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+## Web Server Middleware for Dart |
+ |
+[](https://travis-ci.org/dart-lang/shelf) |
+[](https://coveralls.io/r/dart-lang/shelf) |
+ |
+## Introduction |
+ |
+**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. |
+ |
+## Example |
+ |
+See `example/example_server.dart` |
+ |
+```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()) |
+ .addHandler(_echoRequest); |
+ |
+ io.serve(handler, 'localhost', 8080).then((server) { |
+ print('Serving at http://${server.address.host}:${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. |
+ |
+[handler]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf@id_Handler |
+ |
+[shelf.Request]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request |
+ |
+[shelf.Response]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Response |
+ |
+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. |
+ |
+[middleware]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf@id_Middleware |
+ |
+[shelf.Pipeline]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Pipeline |
+ |
+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`][handlerPath] and [`url`][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()`][change]: |
+ |
+[handlerPath]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_handlerPath |
+[url]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_url |
+[change]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_change |
+ |
+```dart |
+// 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)); |
+``` |
+ |
+## Adapters |
+ |
+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. |
+ |
+[shelf_io.serve]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf-io@id_serve |
+ |
+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][]. |
+ |
+[new shelf.Request]: http://www.dartdocs.org/documentation/shelf/latest/index.html#shelf/shelf.Request@id_Request- |
+ |
+[RFC 2616 section 4.2]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html |
+ |
+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 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. |
+ |
+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: |
+ |
+```dart |
+/// 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(); |
+ } |
+} |
+``` |
+ |
+## Inspiration |
+ |
+* [Connect](http://www.senchalabs.org/connect/) for NodeJS. |
+ * Read [this great write-up](http://howtonode.org/connect-it) to understand |
+ the overall philosophy of all of these models. |
+* [Rack](http://rack.github.io/) for Ruby. |
+* [WSGI](http://legacy.python.org/dev/peps/pep-3333/) for Python. |