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Posted by Edward Diener on 01/25/08 18:20
I am working on a web application which programatically changes an image
on the server side ( via ImageMagick ) on the roundtrip between client
and server. When the web page, in which the img tag resides, is
redisplayed, I need Firefox to reread the web page in order to redisplay
the image correctly rather than take the image from its cache.
I have been told that the only reliable way to do this is by using the
content header of the web page to specify certain values which,
according to W3C, browsers are supposed to follow to tell it how to
treat the cache. I have done this, according to the documentation I have
found, and yet the browser does not redisplay the image from the URL but
continues to display it by using its cache. This displays the image
incorrectly since it has changed on the server in the roundtrip between
client and server and back again.
The problem occurs in Firefox ( 2.0.0.11 ) and with IE6 and IE7 also. I
have not tried to test it with any other browsers since the
specification for the web application is that it is only guaranteed to
work with Firefox and IE. The fact that the problem exists with IE as
well as Firefox suggests that some combination in the content header is
not correct to force browsers to not use the cache but instead to reread
the data from the img URL.
Here is the content header, without the cookies, from such a page, as
extracted live:
--------------------------------------------------------
Content-type: text/plain
HTTP/1.1 200 OK
Server: ""
Date: Thu, 24 Jan 2008 16:01:45 GMT
Content-type: text/html
Pragma: no-cache
Cache-control: private,no-cache,no-store,must-revalidate
Expires: -1
--------------------------------------------------------
If anybody can see what is incorrect in the content header and tell me
what I have to change in order to cause the browser to redisplay the
image from the URL and not from its cache, it would be highly
appreciated by me. Getting browsers to redisplay the image correctly,
from changes made on the server side, is crucial to this particular
application, and this problem has persisted for much of the life of the
application without a resolution.
Thank you !
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