Описание тега app-offline.htm

Starting from ASP.NET 2.0 app-offline.htm is the name of an html file that can be used to put an asp.net based website offline. If file with this name is placed in the root of a web application directory, ASP.NET 2.0 will shut-down the application, unload the application domain from the server, and stop processing any new incoming requests for that application. asp.net will also then respond to all requests for dynamic pages in the application by sending back the content of the app_offline.htm file, so usually this file contains a message stating the reason for site's unavailability.

This provides a convenient way to take down an application for example while lenghty maintenance operations are performed.

When app_offline.htm is present, iis will return this http status code:

HTTP/1.1 503 Service Unavailable

Once the app_offline.htm file is removed, the next request into the application will cause ASP.NET to load the application and app-domain again, and the application will continue to operate in its usual way.