Web Server Middleware for Dart
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
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.
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));
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.
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.
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:
/// 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 for NodeJS.
- Read this great write-up to understand the overall philosophy of all of these models.
- Rack for Ruby.
- WSGI for Python.