Concept of ASP.NET Callbacks
Nikhil blog about the concept and design issues of async callbacks in server control, he really did a great work and be sure to check out his little animated slide in explaining callback. The idea of callback is awesome and I'm sure lots of developers have implemented something like this in v1.* nowadays, especially for those have practical experience in using classic ASP and XMLHTTP.
As commented by Jay, I also have a query in the issues of “concurrency” and “user experience”. Callback is definitely improve the user experience and make sense to page developers (though it involve lots of complicated works behind the scene), but the callback actions are async and do it silently (forget about the fancy Cursors.Wait in client side), the concurrency issues and result of the page are sync between browser and user behaviour/mind? That said, the initial (input) values, ViewState, various event handlers (client and server side) and the result can catch up with user interactions in between these process?