What is a Partial Postback?

Question

I'm an experienced web developer in several different web technology stacks. I currently work in .Net and I've become curious about the Partial Postback.

I understand what the Partial Postback's function is and how to work with it programatically, but I don't like using technology that I don't understand, how is a Partial Postback implemented.

I understand HTTP Requests and Asynchronous Requests, the thing that bugs me about the Partial Postback is that it seems to be both.

Maybe I'm just missing something but it appears to me that the Partial Request does both, first firing off an Asynchronous POST Request but the Browser seems to be aware and the activity indicator starts to spin, which normally only occurs during an HTTP Request Page Render.

So can anyone shed some light on how Microsoft implemented, at the HTTP Request level, the Partial Postback?

Answer 1

From How to write your own partial postback in ASP.NET 2.0:

At the heart of the partial post back construction is the XMLHttpRequest, a DOM API. It can be used inside a web browser scripting language, such as JavaScript, to send an HTTP request directly to a web server without having to reload the entire page and handling the response from the server again within the scripting language. This data, in the form of XML, can then be used to manipulate the page elements on the client side.

When the button on the ASPX page has been clicked, a client side HTTP request is made using the XMLHttpRequest API. This request is handled by an HttpHandler on the web server. The HttpHandler receives the request, processes it, and sends back the response to the XMLHttp object on the ASPX page. The XMLHttp object in turn consumes the response and renders the appropriate UI changes without the browser having to do a full refresh of the page.

An UpdatePanel not only refreshes the controls which it contains, it also updates the ViewState value with the one obtained from the server, after processing.

Answer 2

Take a look at:

http://www.codeproject.com/KB/aspnet/PartialPostback.aspx