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.

 
 

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.

 
 

Direct Web Remoting is an open source Java library which allows for easy integration of AJAX into your web site. It allows you to call Java methods directly from Javascript

 
 

Internet Explorer 5.0 and up, Opera 7.6 and up, Netscape 7.1 and up, Firefox 1.0 and up, Safari 1.2 and up, among others.

 
 

You could build an XML document out of your recordset and send that back to the server, say you had a redord set for a “user” with the following details (name, surname, age, email)

 
 

A good framework is “Zephyr”.
It is used with php and seems to be very easy to use.
Prototype.JS Framework is available with detailed documentation, its very easy to handle AJAX and JavaScript, Introduce $ concept its very simple, For Ajax specially u don’t need to check cross broswer, Open, Close and error handling through traditional AJAX.

 
 

Microsoft first implemented the XMLHTTPRequest object in Internet Explorer 5 for Windows as an ActiveX object. It is the term coined after this support.

Asynchronous javascript and XML. Nobody invented AJAX, Jesse James Garrett of Adaptive Path coined the phrase early 2005 - but the tech was already in place much earlier than that

 
 

Current AJAX applications use polling to communicate changes data between the server and client. Some applications, such as chat applications, stock tickers, or score boards require more immediate notifications of updates to the client. Comet is an event based low latency server side push for AJAX applications. Comet communication keeps one of the two connections available to the browser open to continously communicate events from the server to the client.

 
 

JavaScript is in plain view to the user with by selecting view source of the page. JavaScript can not access the local filesystem without the user’s permission. An AJAX interaction can only be made with the servers-side component from which the page was loaded. A proxy pattern could be used for AJAX interactions with external services.

You need to be careful not to expose your application model in such as way that your server-side components are at risk if a nefarious user to reverse engineer your application. As with any other web application, consider using HTTPS to secure the connection when confidential information is being exchanged

 
 

AJAX requests should use an HTTP GET request when retrieving data where the data will not change for a given request URL. An HTTP POST should be used when state is updated on the server. This is in line with HTTP idempotency recommendations and is highly recommended for a consistent web application architecture