Relic web server

Relic is a web server based on Shelf that supports middleware. It's currently available as a tech preview to gather feedback before we release a stable version. Beware that the API is still subject to change. The best way to provide your feedback is through issues on GitHub here: https://github.com/serverpod/relic/issues

This package was born out of the needs of Serverpod, as we wanted a more modern and performant base for our web server. Relic is based on Shelf, but we have made several improvements:

• We removed all List<int> in favor of Uint8List.
• We made everything type-safe (no more dynamic).
• Encoding types have been moved to the Body of a Request/Response to simplify the logic when syncing up the headers and to have a single source of truth.
• We've added parsers and validation for all commonly used HTTP headers. E.g., times are represented by DateTime, cookies have their own class with validation of formatting, etc.
• Extended test coverage.
• There are lots of smaller fixes here and there.

Although the structure is very similar to Shelf, this is no longer backward compatible. We like to think that a transition would be pretty straightforward, and we are planning put a guide in place.

Before a stable release, we're also planning on adding the following features:

• We want to more tightly integrate a http server (i.e., start with HttpServer from dart:io as a base) with Relic so that everything uses the same types. This will also improve performance as fewer conversions will be needed.
• Routing can be improved by using Radix trees. Currently, there is just a list being traversed, which can be an issue if you have many routes.
• We're planning to add an improved testing framework.
• Performance testing and optimizations.

In addition, we're planning to include Relic in Serverpod, both for powering our RPC and as a base for our web server. This would add support for middleware in our RPC. In our web server integration, we have support for HTML templates and routing. You also get access to the rest of the Serverpod ecosystem in terms of serialization, caching, pub-sub, and database integrations.

Example

See example/example.dart

import 'package:relic/relic.dart';

void main() async {
  var handler =
      const Pipeline().addMiddleware(logRequests()).addHandler(_echoRequest);

  var server = await serve(
    handler,
    RelicAddress.fromHostname('localhost'),
    8080,
  );

  // Enable content compression
  server.autoCompress = true;

  print('Serving at http://${server.address.host}:${server.port}');
}

Response _echoRequest(Request request) {
  return Response.ok(
    body: Body.fromString(
      'Request for "${request.url}"',
    ),
  );
}

Handlers and Middleware

A Handler is any function that handles a Request and returns a 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 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.copyWith():

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

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

Adapters

An adapter is any code that creates Request objects, passes them to a handler, and deals with the resulting Response. For the most part, adapters forward requests from and responses to an underlying HTTP server; 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].
void catchTopLevelErrors(
  void Function() callback,
  void Function(Object error, StackTrace stackTrace) onError,
) {
  if (Zone.current.inSameErrorZone(Zone.root)) {
    return runZonedGuarded(callback, onError);
  } else {
    return callback();
  }
}

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

Request Requirements

If the underlying request uses a chunked transfer coding, the adapter must decode the body before passing it to 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() 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.

Inspiration

• Shelf for Dart.
• Connect for NodeJS.
• Rack for Ruby.