The term Ajax represents a broad group of technologies that can be used to implement a web application. This application can communicate with the server in the background, without interfering with the current state of the page.
The following technologies are used in Ajax:
Ø HTML or XHTML and CSS are used for presentation.
Ø For the dynamic display and interaction with data, the Document Object Model (DOM) is used.
Ø XML is used for the interchange of data.
Ø XSLT is used for the manipulation of the data.
Ø For asynchronous communication, XMLHttpRequest is used.
Ø To merge all these, Java is used. I.e. client-side scripting.
Since then, there have been a number of technical developments in Ajax application. Instead of JavaScript, VBScript can also be used as client-side scripting language. But, JavaScript is popular among the languages used in Ajax programming, taking compatibility with modern browsers into consideration.
Drawbacks:
Ø Clicking the browser’s back button will not return the browser to an earlier state of the Ajax-enabled page because, the dynamically created pages using successive Ajax, do not automatically register with the browser’s History engine. But it may return to the last full page visited before the Ajax-enabled page.
Ø Dynamic web page updates make it difficult to bookmark a particular state of the application. Solution for this problem is to keep track of the URL fragment identifier.
Ø Dynamic web pages may interfere disruptively with user interactions, especially when working on an unstable internet connection. However, it also depends upon the nature of the Ajax application. For instance, editing a search field may send a request to the server for search completions. But the user may not know that a search completion pop-up is forthcoming and he might have proceeded with further action.
Ø Web applications which can be indexed publicly, should provide an alternative means of accessing the content because, most web crawlers do not execute JavaScript code. That would normally be retrieved with Ajax, thereby allowing the search engines to index it.
Ø Pages depending on the Ajax cannot be viewed in a browser with JavaScript or XMLHttpRequest disabled. Hence the user cannot see the Ajax-enabled pages properly. Similarly, devices such as PDAs, mobile phones and screen readers may not have support for the required technologies. Some screen readers even with Ajax may not be able to read properly the dynamically generated content. The easiest way to solve this problem is to switch back to non-JavaScript methods.
Ø The Same Origin Policy prevents some Ajax techniques from being used across domains. There are methods to disable this security feature. We need to use special Cross Domain communications channel embedded as an iframe within a page.
Ø Ajax-enabled interfaces dramatically increase the number of user-generated requests to web servers and their back-ends. This leads to longer response times or additional hardware needs.
Ø Some Ajax based applications are not read by screen-reading technologies. The WAI-ARIA standards help in providing hints for such case.
