Dealing with WebSocket in Dart

WebSocket is a protocol created more than 10 years ago offering a bidirectional communication channel between a client and a server. It fixes the HTTP polling issue.

The WebSocket Protocol attempts to address the goals of existing bidirectional HTTP technologies in the context of the existing HTTP infrastructure; as such, it is designed to work over HTTP ports 80 and 443 as well as to support HTTP proxies and intermediaries, even if this implies some complexity specific to the current environment. However, the design does not limit WebSocket to HTTP, and future implementations could use a simpler handshake over a dedicated port without reinventing the entire protocol. This last point is important because the traffic patterns of interactive messaging do not closely match standard HTTP traffic and can induce unusual loads on some components.

-- RFC6455, section 1

Today, let have some fun with WebSocket in Dart

WebSocket Server with shelf

Unfortunately, like most of the modern language, there is a framework war. Many different implementation exists for doing the same thing. I don't really have time to check all of them, so, I will just do quick table. All of them are supporting WebSockets.

package	version	framework?	deprecated?	notes
shelf	1.4.2	no	no	maintained by dart team
http_server	1.0.0	no	yes	previous official http server
frog	1.2.6	yes	no	minimalist backend framework
shelf_easy	5.2.0	yes	no	lightweight framework
darto	0.0.35	yes	no	micro-web framework
serinus	2.1.12	yes	no	flexible framework
alfred	1.1.3	yes	no	expressjs like web server
vania	1.1.5	yes	yes	high-performance framework
finch	1.3.2	yes	no	lightweight framework (looks popular)

Most of them are using shelf or http_server as backend, not sure if it's really necessary to test them all one by one. Anyway, I will use shelf for one reason: this is not a framework, and it is maintained by the Dart team. I'm a bit sad though, because I can't find a lot of documentation, except the official API. To use WebSocket with shelf, the shelf_web_socket handler is required, again, only the API documentation can easily be found. A full example can be found in server/simple directory from dart source code.

For this exercise, I would like to have something simple:

• long living foreground process
• listening on 127.0.0.0:8080
• only one route: /
• all other routes are redirected to /
• automatic support of web socket
• echo all message sent by the client

No authentication, no encryption, just a simple websocket echo server. First, let try to understand how to use shelf. What kind of packages will be needed there?

• shelf from the shelf package. It includes different kind of helpful classes to deal with the server, like Cascade (managing handlers), Pipeline to compose Middlewares and Handlers, Request to give an abstract view of a request, Response to offers an interface for the HTTP response returned and finally Server as adapter. We will see that probably later.

• shelf_io from the shelf package. This one is an adapter, mostly used to handle HttpRequest, if one wants to create a quick and dirty http server, one can use the server() method.

• shelf_router from the shelf_router package. This is a routing library designed for shelf, offering Route and Router classes to deal with HTTP path routing. In our case, it will be only used to redirect all path to /.

• shelf_web_socket from the shelf_web_socket package. Well, it has already be described before, this is an handler to deal with WebSockets.

• web_socket_channel package, mostly used for few exposed classes.

Knowing that, here the headers used to deal with a simple shelf WebSocket server:

import 'dart:collection';
import 'dart:convert';
import 'dart:io';
import 'package:ws/ws.dart' as ws;
import 'package:shelf/shelf.dart';
import 'package:shelf/shelf_io.dart' as shelf_io;
import 'package:shelf_router/shelf_router.dart' as shelf_router;
import 'package:shelf_web_socket/shelf_web_socket.dart';
import 'package:web_socket_channel/web_socket_channel.dart';

To create a WebSocket handler, a callback function is required, it is called onConnection in the implementation. This function is taking 2 arguments, the first one is a WebSocketChannel object, and the second argument is a String representing an eventual WebSocket subprotocol.

void _onConnection(WebSocketChannel webSocket, String? subprotocol) {
  webSocket.sink.add('{"msg":"ready"}\n');
  webSocket.stream.listen((message) {
    print('received: "$message"');
    webSocket.sink.add('{"msg":"$message"}\n');
  });
}

The WebSocketChannel object got 2 interesting properties:

• sink - an instance of the Sink class - used to send value to the client, it can be used also to close the connection with the close() method. Then, in onConnection() function, when the connection is ready, the server is sending a first message.

• stream - an instance of the Stream class - used to receive the message emitted by the client. In onConnection() function, every time a client is sending a message, the server will print the message locally on STDOUT and will return the same message to the client.
  

Handler _webSocketHandler1() {
  return webSocketHandler(
    _onConnection,
    protocols: null,
    allowedOrigins: null,
    pingInterval: Duration(seconds: 30)
  );
}

A dedicated function to start this first test is also required. Let call it startShelfIO where shelf_io will be used to start the server using serve() method.

void startShelfIO() {
  shelf_io
    .serve(_webSocketHandler1(), 'localhost', 8080)
    .then((server) {
      print('server started');
    });
}

Finally the main() function, it will simply invoke startShelfIO() function and do nothing more.

void main(List<String> arguments) {
  startShelfIO();
}

Hmm. We don't have a WebSocket client available right, it will be coded in the next part of this article. Unluckily, curl does not have a full support for the WebSocket protocol (it can receive but not send data). Instead, we can install the browser-websocket-client open-source plugin available on the Chrome Web Store. Are you Firefox friendly? You can use Weasel and its still open-source. If you prefer a command line tool, you can also check websocat or claws.

$ dart run
Building package executable... 
Built ws:ws.
server started
received: "{"client": "test"}"

It seems to works... What if we want to close the connection from the server side? Simply close() the sink.

webSocket.sink.close();

To do something before closing the connection or if the client is closing it on its side, the onDone attribute can be set while invoking the listen() method.

If this stream closes and sends a done event, the onDone handler is called. If onDone is null, nothing happens.

-- listen abstract method

Am I following the best practices here? Absolutely not! But if you want to fix my mess and add many layers of security, here a small list of references to do that:

• WebSocket Security Cheat Sheet by the OWASP Team;

• WebSocket architecture best practices: Designing scalable realtime systems by Ably

• WebSocket Best Practices for Production Applications from WebSocket.org

• WebSocket Security Testing: Penetration Test Cases & Best Practices by Mahendra Mb
  

• WebSocket: Best Practices by MongoHost

If you simply see other implementation or read more about WebSockets server on Dart:

• Create a WebSocket Server with Dart by MojoAuth

• Working with WebSocket by @itzmeanjan

WebSocket Client with web_socket

I will not do the same error twice and start to check all frameworks/implementations to deal with a WebSocket client in Dart. The first one available is the web_socket package. This one is directly maintained by the Dart team and the source code can be seen in pkgs/web_socket. An unique example is also available. The previous package called web_socket_channel was the one used by most of the project, but due to the complexity, a simpler one was required.

$ dart pub add web_socket
Resolving dependencies... 
Downloading packages... 
  web_socket 1.0.1 (from transitive dependency to direct dependency)
Changed 1 dependency!

Only the web_socket package will be required here. Note: to make things easier, just create a new dart file like bin/wsc.dart.

import 'package:web_socket/web_socket.dart';

Let create an isolated function called webSocketClient to deal with the socket initialization.

void webSocketClient() async {
  final socket = 
    await WebSocket.connect(
      Uri.parse('ws://localhost:8080/')
    );
  socket.sendText('hello');
  socket.events.listen(
    (event) async {
      switch (event) {
        case TextDataReceived(text: final text):
          print("$text");
        case BinaryDataReceived(data: final data):
          print("$data");
        case CloseReceived(code: final code, reason: final reason):
          print("$code, $reason");
      }
      socket.close();
    }
  );
}

• The connect() static method is called with the URL of the server using an Uri object. It returns a Future<WebSocket> object.

• sendText() method is then invoked to send a text message to the server. If binaries are required, then use sendBytes() method. It returns nothing and in case of disconnection, the messages are silently discarded.

• The WebSocketEvent class is then used to deal with the events received on the socket. Those events are usually messages sent by the server, they can are defined in three different classes:

  • a TextDataReceived object is returned when the server is sending text message (e.g. JSON).
  • a BinaryDataReceived object is returned when the server is sending binary message (e.g. protobuf, CBOR).
  • a CloseReceived object is finally returned when the connection is closed.

• The close() method is called to explicitly close the connection from the client side.

The client side here is quite simple, and does not involve complex procedure. To invoke it, just execute the webSocketClient() function in the main() entry-point.

void main() async {
  webSocketClient(socket);
}

Time to test if it is working correctly

$ dart run bin/wsc.dart
{"msg":"ready"}

It seems to work. The client connects to the server, return the message received and simply close the connection.

Another popular WebSocket client package called web_socket_client is also available at pub.dev. I'm not really sure if it adds more features or is more flexible than the default WebSocket client maintained by the Dart team. If you are interested to see more implementations and examples, here a quick list:

• Official example

• Dart WebSocket on ZetCode

• Building a WebSocket Client in Flutter: From Zero to Hero by Punith S Uppar

• Real-time apps with Flutter and WebSockets by Gianfranco Papa

Conclusion

Creating a WebSocket client and a WebSocket server was not so hard, even if the documentation is hard to find, and most of the examples are limited to simple use cases. The subprotocol feature present in WebSocket is kinda interesting and I think I will work on that on another post. Again, the code presented above was made for test purpose only, I can't recommend anyone to use it in production environment.

Anyway, I think in a future publication, we will create something a bit more complex, like creating a custom communication protocol between a client and a server, using binary data.

---

Cover Image by Andrew Knechel on Unsplash