Beginning Active Server Pages

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the bandwidth usage for your site, it also means that if the user has loaded the page before, he/she could be viewing out of date or inaccurate information.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"
Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
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!

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About the author

James Crowley

James Crowley United Kingdom

James first started this website when learning Visual Basic back in 1999 whilst studying his GCSEs. The site grew steadily over the years while being run as a hobby - to a regular monthly audien...

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