Ajax Technology Information

Ajax (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 behaviour 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 JavaScript and XML is not actually required, nor do the requests need to be asynchronous.

Techniques for the asynchronous loading of content date back to the mid 1990s. 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 enables this to be achieved. In 1999, Microsoft created the XMLHTTP ActiveX control in Internet Explorer 5, which is now supported by Mozilla, Safari and other browsers as the native XMLHttpRequest object. However, this feature only became widely known after being used by Gmail (2004) and Google Maps (2005)

The term “Ajax” itself was coined in 2005. Jesse James Garrett thought of the term “Ajax” while in the shower, when he realized the need for a shorthand term to represent the suite of technologies he was proposing to a client.

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.

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 required:

  * XHTML and CSS for presentation
  * the Document Object Model for dynamic display of and interaction with data
  * XML and XSLT for the interchange, and manipulation and display, of data, respectively
  * 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, 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 such as Echo2 and ZK enable fine grained control of a page from the server, using only standard JavaScript libraries.

  * Ajax interfaces are substantially harder to develop properly than static pages.
  * 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 anchor portion of the URL (following a #) when Ajax is run and monitoring it for changes.

  * 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.

  * Because most web crawlers do not execute JavaScript code, publicly indexable web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, to allow search engines to index it.

  * 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. In JavaScript, form submission could then be halted with “return false”.

  * 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.
  * Ajax opens up another attack vector for malicious code that web developers might not fully test for.
  * 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.
  * User interfaces can be confusing or behave inconsistently when normal web patterns are not followed.