Ajax explained

AJAX – ASYNCHRONOUS JAVASCRIPT – CLIENT-SIDE SCRIPTING FOR FASTER LOADING – edited pnoone, source Wikipedia, 2010

Ajax (pronounced /ˈeɪdʒæks/) (shorthand for Asynchronous JavaScript and XML is a group of interrelated web development techniques used on the client-side to create interactive web applications.

With Ajax, web applications can retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing page. The use of Ajax techniques has led to an increase in interactive or dynamic interfaces on web pages. Data is usually retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not actually required, nor do the requests need to be asynchronous.

Like DHTML and LAMP, Ajax is not a technology in itself, but a group of technologies. Ajax uses a combination of HTML and CSS to mark up and style information. The DOM is accessed with JavaScript to dynamically display, and to allow the user to interact with, the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.

HISTORY

In the 1990s, most web sites were based on complete HTML pages; each user action required that the page be re-loaded from the server (or a new page loaded). This process is not efficient, as reflected by the user experience (all page content disappears then reappears, etc.). Each time a page is reloaded due to a partial change, all of the content must be re-sent instead of just the changed information. This can place additional load on the server and use excessive bandwidth.

Asynchronous loading of content first became practical when Java applets were introduced in the first version of the Java language in 1995. These allow compiled client-side code to load data asynchronously from the web server after a web page is loaded. In 1996, Internet Explorer introduced the IFrame element to HTML, which also enabled asynchronous loading.

In 1999, Microsoft created the XMLHTTP ActiveX control in Internet Explorer 5, which was later adopted by Mozilla, Safari, Opera and other browsers as the native XMLHttpRequest object. Microsoft has adopted the native XMLHttpRequest model as of Internet Explorer 7, though the ActiveX version is still supported.

The utility of background HTTP requests to the server and asynchronous web technologies remained fairly obscure until it started appearing in full scale online applications such as Outlook Web Access,(2000)[6] Oddpost (2002), and later, notably Google made a wide deployment of Ajax with Gmail (2004) and Google Maps (2005).

The term “Ajax” was coined in 2005 by Jesse James Garrett. However, a patent application covering this type of user interface was filed on September 3, 2003, thus predating the term itself by two years. This application resulted in US Patent #7,523,401 being issued to Greg Aldridge of Kokomo, IN.

On April 5, 2006 the World Wide Web Consortium (W3C) released the first draft specification for the object in an attempt to create an official web standard.

TECHNOLOGIES

The term Ajax has come to represent a broad group of web technologies that can be used to implement a web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:

*HTML or XHTML and CSS for presentation
*The Document Object Model (DOM) for dynamic display of and interaction with data
*XML for the interchange of data, and XSLT for its manipulation
*The XMLHttpRequest object for asynchronous communication
*JavaScript to bring these technologies together

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and the definition of the term Ajax. In particular, it has been noted that:

JavaScript is not the only client-side scripting language that can be used for implementing an Ajax application. Other languages such as VBScript are also capable of the required functionality.

However JavaScript is the most popular language for Ajax programming due to its inclusion in and compatibility with the majority of modern web browsers.

XML is not required for data interchange and therefore XSLT is not required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange,[10] although other formats such as preformatted HTML or plain text can also be used.

Classic Ajax involves writing ad hoc JavaScript on the client. A simpler if cruder alternative is to use standard JavaScript libraries that can partially update a page, such as ASP.Net’s UpdatePanel. Tools (or web application frameworks) such as Echo and ZK enable fine grained control of a page from the server, using only standard JavaScript libraries.

Introducing XMLHttpRequest and pseudomultithreading, it is possible to swap the role of client and server (web browser may start behaving as a server and web server may start behaving as a client) in Client-Server models.

DRAWBACKS

1) Pages dynamically created using successive Ajax requests do not automatically register themselves with the browser’s history engine, so clicking the browser’s “back” button may not return the user to an earlier state of the Ajax-enabled page, but may instead return them to the last full page visited before it. Workarounds include the use of invisible IFrames to trigger changes in the browser’s history and changing the URL fragment identifier (the portion of a URL after the ‘#’) when Ajax is run and monitoring it for changes.

2) Dynamic web page updates also make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the ‘#’) to keep track of, and allow users to return to, the application in a given state.

3) Depending on the nature of the Ajax application, dynamic page updates may interfere disruptively with user interactions, especially if working on an unstable internet connection. For instance, editing a search field may trigger a query to the server for search completions, but the user may not know that a search completion popup is forthcoming, and if the internet connection is slow, the popup list may show up at an inconvenient time, when the user has already proceeded to do something else.

4) Because most web crawlers do not execute JavaScript code,[14] publicly indexable web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, thereby allowing search engines to index it.

5) Any user whose browser does not support JavaScript or XMLHttpRequest, or simply has this functionality disabled, will not be able to properly use pages which depend on Ajax. Similarly, devices such as mobile phones, PDAs, and screen readers may not have support for the required technologies. Screen readers that are able to use Ajax may still not be able to properly read the dynamically generated content. The only way to let the user carry out functionality is to fall back to non-JavaScript methods. This can be achieved by making sure links and forms can be resolved properly and do not rely solely on Ajax.

6) The same origin policy prevents some Ajax techniques from being used across domains, although the W3C has a draft of the XMLHttpRequest object that would enable this functionality. Techniques exist to sidestep this limitation by using a special Cross Domain Communications channel embedded as an iframe within a page.

7) Ajax has its own set of vulnerabilities that developers must address. Developers familiar with other web technologies may have to learn new testing and coding methods to write secure Ajax applications.

8) Ajax-powered interfaces may dramatically increase the number of user-generated requests to web servers and their back-ends (databases, or other). This can lead to longer response times and/or additional hardware needs.