connecting autobahn websocket server to crossbar.io router

Question

I have an application that connects to a web page that sends and receives text strings over a websocket on port 1234. I do not have access to the front end code, so I cannot change the HTML front end code. I created an autobahn server with a class derived from WebSocketServer protocol that communicates with the web page over port 1234. This works and I am able to send and receive text to the front end. However, I need to process incoming data and would like to publish the received data to a crossbar.io container through the router on port 8080 (or any other port). The port to the web browser is fixed at 1234. It there a way for me to "plug in " the autobahn websocket server into the crossbar router or is there an alternative way to create a websocket server that will allow me to to send and receive the text on port 1234 and at the same time participate in pub/sub and RPC with the crossbar router?

Answer 1

I am assuming you are using Python. If you are not, the answer should still be the same, but depending on the language/library and its implementation the answer can change.

From what you are saying, it does not sound like you really need a "plug in". Crossbar does have these under the description of router components. But unless you really need to attach a Python instance directly to the router either for performance or otherwise, I would recommend keeping your application off the router. It would work perfectly fine as a stand alone instance especially if it is on the same machine where the WAMP router is located where the packets would only require to communicate over loopback (which is VERY fast).

Given that you are using Python:

You can use your WebSocketServer and a WampApplicationServer together. The little hiccup you might run into is starting them up properly. In either scenario Python2.x with twisted or Python3.4 with Asyncio you can only start the reactor/event loop once or an error will ensue. (Both Twisted and Asyncio have the same basic concept) In Asyncio you will get RuntimeError: Event loop is running. if you attempt to start the event loop twice. Twisted has a similar error. Using the ApplicationRunner in twisted, there is an option (second argument in run) not to start up the reactor which you can use after the reactor is already running. In Asyncio, there is no such option, the only way I found out how to do it is to inherit the Application runner and overwrite the run method to start the session to be started as a task. Also, be warned that threads do not cooperate with either event loop unless properly wrapped.

Once you have the two connections set up in one instance you can do whatever you want with the data.

Answer 2

Thanks for the idea, and the problems you mention are exactly what I encountered. I did find a solution however, and thanks to the flexibility of crossbar, created a JavaScript guest that allows me to do exactly what I need. Here is the code:

// crossbar setup
var autobahn = require('autobahn');

var connection = new autobahn.Connection({
        url: 'ws://127.0.0.1:8080/ws',
        realm: 'realm1'
    }
);

// Websocket to Scratch setup
// pull in the required node packages and assign variables for the entities
var WebSocketServer = require('websocket').server;
var http = require('http');

var ipPort = 1234; // ip port number for Scratch to use

// this connection is a crossbar connection
connection.onopen = function (session) {

    // create an http server that will be used to contain a WebSocket server
    var server = http.createServer(function (request, response) {
        // We are not processing any HTTP, so this is an empty function. 'server' is a wrapper for the
        // WebSocketServer we are going to create below.
    });

    // Create an IP listener using the http server
    server.listen(ipPort, function () {
        console.log('Webserver created and listening on port ' + ipPort);
    });

    // create the WebSocket Server and associate it with the httpServer
    var wsServer = new WebSocketServer({
        httpServer: server
    });

    // WebSocket server has been activated and a 'request' message has been received from client websocket
    wsServer.on('request', function (request) {
        // accept a connection request from Xi4S
        //myconnection is the WS connection to Scratch
        myconnection = request.accept(null, request.origin); // The server is now 'online'

        // Process Xi4S messages
        myconnection.on('message', function (message) {

            console.log('message received: ' + message.utf8Data);
            session.publish('com.serial.data', [message.utf8Data]);

            // Process each message type received
            myconnection.on('close', function (myconnection) {
                console.log('Client closed connection');
                boardReset();
            });
        });
    });
};

connection.open();