Forcing a single image on a website not to cache?

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teamsteve

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#31797 31-Mar-2009 09:10

I've got one image on this website which is dynamically generated (ie: <img src="my_dynamic_image.asp" />) but it looks like it's caching for some people, so they aren't seeing the newest version of it all the time

Is there a way to set this one file to not be cached?

Thanks!

freitasm

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#204354 31-Mar-2009 09:19

Since this is a dynamic page, couldn't you add something like a GUID to the end of its name as a parameter, such as my_dynamic_image.asp?v=[GUID]...

This way the browser will see each request as a different one, always...

teamsteve

59 posts

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#204359 31-Mar-2009 09:34

freitasm: Since this is a dynamic page, couldn't you add something like a GUID to the end of its name as a parameter, such as my_dynamic_image.asp?v=[GUID]...

This way the browser will see each request as a different one, always...

It's not really an option, the site is using an off the shelf shopping cart which I'm reluctant to change the code of (although I've just found that my customer doesn't seem so restrained in changing my code

)

I've just added...

    <meta http-equiv="cache-control" content="no-cache" />
    <meta http-equiv="pragma"        content="no-cache" />
    <meta http-equiv="expires"       content="0" />

... to the head of the pages containing the dynamic image which should stop the whole pages from being cached, but if there were a way to localise that to a single file that'd be ace

RedJungle

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#204376 31-Mar-2009 11:16

I would use the method Mauricio outlined as it's the most reliable. Add a random guid or timestamp parameter to the image URL.

freitasm

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#204378 31-Mar-2009 11:17

Fair enough although adding a GUID as a parameter to that dynamically generate image would be one additional line and one change to the output - less than your changes I think...

slartibartfast

61 posts

Master Geek

#204379 31-Mar-2009 11:18

If you don't want to change the code, then you could do a slight variant of what Mauricio is proposing.

Assuming you can put a proxy of some sort in front of the website (Squid perhaps?) and also assuming that the URI you're trying to prevent caching on is constant then you could put a rule in to rewrite any occurence of the URI and append to it ?v={timestamp}.

Not as nice as Mauricio's suggestion but might work

freitasm

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#204380 31-Mar-2009 11:19

set TypeLib = Server.CreateObject("Scriptlet.TypeLib")
?version=[% =left(cstr(TypeLib.Guid),38) %]
set Typelib = nothing

mjb

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#204443 31-Mar-2009 15:39

teamsteve: I've just added...

???
???
???

... to the head of the pages containing the dynamic image which should stop the whole pages from being cached, but if there were a way to localise that to a single file that'd be ace

You want to send extra headers before the image in the dynamic image page. not sure how you do it in asp (getting hives here), but in php it's:

header("Cache-Control: no-cache, must-revalidate");
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // (Just an old date)

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mjb

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#204444 31-Mar-2009 15:40

Oh look.... here it is: http://support.microsoft.com/kb/234067.

So, you want asp code:

<%
Response.CacheControl = "no-cache"
Response.AddHeader "Pragma", "no-cache"
Response.Expires = -1
%>

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