Ajax (programming)

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Ajax (asynchronous JavaScript and XML), or AJAX, is a group of inter-related web development techniques used for creating interactive web applications. A primary characteristic is the increased responsiveness of web pages achieved by exchanging small amounts of data with the server "behind the scenes" so that entire web pages do not have to be reloaded each time there is a need to fetch data from the server. This is intended to increase the web page's interactivity, speed, functionality and usability.

Ajax is asynchronous, in that extra data is requested from the server and loaded in the background without interfering with the display and behavior of the existing page. JavaScript is the scripting language in which Ajax function calls are usually made.[1] Data is retrieved using the XMLHttpRequest object that is available to scripting languages run in modern browsers, or, alternatively, through the use of Remote Scripting in browsers that do not support XMLHttpRequest. In any case, it is not required that the asynchronous content be formatted in XML.

Ajax is a cross-platform technique usable on many different operating systems, computer architectures, and web browsers as it is based on open standards such as JavaScript and the Document Object Model (DOM). There are free and open source implementations of suitable frameworks and libraries.

1 Constituent technologies
2 History
3 Rationale
4 Advantages
- 4.1 Bandwidth usage
- 4.2 Separation of data, format, style, and function
5 Disadvantages
6 Accessibility
7 Patents
8 See also
9 Notes and references
10 External links

Constituent technologies

Ajax uses a combination of:

HTML and CSS for marking up and styling information.
The DOM accessed with JavaScript to dynamically display and interact with the information presented.
A method for exchanging data asynchronously between browser and server, therewith avoiding page reloads. The XMLHttpRequest (XHR) object is usually used, but sometimes an IFrame object or a dynamically added <script> tag is used instead.
A format for the data sent to the browser. Common formats include XML, pre-formatted HTML, plain text, and JSON. This data could be created dynamically by some form of server-side scripting.

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

History

Techniques for the asynchronous loading of content on an existing Web page date back as far as the src attribute, which could take any external URL and load a page containing JavaScript that manipulated the parent page, resulting in Ajax-like effects. This set of client-side technologies was usually grouped together under the generic term DHTML.[clarify][citation needed]

Microsoft's Remote Scripting, 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 4 and Netscape Navigator 4 and onwards. Microsoft then created the XMLHttpRequest object in Internet Explorer 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 for remote scripting to enable consistent results across different browsers, created in 2002 a user-community modification to Microsoft Remote Scripting which was created to replace the Java applet with XMLHttpRequest.[2]

Remote Scripting frameworks such as ARSCIF[3] surfaced in 2003 not long before Microsoft introduced callbacks in ASP.NET.[4]

In addition, the World Wide Web Consortium (W3C) has several recommendations that also allow for dynamic communication between a server and user agent, though few of them are well supported.[citation needed] These would include:

The object element defined in HTML 4 for embedding arbitrary content types into documents (replaces inline frames under XHTML 1.1)
The Document Object Model (DOM) Level 3 Load and Save Specification[5]

Rationale

The core reason to use Ajax-style programming is to overcome the page loading requirements of HTML/HTTP-mediated web pages. Ajax creates the necessary initial conditions for the evolution of complex, intuitive, dynamic, data-centric user interfaces in web pages. Indeed, much of the Web's innovation and evolution during the Web 2.0 era has relied upon and benefited immensely from the capabilities of an Ajax platform.[citation needed]

Web pages, unlike native applications, are loosely coupled, meaning that the data they display are not tightly bound to data sources and must be first marshaled (set out in proper order) into an HTML page format before they can be presented to a user agent on the client machine.[clarify] For this reason, web pages have to be re-loaded each time a user needs to view different datasets. By using the XMLHttpRequest object to request and return data without a re-load, a programmer bypasses this requirement and makes the loosely coupled web page behave much like a tightly coupled application, but with a more variable lag time for the data to pass through a longer "wire" to the remote web browser.

For example, in a classic desktop application, a programmer has the choice of populating a tree view control with all the data needed when the form initially loads, or with just the top-most level of data—which would load more quickly, especially when the dataset is very large.[clarify] In the second case, the application would fetch additional data into the tree control depending on which item the user selects. This functionality is difficult to achieve in a web page without Ajax.[citation needed] To update the tree based on a user's selection would require the entire page to re-load, leading to a very jerky, non-intuitive feel for the web user who is browsing the data in the tree.

As a good practice, AJAX should only be used in those pages where communication is done by POST method.[citation needed] Since POST form is generally used to avoid user bookmarking a page, the use of AJAX here perfectly sync with the purpose.

Advantages

Bandwidth usage

By generating the HTML locally within the browser, and only bringing down JavaScript calls and the actual data, Ajax web pages can appear to load relatively quickly since the payload coming down is much smaller in size, and the rest of the layout does not have to be redrawn on each update. An example of this technique is a large result set where multiple pages of data exist. With Ajax, the HTML of the page (e.g., a table structure with related <tr> and <td> tags) can be produced locally in the browser, not brought down with the first page of the document. In addition to "load on demand" of contents, some web-based applications load stubs of event handlers and then load the functions on the fly. This technique significantly cuts down the bandwidth consumption for web applications.[citation needed] In addition Ajax works on the client and shares some work of the server, reducing the server load.

Separation of data, format, style, and function

A less specific benefit of the Ajax approach is that it tends to encourage programmers to clearly separate the methods and formats used for the different aspects of information delivery via the web. Although Ajax can appear to be a jumble of languages and techniques, and programmers are free to adopt and adapt whatever works for them, they are generally propelled by the development motive itself to adopt separation among the following:[citation needed]

Raw data or content to be delivered, which is normally embedded in XML and sometimes derived from a server-side database.
Format or structure of the webpage, which is almost always built in HTML or XHTML and is then reflected and made available to dynamic manipulation in the DOM.
Style elements of the webpage: everything from fonts to picture placement are derived by reference to embedded or referenced CSS.
Functionality of the webpage, which is provided by a combination of:
1. Javascript on the client browser (Also called DHTML),
2. Standard HTTP and XMLHttp or client-to-server communication, and
3. Server-side scripting and/or programs using any suitable language preferred by the programmer to receive the client's specific requests and respond appropriately.

Disadvantages

Browser integration

The dynamically created page does not register itself with the browser history engine, so triggering the "Back" function of the users' browser might not bring the desired result.[clarify]

Developers have implemented various solutions to this problem. These solutions can involve using invisible IFrames to invoke changes that populate the history used by a browser's back button. Google Maps, for example, performs searches in an invisible IFRAME and then pulls results back into an element on the visible web page. The World Wide Web Consortium (W3C) did not include an iframe element in its XHTML 1.1 Recommendation; the Consortium recommends the object element instead.

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 '#')[6] 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. It is not, however, a complete solution.[citation needed]

Response-time

Network latency, the interval between user request and server response, must be considered carefully during Ajax development. Without clear feedback to the user,[7] preloading of data and proper handling of the XMLHttpRequest object, users might experience delays in the interface of the web application, something which they might not expect or understand. Additionally, when an entire page is rendered there is a brief moment of re-adjustment for the eye as content changes. The lack of this re-adjustment with smaller portions of the screen changing makes the latency more apparent.[clarify] The use of visual feedback (such as throbbers) to alert the user of background activity and/or preloading of content and data are often-suggested solutions to these latency issues.

Search engine optimization

Websites that use Ajax to load data which should be indexed by search engines must be careful to provide equivalent data for clients without JavaScript, as search engines do not generally execute the JavaScript code required for Ajax functionality.[citation needed] This problem is not specific to Ajax, as the same issue occurs with sites that provide dynamic data as a full-page refresh in response to, say, a form submit. (The general problem is sometimes called the deep web.)

Reliance on JavaScript and the DOM

Ajax relies on JavaScript and the browser's Document Object Model (DOM), which is often implemented differently by different browsers or versions of a particular browser. Because of this, sites that use JavaScript may need to be tested in multiple browsers to check for compatibility issues. It's common to see JavaScript code written twice—one part for Internet Explorer, another part for Mozilla compatibles[citation needed]—although this is less true with the release of IE7 and with the now-common use of JavaScript libraries such as Prototype or jQuery, which abstract browser-specific differences from the web developer.

The level of IDE support for JavaScript used to be poor,[citation needed] although it is changing with more wide-spread use of tools like Drosera, Firebug, Microsoft Script Debugger/Editor, Venkman and Dragonfly.

Finally, if the user has switched off JavaScript support, pages designed to use Ajax may not function.

Web analytics

Many web analytics solutions expect a new page to be loaded whenever new or updated content is displayed to the user, and after each step in a process such as a check-out.[citation needed] Since Ajax alters this process, developers must take care to instrument a page or a portion thereof so that it can be accurately tracked. Analytics systems which can track events other than simple page views, such as clicks of buttons or links, are most able to accommodate Ajax-using sites.[citation needed][clarify]

Accessibility

Main article: WAI-ARIA

Non-Ajax users would ideally continue to load and manipulate the whole page as a fall back, enabling 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.[clarify] For this reason it is advised to first develop a full application without Ajax, and implement Ajax enhancements as an addition only (see: Progressive enhancement methodology).[citation needed] The same counts for JavaScript in general, which can be disabled in most browsers.

Because many people[weasel words] consider it unreasonable to advise against the use of JavaScript in modern web applications, the W3C Web Accessibility Initiative (WAI) is working on a specification for Accessible Rich Internet Applications (ARIA). In effect, ARIA allows web pages, or portions of web pages, to declare themselves as applications rather than as documents. This allows Ajax interactions and other types of JavaScript-enabled UI controls to interact with assistive technology, like screen readers for the vision-impaired.

One early user agent implementation is Firefox 3, which maps WAI-ARIA to the accessibility API of the underlying operating system, making it possible for screen readers to treat ARIA widgets in much the same way as the widgets found in client-side software. Another early implementation is Fire Vox, an extension that turns Firefox into a self-voicing browser. Since Fire Vox operates directly on the DOM and does not use the operating system's accessibility API, it can handle WAI-ARIA features, such as live regions, which have no mapping to traditional desktop applications.

Ajax libraries and toolkits are also adopting ARIA. Notable examples include Google Web Toolkit, Backbase and Dojo Toolkit.[citation needed] In addition, AxsJAX, a JavaScript library which focuses on web-application accessibility, includes several abstractions built on top of ARIA.

Patents

Since the beginning of 2006, there have been 2 patents claiming Ajax domains.

The first, US patent No. 7,000,180, filed in 2001, was issued on Feb 14, 2006. In brief, is entitled methods, systems, and processes for the design and creation of Rich-Media applications via the Internet. It contains 83 claims that encompass a wide range of rich-media net application methods, systems, and processes.[citation needed]

The other, US patent No. 6,418,462, filed in 1999, was issued on July 9, 2002. It is said to "disclose methods allowing clients to perform tasks through a sideband communication channel, in addition to the main communication channel between a client and server".

The validity, effectiveness and enforceability of the claimed patents are unverified.