Capacity Planning

Caching and the Refresh Process

In a refresh request, Internet Explorer (the client) tells the server the datestamp of the version of a file it has in its cache, using the “If-Modified-Since” header. IIS 5.0 then determines whether the file has been modified since that time. If it has not, IIS 5.0 replies with a brief “Not Modified” response.

A static HTML page is not retrieved during a screen refresh if it has not been updated. Some publication processes copy files that haven’t been modified, which gives them new timestamps and thus “updates” them as far as the system is concerned. These files are downloaded even though they haven’t really changed. When you set up your publication process, you should make every effort to avoid this waste of resources.

By default, IIS 5.0 sets HTTP cache-control to prevent browsers from caching processed page scripts in Active Server Pages (ASP), because there is no way to guarantee that an ASP page will be the same the next time it is requested. (IIS 5.0 caches compiled, readyto-process scripts in ASP pages in its Script Engine Cache.)

Thus, under ordinary circumstances, just changing a file’s extension from .htm to .asp, without putting any script on the page, causes a screen refresh to take longer. Because IIS 5.0 checks for this condition, the extra time is minimized but not entirely eliminated.