There is an AJAX engine between User and Server which was not the case with standard web application model. So, first of all, AJAX engine is loaded in the browser which is responsible for user interface interaction and server communications.

HTTP request sent to server via AJAX engine, which will be fired using a Javascript call to AJAX engine for user interactions. There will be cases where some interaction doesn't require to be passed to server such as data validation, data editing in memory etc. For such actions, AJAX engine will take care off. But for actions where AJAX engine needs inputs from the server like data processing, retrieving new data etc and makes those requests asynchronously - independent of communication with the server, using XML.

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