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Posted 24 Apr 2017


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24 Apr 2017CPOL8 min read
A look at the (Autobahn messaging platform)


A while ago someone posted another SignalR article on, and I stated hey you should take a look at this stuff :, and not liking to me someone that is not willing to die by the sword (afting living by it of course) I decided to put my money where my mouth was/is, and write a small article on


So what its Well quite simply it is a message broker that has many language bindings, that should all be able to communicate together seamlessly. 

Here is what the people being have to say about their own product is an open source networking platform for distributed and microservice applications. It implements the open Web Application Messaging Protocol (WAMP), is feature rich, scalable, robust and secure. Let take care of the hard parts of messaging so you can focus on your app's features.



So tell me more about Crossbar.IO claim to have these language bindings:

  • JavaScript in the browser *
  • JavaScript in Node *
  • Python
  • PHP
  • Java
  • C#
  • Erlang


So on the surface of it, this looks like quite a cool library allowing many different disparate applications talk to each other.

It is worth pointing out that these language bindings without a * are written by the community, they are NOT maintained by the developers, as such you may find differences between what works in the main actual maintained language binding.

Unfornately this is life, its exactly the same when you use other messaging frameworks with multi-language bindings like RabbitMQ and Kafka (which we will be looking at more later).



Where is the code?

For the little demo app that this article uses all the demo code is at my GitHub account :



This section will guide you through the installation process


Installing Python

  1. Download python, install it to default not to "Program Files", make sure "Add to path" is YES
  2. Open a cmd line where you installed python
  3. using pip from python command line: pip install pypiwin32
  4. using pip from python command line: pip install crossbar

Verifying installation

1. Validate install command line : which crossbar 

Image 1



2. Validate install command line : crossbar version

Image 2





The Demo

This section will outline some core idea before we dive into the demo code

Core Concepts

crossbar node has this concept of  "node". A "node" is basically a running instance of the crossbar.exe process which is running an active configuration. Typically this would be once per machine/VM.

.crossbar folders

In order to configure the running crossbar node, there is a special folder where it will look, which is called ".crossbar". In this folder, there are a number of files, the MOST important of which is "config.json", which is what is used to actual configure the crossbar.exe process when it starts. This folder is magical/special and will be examined when you use the crossbar start command line (which we don't actually want to do, but more on this in just a moment).

When we issue a crossbar start command line, crossbar.exe will pick up the config file it finds at within the folder specified by the .crossbar name.

So what's it one of these "config.json" files.

Here is one that comes with the  .NET demos :, namely the demo (the  web site says you should be able to use the command line to scaffold a new project for your language of choice, but you xan't they found it too much work to manatin the templates so you need to grab them from the examples folder).

    "version": 2,
    "controller": {},
    "workers": [
            "type": "router",
            "options": {
                "pythonpath": [
            "realms": [
                    "name": "realm1",
                    "roles": [
                            "name": "anonymous",
                            "permissions": [
                                    "uri": "",
                                    "match": "prefix",
                                    "allow": {
                                        "call": true,
                                        "register": true,
                                        "publish": true,
                                        "subscribe": true
                                    "disclose": {
                                        "caller": false,
                                        "publisher": false
                                    "cache": true
            "transports": [
                    "type": "web",
                    "endpoint": {
                        "type": "tcp",
                        "port": 8080
                    "paths": {
                        "/": {
                            "type": "static",
                            "directory": "../src/Web"
                        "ws": {
                            "type": "websocket"
            "type": "guest",
            "executable": "Hello.exe",
            "arguments": [
            "options": {
                "workdir": "../src/DotNet/Hello/bin/Debug/"


There are actual 2 workers in this one config file

  • A web site
  • A .NET application (Hello.exe)


So lets think about that for a minute, we have this config file, and we are supposed to run crossbar.exe with this configuration file, and it starts Hello.exe and some web site. mmmm, How do I debug Hello.exe if Crossbar.exe is running it.

Seems to me that this is the opposite of what we want, how do we debug. There are a few posts on this

I decided to take an alternative route to this, I thought I don't mind using the web dev tools (F12 in Chrome) to debug JavaScripts, but for .NET I want to use Visual Studio, so I get my .NET code to start as an extra process, where I config the .NET worker in code, and pass the new Crossbar.exe process a modified version of the file above, that has the .NET Hello.exe config taken out of it.

I have to say if I had of been using RabbitMQ OR Kafka this would have been childs play. I prefer embedded APIs rather than having to submit my code to something else to run. Sure you MUST be able to configure stuff yourself, but why not start out that way.


RPC allows workers to make Remote Procedure Calls to other workers in different languages


Pub/Sub allows workers to publish and receive to/from other workers in different languages


The Demo Code

The demo code will show a single .NET worker and single JavaScript worker that will pub/sub and RPC to each other


The .NET code

Here is the entire .NET code for a publisher/subscriber, and RPC callable .NET program

using System;
using System.Reactive.Subjects;
using System.Threading.Tasks;
using WampSharp.Core.Listener;
using WampSharp.V2;
using WampSharp.V2.Client;
using WampSharp.V2.Core.Contracts;
using WampSharp.V2.Realm;
using WampSharp.V2.Rpc;
using System.Diagnostics;

namespace Hello
    public class Program
        static void Main(string[] args)

            //use this for debugging
            Task.Factory.StartNew(() =>
                Process process = new Process();
                process.StartInfo.FileName = @"c:\Users\sacha\AppData\Local\Programs\Python\Python36-32\Scripts\crossbar.exe";
                process.StartInfo.Arguments = @"start --cbdir C:\Users\sacha\Desktop\CrossbarIOExample\CrossBarDotNetExample\.crossbar";
                process.StartInfo.WindowStyle = ProcessWindowStyle.Maximized;
                // Waits here for the process to exit, but since this is 
                // dedicated thread main thread is not blocked

            }, TaskCreationOptions.LongRunning);

            //give process time to start
            System.Threading.Thread.Sleep(1000 * 20);

            Console.WriteLine("WampSharp Hello demo starting ...");

            string wsuri = "ws://";
            string realm = "realm1";
            if (args.Length > 0) {
               wsuri = args[0];
               if (args.Length > 1) {
                  realm = args[1];
            Task runTask = Run(wsuri, realm);


        private async static Task Run(string wsuri, string realm)
            Console.WriteLine("Connecting to {0}, realm {1}", wsuri, realm);

            DefaultWampChannelFactory factory = new DefaultWampChannelFactory();

            IWampChannel channel =
                factory.CreateJsonChannel(wsuri, realm);

            IWampClientConnectionMonitor monitor = channel.RealmProxy.Monitor;
            monitor.ConnectionBroken += OnClose;
            monitor.ConnectionError += OnError;

            await channel.Open().ConfigureAwait(false);

            IWampRealmServiceProvider services = channel.RealmProxy.Services;

            // SUBSCRIBE to a topic and receive events
            ISubject<string> helloSubject = 

            IDisposable subscription =
                    (x => Console.WriteLine("event for 'onhello' received: {0}", x));

            Console.WriteLine("subscribed to topic 'onhello'");

            // REGISTER a procedure for remote calling
            Add2Service callee = new Add2Service();

            await services.RegisterCallee(callee)
            Console.WriteLine("procedure add2() registered");

            // PUBLISH and CALL every second... forever
            ISubject<int> onCounterSubject =

            ISubject<int> onDotNetCounterSubject =

            IMul2Service proxy =

            int counter = 0;

            while (true)
                // PUBLISH an event
                Console.WriteLine("published to 'oncounter' with counter {0}", counter);

                Console.WriteLine("published to 'ondotnetcounter' with counter {0}", counter);

                // CALL a remote procedure
                    int result = await proxy.Multiply(counter, 3)

                    Console.WriteLine("mul2() called with result: {0}", result);
                catch (WampException ex)
                    if (ex.ErrorUri != "wamp.error.no_such_procedure")
                        Console.WriteLine("call of mul2() failed: " + ex);

                await Task.Delay(TimeSpan.FromSeconds(1))

        #region Callee

        public interface IAdd2Service
            int Add(int x, int y);

        public class Add2Service : IAdd2Service
            public int Add(int x, int y)
                Console.WriteLine("add2() called with {0} and {1}", x, y);
                return x + y;


        #region Caller

        public interface IMul2Service
            Task<int> Multiply(int x, int y);             


        private static void OnClose(object sender, WampSessionCloseEventArgs e)
            Console.WriteLine("connection closed. reason: " + e.Reason);

        private static void OnError(object sender, WampConnectionErrorEventArgs e)
            Console.WriteLine("connection error. error: " + e.Exception);

This uses the ONLY .NET binding WampSharp Nuget package. This is developed by a community member, so updates may be patchy. Here be dragons (well possibly anyway)


The JavaScript Code

Here is the entire JavaScript code for a publisher/subscriber, and RPC callable JavaScript program

<!DOCTYPE html>
      <h1>Hello WAMP</h1>
      <p>Open JavaScript console to watch output.</p>
      <script>AUTOBAHN_DEBUG = false;</script>
       <script src="js/autobahn.min.js"></script>

         // the URL of the WAMP Router (
         var wsuri;
         if (document.location.origin == "file://") {
            wsuri = "ws://";

         } else {
            wsuri = (document.location.protocol === "http:" ? "ws:" : "wss:") + "//" +
               + "/ws";

         // the WAMP connection to the Router
         var connection = new autobahn.Connection({
            url: wsuri,
            realm: "realm1"

         // timers
         var t1, t2;

         // fired when connection is established and session attached
         connection.onopen = function (session, details) {


            // SUBSCRIBE to a topic and receive events
            function on_counter (args) {
               var counter = args[0];
               console.log("on_counter() event received with counter " + counter);
            session.subscribe('com.example.oncounter', on_counter).then(
               function (sub) {
                  console.log('subscribed to topic');
               function (err) {
                  console.log('failed to subscribe to topic', err);
	//SUBSCRIBE to a topic and receive events
            function on_dotnetcounter (args) {
               var counter = args[0];
               console.log("DOTNET : on_dotnetcounter() event received with counter " + counter);
            session.subscribe('com.example.ondotnetcounter', on_dotnetcounter).then(
               function (sub) {
                  console.log('subscribed to topic');
               function (err) {
                  console.log('failed to subscribe to topic', err);

            // PUBLISH an event every second
            t1 = setInterval(function () {

               session.publish('com.example.onhello', ['Hello from JavaScript (browser)']);
               console.log("published to topic 'com.example.onhello'");
            }, 1000);

            // REGISTER a procedure for remote calling
            function mul2 (args) {
               var x = args[0];
               var y = args[1];
               console.log("mul2() called with " + x + " and " + y);
               return x * y;
            session.register('com.example.mul2', mul2).then(
               function (reg) {
                  console.log('procedure registered');
               function (err) {
                  console.log('failed to register procedure', err);

            // CALL a remote procedure every second
            var x = 0;

            t2 = setInterval(function () {

     'com.example.add2', [x, 18]).then(
                  function (res) {
                     console.log("add2() result:", res);
                  function (err) {
                     console.log("add2() error:", err);

               x += 3;
            }, 1000);

         // fired when connection was lost (or could not be established)
         connection.onclose = function (reason, details) {
            console.log("Connection lost: " + reason);
            if (t1) {
               t1 = null;
            if (t2) {
               t2 = null;

         // now actually open the connection


As JavaScript IS AN OFFICIALLY supported binding, this should be kept up to date by the  folks.


Runtime Screenshots

And this is what we see when we run the solution in Visual Studio

Image 3



How do I debug and still run crossbar?

  • For the .NET code you can just use my trick where I start an extra crossbar.exe process and tell it what modified config file to use
  • For the web app just use the dev tools of your browser.



Comparison with others

It seems only fair that I do a comparison of it next to a few other messaging frameworks that I like/have used. Its worth noting that these all need something extra installed before you even get into any comparison

  • Crossbar needs python
  • Rabbit need Erlang
  • Kafka needs a JDK


 Multi LanguagePersisted messagedClusteredConfigurationDebugging ExperiencePub/SubRPC
Crossbar.ioYesNo (1)NoVia some weird .crossbar folder. May be possible to do same in code but probably a right nightmareBit naff to be honest. As crossbar wants to use the .crossbar folder config to start your "crossbar node". Which means it is running the app not your IDE. So you are forced to start crossbar.exe from your app, and pass it a config command line arg, and wait for it to be running.

Yes, with easeYes, with ease
RabbitMQYesYesYesVia standard code/configJust run browser/IDE and put in breakpointsYes, with ease (queues)Yes, with correlationId
KafkaYesYesYesVia standard code/configJust run browser/IDE and put in breakpointsYes, with ease (topics)Yes, with correlationId + extra topic, not as easy as others


1 One way to deal with this is slap a database in the way so all producers write to the database first, then you just use event sourcing to supply the node with events, which it will broadcast out.

So as you can see there are far better solutions out there, but it really does depend on what your needs are.



This is a tricky part to write, as on one hand I like the fact that this is fairly easy to use, but on the other hand there are just too many factors that I don't like to make this my defacto goto messaging solution.

I like how different things can talk to each other fairly easily (all be it each language bindding offers a VERY different API). I don't like how debugging is hard, and I have to use this .crossbar folder to configure things.

I am also not a fan of the messaging not being durable. Sure you can get round it as I've stated but there are better solutions out there.

So for me its ok if all you really want is fire and forget, non persisted messaging, for anything else I would use RabbitMQ or Kafka




This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Software Developer (Senior)
United Kingdom United Kingdom
I currently hold the following qualifications (amongst others, I also studied Music Technology and Electronics, for my sins)

- MSc (Passed with distinctions), in Information Technology for E-Commerce
- BSc Hons (1st class) in Computer Science & Artificial Intelligence

Both of these at Sussex University UK.


I am lucky enough to have won a few awards for Zany Crazy code articles over the years

  • Microsoft C# MVP 2016
  • Codeproject MVP 2016
  • Microsoft C# MVP 2015
  • Codeproject MVP 2015
  • Microsoft C# MVP 2014
  • Codeproject MVP 2014
  • Microsoft C# MVP 2013
  • Codeproject MVP 2013
  • Microsoft C# MVP 2012
  • Codeproject MVP 2012
  • Microsoft C# MVP 2011
  • Codeproject MVP 2011
  • Microsoft C# MVP 2010
  • Codeproject MVP 2010
  • Microsoft C# MVP 2009
  • Codeproject MVP 2009
  • Microsoft C# MVP 2008
  • Codeproject MVP 2008
  • And numerous codeproject awards which you can see over at my blog

Comments and Discussions

Questionguest workers vs stand alone workers Pin
Member 131647841-May-17 10:03
Member 131647841-May-17 10:03 
AnswerRe: guest workers vs stand alone workers Pin
Sacha Barber7-May-17 9:17
Sacha Barber7-May-17 9:17 
GeneralMy vote of 5 Pin
FranckBerthier24-Apr-17 18:03
FranckBerthier24-Apr-17 18:03 

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