Making Content Expire
A common problem when creating ASP pages is that although the data usually
comes 'live' from the database, the browser often caches that information. Although
this can help to reduce the bandwidth usage for your site, it also means that
if the user has loaded the page before, he/she could be viewing out of date
or inaccurate information.
There are a number of ways around this. First, you can set
Response.Expires = -1
which means that the page will always expire, and the browser will load a
new copy. Alternatively, you can set a date on which the page will expire:
Response.ExpiresAbsolute = "5 December 2001"
or, simply the number of seconds in the future:
Response.Expires = 1000 'expires after 1000 seconds
However, in situations such as a shopping cart, even this is not enough. You
will find that if the user presses the back button on their browser, more often
that not, it doesn't bother re-loading the page... which can cause problems
if the user has just completed the order, and then goes back to a full shopping
cart. To get around this, there is one last trick... add this code:
'date in the past...
Response.AddHeader "Expires", "Mon, 26 Jul 1997 05:00:00 GMT"
'always modified
Response.AddHeader "Last-Modified", Now & " GMT"
'HTTP/1.1
Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
'HTTP/1.0
Response.AddHeader "Pragma", "no-cache"
'last ditch attempt!
Response.Expires = -1
This code adds a number of fields to the header information sent to the browser,
and seems to be effective whatever the browser!