Wednesday, August 3, 2016

Porting node-simple-xmpp from the Node.js ecosystem to Titanium

As may have become obvious by reading some of my previous blog posts, I am frequently using JavaScript for a variety of programming purposes. Although JavaScript was originally conceived as a programming language for use in web browsers (mainly to make web pages more interactive), it is also becoming increasingly more popular in environments outside the browser, such as Node.js, a runtime environment for developing scalable network applications and Appcelerator Titanium, an environment to develop cross-platform mobile applications.

Apart from the fact that these environments share a common programming language -- JavaScript -- and a number of basic APIs that come with the language, they all have their own platform-specific APIs to implement most of an application's basic functionality.

Moreover, they have their own ecosystem of third-party software packages. For example, in Node.js the NPM package manager is the ubiquitous way of publishing and obtaining software. For web browsers, bower can be used, although its adoption is not as widespread as NPM.

Because of these differences, reuse between JavaScript environments is not optimal, in particular for packages that have dependencies on functionality that is not part of JavaScript's core API.

In this blog post, I will describe our experiences with porting the simple-xmpp from the Node.js ecosystem to Titanium. This library has dependencies on Node.js-specific APIs, but with our porting strategy we were able to integrate it in our Titanium app without making any modifications to the original package.

Motivation: Adding chat functionality


Not so long ago, me and my colleagues have been looking into extending our mobile app product-line with chat functionality. As described in an earlier blog post, we use Titanium for developing our mobile apps and one of my responsibilities is automating their builds with Nix (and Hydra, the Nix-based continuous integration server).

Developing chat functionality is quite complex, and requires one to think about many concerns, such as the user-experience, security, reliability and scalability. Instead of developing a chat infrastructure from scratch (which would be much too costly for us), we have decided to adopt the XMPP protocol, for the following reasons:

  • Open standard. Everyone is allowed to make software implementing aspects of the XMPP standard. There are multiple server implementations and many client libraries available, in many programming languages including JavaScript.
  • Decentralized. Users do not have to connect to a single server -- a server can relay messages to users connected to another server. A decentralized approach is good for reliability and scalability.
  • Web-based. The XMPP protocol is built on technologies that empower the web (XML and HTTP). The fact that HTTP is used as a transport protocol, means that we can also support clients that are behind a proxy server.
  • Mature. XMPP has been in use for a quite some time and has some very prominent users. Currently, Sony uses it to enrich the PlayStation platform with chat functionality. In the past, it was also used as the basis for Google and Facebook's chat infrastructure.

Picking a server implementation was not too hard, as ejabberd was something I had experience with in my previous job (and as an intern at Philips Research) -- it supports all the XMPP features we need, and has proven to be very mature.

Unfortunately, for Titanium, there was no module available that implements XMPP client functionality, except for an abandoned project named titanium-xmpp that is no longer supported, and no longer seems to work with any recent versions of Titanium.

Finding a suitable library


As there was no working XMPP client module available for Titanium and we consider developing such a client for Titanium from scratch too costly, we first tried to fix titanium-xmpp, but it turned out that too many things were broken. Moreover, it used all kinds of practices (such as an old fashioned way of module loading through Ti.include()) that have been deprecated a long time ago.

Then we tried porting other JavaScript-based libraries to Titanium. The first candidate was strophe.js which is mainly browser-oriented (and can be used in Node.js through phantomjs, an environment providing a non-interactive web technology stack), but had too many areas that had to be modified and browser-specific APIs that require substitutes.

Finally, I discovered node-xmpp, an XMPP framework for Node.js that has a very modular architecture. For example, the client and server aspects are very-well separated as well as the XML parsing infrastructure. Moreover, we discovered simple-xmpp, a library built on top of it to make a number of common tasks easier to implement. Moreover, the infrastructure has been ported to web browsers using browserify.

Browserify is an interesting porting tool -- its main purpose is to provide a replacement for the CommonJS module system, which is a first-class citizen in Node.js, but unavailable in the browser. Browserify statically analyzes closures of CommonJS modules, and packs them into a single JavaScript file so that the module system is no longer needed.

Furthermore, browserify provides browser-equivalent substitutes for many core Node.js APIs, such as events, stream and path, making it considerably easier to migrate software from Node.js to the browser.

Porting simple-xmpp to Titanium


In addition to browserify, there also exists a similar approach for Titanium: titaniumifier, that has been built on top of the browserify architecture.

Similar to browserify, titaniumifier also packs a collection of CommonJS modules into a single JavaScript file. Moreover, it constructs a Titanium module from it, packs it into a Zip file that can be distributed to any Titanium developer so that it can be used by simply placing it into the root folder of the app project and adding the following module requirement to tiapp.xml:

<module platform="commonjs">ti-simple-xmpp</module>

Furthermore, it provides Titanium-equivalent substitute APIs for Node.js core APIs, but its library is considerably more slim and incomplete than browserify.

We can easily apply titatiumifier to simple-xmpp, by creating a new NPM project (package.json file) that has a dependency on simple-xmpp:

{
  "name": "ti-simple-xmpp",
  "version": "0.0.1",
  "dependencies": {
    "simple-xmpp": "1.3.x"
  }
}

and a proxy CommonJS module (index.js) that simply exposes the Simple XMPP module:

exports.SimpleXMPP = require('simple-xmpp');

After installing the project dependencies (simple-xmpp only) with:

$ npm install

We can attempt to migrate it to Titanium, by running the following command-line instruction:

$ titaniumifier

In my first titaniumify attempt, the tool says that some mandatory Titanium specific properties, such as a unique GUID identifier, are missing that need to be added to package.json:

"titaniumManifest": {
    "guid": "76cb731c-5abf-3b79-6cde-f04202e9ea6d"
},

After adding the missing GUID property, a CommonJS titanium module gets produced that we can integrate in any Titanium project we want:

$ titaniumifier
$ ls
ti-simple-xmpp-commonjs-0.0.1.zip

Fixing API mismatches


With our generated CommonJS package, we can start experimenting by creating a simple app that only connects to a remote XMPP server, by adding the following lines to a Titanium app's entry module (app.js):

var xmpp = require('ti-simple-xmpp').SimpleXMPP;

xmpp.connect({
    websocket: { url: 'ws://myserver.com:5280/websocket/' },
    jid : 'username@myserver.com',
    password : 'password',
    reconnect: true,
    preferred: 'PLAIN',
    skipPresence: false
});

In our first trial run, the app crashed with the following error message:

Object prototype may only be an Object or null

This problem seemed to be caused by the following line that constructs an object with a prototype:

ctor.prototype = Object.create(superCtor.prototype, {

After adding a couple of debugging statements in front of the Object.create() line that print the constructor and the prototype's properties, I noticed that it tries to instantiate a stream object (not a constructor function) without a prototype member. Referring to a prototype that is undefined, is apparently not allowed.

Deeper inspection revealed the following code block:

/*<replacement>*/
var Stream;
(function() {
  try {
    Stream = require('st' + 'ream');
  } catch (_) {} finally {
    if(!Stream) Stream = require('events').EventEmitter;
  }
})();
/*</replacement>*/

The above code block attempts to load the stream module, and provides the event emitter as a fallback if it cannot be loaded. The stream string has been scrambled to prevent browserify to statically bundle the module. It appears that the titaniumifier tool provides a very simple substitute that is an object. As a result, it does not use the event emitter as a fallback.

We can easily fix the stream object's prototype problem, by supplying it with an empty prototype property by creating a module (overrides.js) that modifies it:

try {
    var stream = require('st' + 'ream');
    stream.prototype = {};
} catch(ex) {
    // Just ignore if it didn't work
}

and by importing the overrides module in the index module (index.js) before including simple-xmpp:

exports.overrides = require('./overrides');
exports.SimpleXMPP = require('simple-xmpp');

After fixing the prototype problem, the next trial run crashed the app with the following error message:

undefined is not an object (evaluation process.version.slice)

which seemed to be caused by the following line:

var asyncWrite = !process.browser && [ 'v0.10', 'v0.9.'].indexOf(process.version.slice(0, 5)) > -1 ? setImmediate : processNextTick;

Apparently, titaniumifier does not provide any substitute for process.version and as a result invoking the slice member throws an exception. Luckily, we can circumvent this by making sure that process.browser yields true, by adding the following line to the overrides module (overrides.js):

process.browser = true;

The third trial run crashed the app with the following message:

Can't find variable: XMLHttpRequest at ti-simple.xmpp.js (line 1943)

This error is caused by the fact that there is no XMLHttpRequest object -- an API that a web browser would normally provide. I have found a Titanium-based XHR implementation on GitHub that provides an identical API.

By copying the xhr.js file into our project, wrapping it in a module (XMLHttpRequest.js), we can provide a constructor that is identical to the browser API:

exports.XMLHttpRequest = require('./xhr');

global.XMLHttpRequest = module.exports;

By adding it to our index module:

exports.overrides = require('./overrides');
exports.XMLHttpRequest = require('./XMLHttpRequest');
exports.SimpleXMPP = require('simple-xmpp');

we have provided a substitute for the XMLHttpRequest API that is identical to a browser.

In the fourth run, the app crashed with the following error message:

Can't find variable: window at ti-simple-xmpp.js (line 1789)

which seemed to be caused by the following line:

var WebSocket = require('faye-websocket') && require('faye-websocket').Client ? require('faye-websocket').Client : window.WebSocket

Apparently, there is no window object nor a WebSocket constructor, as they are browser-specific and not substituted by titaniumifier.

Fortunately, there seems to be a Websocket module for Titanium that works both on iOS and Android. The only inconvenience is that its API is similar, but not exactly identical to the browser's WebSocket API. For example, creating a WebSocket in the browser is done as follows:

var ws = new WebSocket("ws://localhost/websocket");

whereas with the TiWS module, it must be done as follows:

var tiws = require("net.iamyellow.tiws");

var ws = tiws.open("ws://localhost/websocket");

These differences make it very tempting to manually fix the converted simple XMPP library, but fortunately we can create an adapter that has an identical interface to the browser's WebSocket API, translating calls to the Titanium WebSockets module:

var tiws = require('net.iamyellow.tiws');

function WebSocket() {
    this.ws = tiws.createWS();
    var url = arguments[0];
    this.ws.open(url);

    var self = this;
    
    this.ws.addEventListener('open', function(ev) {
        self.onopen(ev);
    });
    
    this.ws.addEventListener('close', function() {
        self.onclose();
    });
    
    this.ws.addEventListener('error', function(err) {
        self.onerror(err);
    });
    
    this.ws.addEventListener('message', function(ev) {
        self.onmessage(ev);
    });
}

WebSocket.prototype.send = function(message) {
    return this.ws.send(message);
};

WebSocket.prototype.close = function() {
    return this.ws.close();
};

if(global.window === undefined) {
    global.window = {};
}

global.window.WebSocket = module.exports = WebSocket;

Adding the above module to the index module (index.js):

exports.overrides = require('./overrides');
exports.XMLHttpRequest = require('./XMLHttpRequest');
exports.WebSocket = require('./WebSocket');
exports.SimpleXMPP = require('simple-xmpp');

seems to be the last missing piece in the puzzle. In the fifth attempt, the app seems to properly establish an XMPP connection. Coincidentally, all the other chat functions also seem to work like a charm! Yay! :-)

Conclusion


In this blog post, I have described a process in which I have ported simple-xmpp from the Node.js ecosystem to Titanium. The process was mostly automated, followed by a number of trial, error and fix runs.

The fixes we have applied are substitutes (identical APIs for Titanium), adapters (modules that translate calls to a particular API into a calls to a Titanium-specific API) and overrides (imperative modifications to existing modules). These changes did not require me to modify the original package (the original package is simply a dependency of the ti-simple-xmpp project). As a result, we do not have to maintain a fork and we have only little maintenance on our side.

Limitations


Although the porting approach seems to fit our needs, there are a number of things missing. Currently, only XMPP over WebSocket connections are supported. Ordinary XMPP connections require a Titanium-equivalent replacement for Node.js' net.Socket API, which is completely missing.

Moreover, the Titanium WebSockets library has some minor issues. The first time we tested a secure web socket wss:// connection, the app crashed on iOS. Fortunately, this problem has been fixed now.

References


The ti-simple-xmpp package can be obtained from GitHub. Moreover, I have created a bare bones Alloy/Titanium-based example chat app (XMPPTestApp) exposing most of the library's functionality. The app can be used on both iOS and Android:


Acknowledgements


The work described in this blog post is a team effort -- Yiannis Tsirikoglou first attempted to port strophe.js and manually ported simple-xmpp to Titanium before I managed to complete the automated approach described in this blog post. Carlos Henrique Lustosa Zinato provided us Titanium-related advice and helped us diagnosing the TiWS module problems.

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