JavaScript 102 : Becoming Live

AJAX

AJAX (Aysnchronous JavaScript and XML) is a technique for making a new request to a server through JavaScript without reloading an entirely new page from the server.

It is made possible by the XMLHttpRequest object although more recently the Fetch API was introduced which makes it easier to work with. 

fetch('http://example.com/something.json').then(function(response) {
  return response.json();
}).then(function(data) {
  console.log(data);
});
Here is a quick AJAX example which loads content from a text file on a server (repeatedly)

This example is very simplistic but a illustrates many points. First of all, the browser loads the HTML and executes the JavaScript contained within ajax_example.html. At the bottom of the JavaScript there is an "window.addEventListener" for the "load" event which is assigned to run a function called "setup_ajax". This ensures that the "setup_ajax" function will run after the HTML page is rendered and the JavaScript functions are all defined.

Taking this a step further, we could modify the text file that is being called repeatedly by the AJAX at any time and the change would be reflected on the user's browser.

Consequently we could make a platform which enabled many people at once to experience changes to a file on the server. This could be the basis of a chat room or any other live platform that we choose.

Add in a bit of server side programming and the sky is the limit.

More Information: A Brief History of Ajax by Aaron Swartz

WebSockets

One of the drawbacks to the above method is that it can never be truly live, there is a delay in between the requests to the server (pull request). Also, each of these requests has quite a bit of overhead, each involves the creation of a socket, the sending of the request, the server handling that request, spawning whatever process it needs to and so on.

A WebSocket offers a solution to the above issues. It is a persistent connection between the client and server any change in state or new data can be instantly sent from client to server or server to client with very little latency.

WebSockets have very little to do with traditional HTTP as it is a thin wrapper on a normal TCP socket. It is however supported via JavaScript by most modern browsers (and in just about every other language on just about any other platform available).

Here is some JavaScript which illustrates WebSockets 
	var websocket = new WebSocket("ws://echo.websocket.org");  // this server no longer exists 			

	// Callbacks 
	websocket.onopen = function(evt) { alert("onopen " + evt) }; 
	websocket.onclose = function(evt) { alert("onclose " + evt) }; 
	websocket.onmessage = function(evt) { alert("onmessage " + evt.data) }; 
	websocket.onerror = function(evt) { alert("onerror " + evt) };

	// Sending
	websocket.send("hi");


As you can see, this example is connecting to a server called echo.websocket.org with the "ws" protocol. In order to support WebSockets with our own servers, we need to write an application that can understand the protocol and handle the connections. While there are a variety of different languages and servers that can do so, I prefer one, one that through it's design handles socket event style programming well.

Node.js

JavaScript (ECMAScript) engine for building server side apps
http://nodejs.org/
Uses V8

Event/Callback driven - A callback function is registered for a specific event. When that event occurs the callback method is run.

A public server

In order to run node servers that are accessible to others via the internet, we'll need somewhere to host it. At the moment, the easiest way to run a Node.js server is to use a company such as Digital Ocean which provides Virtual Private Servers. Digital Ocean has a few things going for them, first is the cost, second is the fact that you can get one running with Node.js already installed very quickly.

In order to get started, you'll need to create an account with Digital Ocean.

After that, you can go ahead and create a "droplet". A "droplet" is Digital Ocean's term for a VPS. After coming up with a hostname, I would choose the smallest size, an appropriate region (one close to you, perhaps), choose a "root" password and click "Create" at the bottom.

Take note of the IP Address of your new server, you'll need to use that quite a bit.

The Command line

Next you'll want to log into the server via the command line via SSH. To do so, open up Terminal on your Mac (or use PuTTY if you are on a PC). To connect on from the Terminal app on the Mac, you type the following: 

ssh root@ip_address

You'll need to go through the following steps to install node: 

apt-get update
apt-get install nodejs
apt-get install npm

Once you have done that, we can move on although it is probably a good idea to keep the terminal open as we'll be using it again shortly.

Uploading

In order to upload to your server, we need to use a tool that will allow us to connect via SSH to transfer the files. I have found that Fetch and Cyberduck are good tools.

To connect, choose "SFTP", under "Hostname" or "Server" use the IP address for your droplet Digital Ocean. The "username" is "root" and the "password" is what you just created in Terminal.

Chat

Here is a full chat example that we can start with:

server.js

// Express is a node module for building HTTP servers
var express = require('express');
var app = express();

// Tell Express to look in the "public" folder for any files first
app.use(express.static('public'));

// If the user just goes to the "route" / then run this function
app.get('/', function (req, res) {
  res.send('Hello World!')
});

// Here is the actual HTTP server 
var http = require('http');
// We pass in the Express object
var httpServer = http.createServer(app);
// Listen on port 80
httpServer.listen(80);

// WebSocket Portion
// WebSockets work with the HTTP server
var io = require('socket.io')(httpServer);

// Register a callback function to run when we have an individual connection
// This is run for each individual user that connects
io.sockets.on('connection', 
	// We are given a websocket object in our function
	function (socket) {
	
		console.log("We have a new client: " + socket.id);
		
		// When this user emits, client side: socket.emit('otherevent',some data);
		socket.on('chatmessage', function(data) {
			// Data comes in as whatever was sent, including objects
			console.log("Received: 'chatmessage' " + data);
			
			// Send it to all of the clients
			socket.broadcast.emit('chatmessage', data);
		});
		
		socket.on('disconnect', function() {
			console.log("Client has disconnected " + socket.id);
		});
	}
);

index.html

<html>
	<head>
		<script type="text/javascript" src="/socket.io/socket.io.js"></script>
		<script type="text/javascript">
		
			var socket = io.connect();
			
			socket.on('connect', function() {
				console.log("Connected");
			});

			// Receive from any event
			socket.on('chatmessage', function (data) {
				console.log(data);
				document.getElementById('messages').innerHTML = "" + data + 
 + "" + document.getElementById('messages').innerHTML;
			});
			
			var sendmessage = function(message) {
				console.log("chatmessage: " + message);
				socket.emit('chatmessage', message);
			};

	
		</script>	
	</head>
 <body>
 <div id="messages">
 No Messages Yet
 </div>
 <input type="text" id="message" name="message">
 <input type="submit" value="submit" onclick="sendmessage(document.getElementById('message').value);">
 </body>
</html>

Running it

To run, upload these files to a new folder on your your server (using Cyberduck)
Next, you'll have to install the socket.io module via terminal
Finally you can run the server via node. 

ssh root@YOUR_IP
cd directory_you_just_created
npm install socket.io
node server.js

Assuming you don't get any errors, you should be able to connect via your browser: http://YOUR_IP:8080/index.html