Q246960: HOWTO: Force Graphics to Always Download to Client

Article: Q246960
Product(s): Internet Information Server
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbVisID600 kbDSupport kbIIS kbiis400
Last Modified: 11-MAY-2000

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The information in this article applies to:

- Microsoft Internet Information Server 4.0 
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SUMMARY
=======

Web sites that change graphic images frequently but utilize the same names for
those images face a potential problem because browsers may cache the images and
not rerequest them when they display the page.

A scenario that is becoming more common is a site that generates real-time
reports every few minutes and saves each report as an image. If browsers cache
the image, the updated reports may not be displayed.

This article describes two recommended solutions and one that is generally not
recommended.

MORE INFORMATION
================

Use one of the following two methods to make sure that graphic files expire so
that they are not cached by browsers:

- Use Internet Information Server's "Custom HTTP Headers" feature to add
  standard headers that tell the browser to either "expire" or to not cache the
  images. This technique works best when all the images are stored on the
  server's hard disk in a few folders. Configure the folders so that the
  desired headers are applied to all content served out of those folders.

  To do so, open the Internet Services Manager and navigate to the folder that
  contains the images. In the Properties dialog box for the folder, select HTTP
  Headers, and then click Add. Type a header name, such as "cache-control"
  (without the quotation marks) and its value, such as "no-cache" (without the
  quotation marks).

  Headers required to ensure that graphics are not cached include:

  cache-control:no-cache
  pragma:no-cache
  expires:0

- With applications for which image files do not have a centralized location,
  you can write an Internet Server API (ISAPI) filter that examines the mime
  type of the outgoing responses and applies a specific set of HTTP headers to
  certain packets.

  For additional information regarding ISAPI, search http://msdn.microsoft.com
  for "ISAPI" or visit the support site
  http://support.microsoft.com/support/isapi.

Note that if the browser already has a cached copy of a particular graphic when
the headers are implemented, it may be necessary to empty the browser's cache
before the browser will consistently rerequest a given image. To empty the cache
in Internet Explorer 5 or later, from the Tools menu, select Internet Options,
and then click Delete Files.

Not Recommended Due to Performance Lag
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There is one technique that was commonly used in the past that is no longer
preferred because it degrades performance. This method is to set HTTP headers on
graphic images to have the client image tag reference an Active Server Pages
(ASP) page as such:

  <img src=graphicserver.asp?id=234>

In this technique, the ASP page then uses the ID to read a specific image from
the hard drive or database and uses Response.AddHeader and Response.BinaryWrite
to stream the appropriate bits to the client with the desired HTTP headers.

This technique does offer some advantages, such as the ability to manipulate the
binary stream on the fly. But if your goal is merely to add HTTP headers, avoid
it because of the performance loss.

Additional query words: no-cache

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Keywords          : kbVisID600 kbDSupport kbIIS kbiis400 
Technology        : kbiisSearch kbiis400
Version           : winnt:4.0
Issue type        : kbhowto

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