I have a requirement in a web application that I am developing in VB.Net to have the ability to take the site offline for maintenance. I have that part down by storing a Boolean value in a table and checking it on the page load of the pages that should be offline. This works fine as it only allows people in the proper roles to access the necessary pages.
However, I need to warn the users that the site is going down in specified amount of time regardless of what page they are on and then have it be unavailable. Can someone direct me on the best way to give the logged in users a pop-up message or other visual notification that they need to save their work because the site is going down?