What is the difference among synchronous postback and asynchronous postback?
The difference between synchronous and asynchronous postback is as follows:
Asynchronous postback renders only the needed part of the page; whereas, synchronous postback renders the whole page for any postback.
Asynchronous postback implements only one postback at a time, that is, if you have two buttons doing asynchronous postback, the actions will be performed one by one; whereas, synchronous postback executes all the actions at once.
Asynchronous postback only changes the update panel that raises the postback; whereas, synchronous postback modifies the entire page.