* Map server logic into a simple function: a single argument for the request,
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| +the response is the return value.
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| +* Trivially mix and match synchronous and asynchronous processing.
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| +* Flexibility to return a simple string or a byte stream with the same model.
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| +
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| +## Example
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| +
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| +See `example/example_server.dart`
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| +
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| +```dart
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| +import 'package:shelf/shelf.dart' as shelf;
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| +import 'package:shelf/shelf_io.dart' as io;
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| +
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| +void main() {
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| + var handler = const shelf.Pipeline().addMiddleware(shelf.logRequests())
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| + .addHandler(_echoRequest);
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| +
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| + io.serve(handler, 'localhost', 8080).then((server) {
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| + print('Serving at http://${server.address.host}:${server.port}');
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| + });
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| +}
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| +
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| +shelf.Response _echoRequest(shelf.Request request) {
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| + return new shelf.Response.ok('Request for "${request.url}"');
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| +}
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| +```
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| +
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| +## Handlers and Middleware
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| +
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| +A [handler][] is any function that handles a [shelf.Request][] and returns a
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| +[shelf.Response][]. It can either handle the request itself--for example, a
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| +static file server that looks up the requested URI on the filesystem--or it can
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| +do some processing and forward it to another handler--for example, a logger that
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| +prints information about requests and responses to the command line.
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| +
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| +[handler]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf@id_Handler
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| +
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| +[shelf.Request]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request
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| +
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| +[shelf.Response]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Response
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| +
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| +The latter kind of handler is called "[middleware][]", since it sits in the
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| +middle of the server stack. Middleware can be thought of as a function that
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| +takes a handler and wraps it in another handler to provide additional
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| +functionality. A Shelf application is usually composed of many layers of
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| +middleware with one or more handlers at the very center; the [shelf.Pipeline][]
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| +class makes this sort of application easy to construct.
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| +
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| +[middleware]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf@id_Middleware
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| +
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| +[shelf.Pipeline]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Pipeline
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| +
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| +Some middleware can also take multiple handlers and call one or more of them for
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| +each request. For example, a routing middleware might choose which handler to
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| +call based on the request's URI or HTTP method, while a cascading middleware
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| +might call each one in sequence until one returns a successful response.
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| +
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| +Middleware that routes requests between handlers should be sure to update each
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| +request's [`handlerPath`][handlerPath] and [`url`][url]. This allows inner
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| +handlers to know where they are in the application so they can do their own
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| +routing correctly. This can be easily accomplished using
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| +[`Request.change()`][change]:
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| +
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| +[handlerPath]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_handlerPath
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| +[url]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_url
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| +[change]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_change
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| +
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| +```dart
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| +// In an imaginary routing middleware...
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| +var component = request.url.pathComponents.first;
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| +var handler = _handlers[component];
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| +if (handler == null) return new Response.notFound(null);
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| +
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| +// Create a new request just like this one but with whatever URL comes after
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| +// [component] instead.
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| +return handler(request.change(script: component));
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| +```
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| +
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| +## Adapters
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| +
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| +An adapter is any code that creates [shelf.Request][] objects, passes them to a
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| +handler, and deals with the resulting [shelf.Response][]. For the most part,
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| +adapters forward requests from and responses to an underlying HTTP server;
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| +[shelf_io.serve][] is this sort of adapter. An adapter might also synthesize
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| +HTTP requests within the browser using `window.location` and `window.history`,
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| +or it might pipe requests directly from an HTTP client to a Shelf handler.
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| +
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| +[shelf_io.serve]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf-io@id_serve
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| +
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| +When implementing an adapter, some rules must be followed. The adapter must not
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| +pass the `url` or `handlerPath` parameters to [new shelf.Request][]; it should
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| +only pass `requestedUri`. If it passes the `context` parameter, all keys must
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| +begin with the adapter's package name followed by a period. If multiple headers
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| +with the same name are received, the adapter must collapse them into a single
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| +header separated by commas as per [RFC 2616 section 4.2][].
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| +
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| +[new shelf.Request]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_Request-
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| +
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| +[RFC 2616 section 4.2]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html
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| +
|
| +An adapter must handle all errors from the handler, including the handler
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| +returning a `null` response. It should print each error to the console if
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| +possible, then act as though the handler returned a 500 response. The adapter
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| +may include body data for the 500 response, but this body data must not include
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| +information about the error that occurred. This ensures that unexpected errors
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| +don't result in exposing internal information in production by default; if the
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| +user wants to return detailed error descriptions, they should explicitly include
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| +middleware to do so.
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| +
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| +An adapter should include information about itself in the Server header of the
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| +response by default. If the handler returns a response with the Server header
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| +set, that must take precedence over the adapter's default header.
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| +
|
| +An adapter should include the Date header with the time the handler returns a
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| +response. If the handler returns a response with the Date header set, that must
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| +take precedence.
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| +
|
| +An adapter should ensure that asynchronous errors thrown by the handler don't
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| +cause the application to crash, even if they aren't reported by the future
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| +chain. Specifically, these errors shouldn't be passed to the root zone's error
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| +handler; however, if the adapter is run within another error zone, it should
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| +allow these errors to be passed to that zone. The following function can be used
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| +to capture only errors that would otherwise be top-leveled:
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| +
|
| +```dart
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| +/// Run [callback] and capture any errors that would otherwise be top-leveled.
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| +///
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| +/// If [this] is called in a non-root error zone, it will just run [callback]
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| +/// and return the result. Otherwise, it will capture any errors using
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| +/// [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 {
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| + return callback();
|
| + }
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| +}
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| +```
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| +
|
| +## Inspiration
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| +
|
| +* [Connect](http://www.senchalabs.org/connect/) for NodeJS.
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| + * Read [this great write-up](http://howtonode.org/connect-it) to understand
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| + the overall philosophy of all of these models.
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| +* [Rack](http://rack.github.io/) for Ruby.
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| +* [WSGI](http://legacy.python.org/dev/peps/pep-3333/) for Python.
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|
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