AJAX is not a new technology but a combination of several existing technologies in a new way. These include HTML, CSS, DOM, XML, XSLT, XMLHttpRequest and Javascript. The acronym AJAX stands for Asynchronous Javascript and XML. AJAX is based on open standards supported by many browsers and platforms. AJAX is a new paradigm for building web application.

It depends. With AJAX the answer is more in between. Control can be more centralized in a server-side component or as a mix of client-side and server-side controllers.

Centralized server-side controller - When having a more centralized controller the key is to make sure the data in client-side page is in sync with that of the server. Some applications may keep all the state on the server and push all updates to client DOM via a simple JavaScript controller.
Client and server-side controllers - This architecture would use JavaScript to do all presentation related control, event processing, page manipulation, and rendering of model data on the client. The server-side would be responsible for things such as business logic and pushing updated model data to the client. In this case the server would not have intimate knowledge of the presentation short of the initial page that would be sent to the client page request.

There are some use cases where an entire AJAX application can be written in a single page. Keep in mind if you choose this type of architecture that navigation and bookmarking should be considered.

How Ajax Can Be Used To Build Java Applications
Why Ajax Can be Useful For High Quality Web Applications
One thing that most people are looking for on the web these days is interactivity. The ability for companies to create web applications which are highly interactive will play a prominent role in their success over the web. In fact, the very nature of Ajax makes it useful for the construction of high end web applications. Ajax, which stands for Asynchronous JavaScript and XML, is an approach to web application design that differs from traditional methods. In the past, most developers had to use plug-ins and frames if they wanted their applications to work with prominent browsers. They were greatly limited in their ability to create interactive applications.

However, this was only a part of the problem. Not only were developers limited in their ability to create highly interactive web applications, they were not able to create an application that had the functionality and interactivity of a traditional desktop application. While the limitations of older web tools may not be easily noticeable to the average user, developers were very well aware of them. Understanding these limitations has been the key to their success when it comes to the introduction of Ajax.

Traditional Barriers to Ajax Applications
The page reload or refresh is testament to the limitations that were prevalent on the web before Ajax was introduced. It was a great barrier to usability, and developers were limited in their ability to correct the issue. Since the inception of the Internet, the goal of many developers has been to create web applications which are very interactive. In addition to being interactive, these applications should also be dynamic. When the user makes a change to the content on a web page, that change should be seen in near real time.

The user should not have to reload the page if they wish to see immediate changes in their actions. However, this is the way it was in the past, and it is still prominent among older websites today. However, the introduction of Ajax has changed everything. Rather than being a technology, Ajax is a technique that allows you to combine existing Java technologies in a way that allows you to bypass the page reload barrier. The fundamental concept behind Ajax is the fact that it uses client side scripting to transfer data to the web server.

Ajax Fundamentals
Not only will Ajax allow developers to create web applications with a great deal of interactivity, these programs will be so interactive that they can be likened to traditional desktop applications. Ajax is basically a pattern, a method of finding and utilizing a design technique that is efficient. While the concept of Ajax may not be new, it is new in the sense that many web developers have not yet taken advantage of it. One reason for Ajax’s sudden popularity is the fact that many popular applications such as Gmail utilize it.

The new Ajax applications are so powerful, so different from their contemporaries that many people have begin calling it a new technology altogether. These technologies have often been placed under the umbrella term which is known as Web 2.0. Since this time, Ajax has become a household name in many web development communities. Even then, its usage has not fully proliferated the web as a whole. There are a number of things which Ajax promises to create.

Ajax Tools and Applications
One of the things that Ajax can enhance are shopping carts. Since you don’t have to worry about reloading a page, a shopping cart designed with Ajax can be updated in real time. As soon as you add an item to it, this change in data will be shown automatically. One of the most important objects that has allowed Ajax to become so prominent is the XMLHttpRequest. It gives a client side script the ability to carry out requests for HTTP, and it will generate a response from the XML server. It is the interactive nature of Ajax that has allowed it to become so powerful.

There is a reason why “asynchronous” is used to describe Ajax. When an HTTP request is sent out, the user doesn’t want to wait for the browser to respond. You would much rather have it react to the direct interaction on the page.

Basic Things That Developers Must Avoid
While Ajax has been the subject of hype for its ability to make web applications much more interactive, with power comes responsibility, and Ajax can be dangerous in the hands of developers who do not know how to use it properly. While Ajax has a great deal of potential, it is far from being perfect. There are a number of UI problems that can occur with Ajax applications, and some of these problems are server sided in nature. There are a lot of mistakes that developers can make when using Ajax, and it is important for you to avoid them. One of the most common mistakes that developers make is using Ajax just because it is one of the hottest tools available on the web.

If you’re going to use Ajax, you should take into consideration more than just the hype which surrounds the tool. While Ajax may be the latest web application tool, it is not a toy. However, many of the applications which are being developed by Ajax have no useful purpose. They are designed simply because the developer things they are cool, but they offer absolutely nothing to the common user. While these applications might be fun for a certain period of time, I think it could be argued that they don’t have a great deal of longevity as far as applications are concerned.

Dealing With The Back Button
The back button is not a bad feature for most web page interfaces. The problem is, the back button doesn’t work well with JavaScript, one of the technologies which make up Ajax. A wish to maintain the back button function is one of the key reasons why it may not be a good idea to create an app that is 100 percent JavaScript. Another thing that developers will want to pay attention to is a failure to give instant visual indicators for clicking on a widget. If a user click on something that creates an Ajax action, that user should be given a clue that something is going on.

One example of a program that uses this properly is Gmail. The button is located on the right side of the top page, and when you do something in Gmail, a small red box will show that the page is loading. This is done because Ajax doesn’t use the standard cues which show that a page is being loaded. Doing this is very important, for it will show that the application is user friendly. Now that I’ve talked about this, I will next want to address the issue of leaving behind people who are offline. Web programs are pushing the boundaries farther each day, and some have said that all applications must be moved online.

The Transition From Offline to Online Applications
There are a number of reasons why many developers feel that virtually all applications should be moved from offline to online. Some of these reasons include global access, the configuration and maintenance, and the learning curve, which has a tendency to be shorter for web applications when compared to desktop applications. However, there are a number of issues that must be considered. First, not everyone has a broadband Internet connection. The vast majority of the people in the world are still using dial up, and only a small percentage are using Internet connections which are broadband or faster.

For the people who use slower connections, being force to only use web based applications can be tedious and irritating. Also, there is the possibility that some people will prefer to use these applications offline. Developers who want to succeed with Ajax must prepare for this. If you decide to simply leave these people behind, you will dramatically reduce your potential user base. Offline access should always be considered even when a web based application is being developed. Anyone who is familiar with Gmail knows that it uses POP, a feature that is not web based.

Users Don’t Want to Wait For Ajax
The biggest mistake that developers can make is developing their applications in a way that forces users to wait. This is one of the biggest downsides to Flash. A lot of webmasters build Flash intros to their site, and while users may watch these intros the first time around, they can become annoying after a few visits, because all they do is waste time. Even if you are able to click on a button to skip the Flash intro, this still takes time. With Ajax, developers must get past fancy animations and focus on functions.