Ajax (programming)

Ajax, shorthand for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user makes a change. This is meant to increase the web page's interactivity, speed, and usability.

The Ajax technique uses a combination of:

XHTML (or HTML) and CSS, for marking up and styling information.
The DOM accessed with a client-side scripting language, especially ECMAScript implementations such as JavaScript and JScript, to dynamically display and interact with the information presented.
The XMLHttpRequest object to exchange data asynchronously with the web server. In some Ajax frameworks and in certain situations, an IFrame object is used instead of the XMLHttpRequest object to exchange data with the web server.
XML is sometimes used as the format for transferring data between the server and client, although any format will work, including preformatted HTML, plain text, JSON and even EBML.

Like DHTML, LAMP and SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together.

History

The first use of the term in public was by Jesse James Garrett in his February 2005 article Ajax: A New Approach to Web Applications. Garrett thought of the term 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. At subsequent talks and seminars Garrett has made the point that Ajax is not an acronym.

Although the term "Ajax" was coined in 2005, most histories of the technologies that enable Ajax start a decade earlier with Microsoft's initiatives in developing Remote Scripting. However techniques for the asynchronous loading of content on an existing web page without requiring a full reload date back as far as the IFRAME element type (introduced in Internet Explorer 3 in 1996) and the LAYER element type (introduced in Netscape 4 in 1997, abandoned during early development of Mozilla). Both element types had a src attribute that could take any external URL, and by loading a page containing JavaScript that manipulated the parent page, Ajax-like effects could be attained. Macromedia's Flash could also, from version 4, load XML and CSV files from a remote server without requiring a browser refresh.

Microsoft's Remote Scripting (or MSRS, introduced in 1998) acted as a more elegant replacement for these techniques, with data being pulled in by a Java applet with which the client side could communicate using JavaScript. This technique worked on both Internet Explorer version 4 and Netscape Navigator version 4 onwards. Microsoft then created the XMLHttpRequest object in Internet Explorer version 5 and first took advantage of these techniques using XmlHttpRequest in Outlook Web Access supplied with the Microsoft Exchange Server 2000 release.

The Web development community, first collaborating via the microsoft.public.scripting.remote newsgroup and later through blog aggregation, subsequently developed a range of techniques for remote scripting in order to enable consistent results across different browsers. In 2002, a user-community modification to Microsoft Remote Scripting was made to replace the Java applet with XMLHttpRequest.

Remote Scripting Frameworks such as ARSCIF surfaced in 2003 not long before Microsoft introduced Callbacks in ASP.NET.

Since XMLHttpRequest is now implemented across the majority of browsers in use, alternative techniques are used infrequently. However, they are still used where compatibility with older Web sites or legacy applications is required.

Pros and cons

Pros

Performance

Ajax solves one of the problems with web pages - the full screen refresh when a user submits a form or clicks a link to go to another page. Depending on the design, this may result in increased performance, as smaller amounts of data can be transferred rather than whole pages of HTML, and the backend code and database calls executed are restricted to those needed to generate that data rather than generating the HTML for the whole page. However if the page using Ajax is not designed properly, e.g., large amounts of HTTP calls, overall performance gains may not be achieved.

Interactivity

Ajax applications are mainly executed on the user's machine, by manipulating the current page within their browser using document object model methods. Ajax can be used for a multitude of tasks such as updating or deleting records; expanding web forms; returning simple search queries; or editing category trees—all without the requirement to fetch a full page of HTML each time a change is made. Generally only small requests need to be sent to the server, and relatively short responses are sent back. This permits the development of more interactive applications featuring more responsive user interfaces due to the use of DHTML techniques.

While the Ajax platform is more restricted than the Java platform, current Ajax applications effectively fill part of the niche first served by Java applets: extending the browser with lightweight mini-applications.

Cons

Usability

One major complaint voiced against the use of Ajax in web applications is that it might easily break the expected behavior of the browser's back button. The difference between returning to a previous state of the current, dynamically modified page versus going back to a previous static page might be a subtle one; but users generally expect that clicking the back button in web applications will move their browser to the last page it loaded, and in Ajax applications this might not be the case. Developers have implemented various solutions to this problem. These solutions can involve using invisible IFRAMEs to invoke changes that do not populate the history used by a browser's back button. (However, the IFRAME technique is regarded by some as outmoded. Although the World Wide Web Consortium (W3C), a standards-setting body, has not formally deprecated IFRAME, it recommends the more general OBJECT instead.) Google Maps, for example, performs searches in an invisible IFRAME and then pulls results back into an element on the visible web page; it is possible to track user behaviour via callbacks which are called whenever the back button is pressed, restoring the application state that existed at the time.

An issue is that dynamic web page updates 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 '#' [http://www.ietf.org/rfc/rfc3986.txt) to keep track of, and allow users to return to, the application in a given state. This is possible because many browsers allow JavaScript to update the fragment identifier of the URL dynamically, so that Ajax applications can maintain it as the user changes the application's state. This solution also improves back-button support.

Response-time concerns

Network latency — or the interval between user request and server response — needs to be considered carefully during Ajax development. Without clear feedback to the user smart preloading of data and proper handling of the XMLHttpRequest object, users might experience delay in the interface of the web application, something which users might not expect or understand [http://www.lastcraft.com/blog/index.php?p=19. The use of visual feedback to alert the user of background activity and/or preloading of content and data are often suggested solutions to these latency issues.

In general the potential impact of latency has not been "solved" by any of the open source Ajax toolkits and frameworks available today, such as the effect of latency variance over time.

Accessibility

Using Ajax technologies in web applications provides many challenges for developers interested in adhering to WAI accessibility guidelines. In addition there are numerous development groups working on USA government projects which require strict adherence to Section 508 Compliance standards. Failure to comply with these standards can often lead to cancellation of contracts or lawsuits intended to ensure compliance.

Because of this, developers need to provide fallback options for users on other platforms or browsers, as most methods of Ajax implementation rely on features only present in desktop graphical browsers.

Web developers use Ajax in some instances to provide content only to specific portions of a web page, allowing data manipulation without incurring the cost of re-rendering the entire page in the web browser. Non-Ajax users would ideally continue to load and manipulate the whole page as a fallback, allowing the developers to preserve the experience of users in non-Ajax environments (including all relevant accessibility concerns) while giving those with capable browsers a much more responsive experience.

References

External links

Articles

Ajax: A New Approach to Web Applications, by Jesse James Garrett. The original article which coined the term.
Why Ajax Matters Now by Jesse James Garrett.
Weighing the Alternatives How Ajax stacks up against competing RIA approaches.
JavaScript refactoring for safer, faster, better Ajax.
Techniques to allow screen readers to read off dynamically added content

Tutorials

Ajax:Getting Started by Mozilla Developer Center.
Cross-browser XMLHttpRequest Tutorial using the Sarissa library.
Building an Ajax page with get, post, text, XML example.

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