Ajax Programming

Brief history
Ajax is only a name given to a set of tools that were previously existing.
The main part is XMLHttpRequest, a class usable in JavaScript , that was implemented into Internet Explorer since the 4.0 version.
The same concept was named XMLHTTP some times, before the Ajax name becomes commonly used.
The use of XMLHttpRequest in 2005 by Google, in Gmail and GoogleMaps has contributed to the success of this format. But this is the name Ajax itself that made the technology so popular.

Why to use Ajax?
Mainly to build a fast, dynamic website, but also to save resources.
For improving sharing of resources, it is better to use the power of all the client computers rather than just an unique server and network. Ajax allows to perform processing on client computer (in JavaScript) with data taken from the server.
The processing of web page formerly was only server-side, using web services or PHP scripts, before the whole page was sent within the network.
But Ajax can selectively modify a part of a page displayed by the browser, and update it without the need to reload the whole document with all images, menus, etc...
For example, fields of forms, choices of user, may be processed and the result displayed immediately into the same page.

What is Ajax in depth?
Ajax is a set of technologies, supported by a web browser, including these elements:
HTML and CSS for presenting.
JavaScript (ECMAScript) for local processing, and DOM (Document Object Model) to access data inside the page or to access elements of XML file read on the server (with the getElementByTagName method for example)...
The XMLHttpRequest class read or send data on the server asynchronously.
optionally...
The DomParser class may be used
PHP or another scripting language may be used on the server.
XML and XSLT to process the data if returned in XML form.
SOAP may be used to dialog with the server.

The "Asynchronous" word, means that the response of the server while be processed when available, without to wait and to freeze the display of the page.

How does it works?
Ajax uses a programming model with display and events. These events are user actions, they call functions associated to elements of the web page.
Interactivity is achieved with forms and buttons. DOM allows to link elements of the page with actions and also to extract data from XML files provided by the server.
To get data on the server, XMLHttpRequest provides two methods:
- open: create a connection.
- send: send a request to the server.
Data furnished by the server will be found in the attributes of the XMLHttpRequest object:
- responseXml for an XML file or
- responseText for a plain text.

Take note that a new XMLHttpRequest object has to be created for each new file to load.

We have to wait for the data to be available to process it, and in this purpose, the state of availability of data is given by the readyState attribute of XMLHttpRequest.

States of readyState follow (only the last one is really useful):
0: not initialized.
1: connection established.
2: request received.
3: answer in process.
4: finished.