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perl clear cache

Posted on 2003-11-05
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Last Modified: 2012-06-27
hi all

I am working on a project that generates financial reports for a set of companies using perl and apache on linux. The problem that i'm facing is that the pages are cached and therefor if i call a report for clent A,the details of client B are displayed.

I have tried putting the foll:

#!/usr/bin/perl -w
my $month = param('month');
use lib '/var/www/cgi-bin';
use modules::module;

use CGI qw(:standard);
use CGI::Carp qw(fatalsToBrowser);

print header;
print start_html('Client A Report');
print"<meta http-equiv=expires content=0> ";
print"<META HTTP-EQUIV=pragma CONTENT=no-cache> ";
print"<body bgcolor=#E6E7E6>";
print "<table border=1 cellpadding=1 cellspacing=1 bgcolor=white>";
:
:

But this is not helping me. If I hit ctrl-F5 the correct data is dislayed,but obviously i cant ask my clent to do the same.

plz help!!!!

Anuvc

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Question by:anuvc
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Expert Comment

by:kandura
ID: 9685367
you could add an 'expires' header to the response, by using this:

print header( -expires => '-1d' );

But are you sure it is a problem in the first place? I don't think client A would have the report for client B in his cache in the first place. Not unless he requested it before.
It may be just you who is having the problem.
It's not obvious from your code, but shouldn't there be a parameter in there that tells you which client is making the request? How do you know which report to generate?
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Accepted Solution

by:
jhurst earned 63 total points
ID: 9726433
the problem with the headers such as expire and no-cache is that they just don't work erliably since many browsers ignore tham and there are intermediate places such as ISPs that sometimes cache.

When I have this problem I make data be displayed by a script such as:
showresults.cgi


That script also uses one additional parameter, not otherwise needed, I call it onetime.  So the invocation of the script is something like:
showresults.cgi?onetime=1068585726.3005
Now, the value 106.... is actually the time that the page making the invocation was created with the process id added to it.  In this manner I can ensure that no two users ever ask for the same page.  
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Assisted Solution

by:elsamman
elsamman earned 62 total points
ID: 9728328
Anuvc,

How are your clients referencing your site?  If they all use the same URL and the client identifier is in a form field you can have problems with caching since a cache only looks at the URL.  Also you want to use cache tags that are part of the header rather than meta tags.  

The bottom line is that caches are not complicated but you really need to understand how they work - then you can work out a relatively simple approach to using them.  It took me a few trials.  Check out these couple of links on caches.

http://www.mnot.net/cache_docs/
http://linux.oreillynet.com/pub/a/linux/2002/02/28/cachefriendly.html
http://www.wdvl.com/Internet/Cache/tips.html#SCRIPT

....Sam

 
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Expert Comment

by:jhurst
ID: 9728356
I do not agree with Sam, sadly there are far too many systems that do not fllow the documented rules as far as cache tags are concerned.  He is right, they are not complicated, sadly however, the rules are not observed.
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Expert Comment

by:elsamman
ID: 9728552
"I do not agree with Sam, sadly there are far too many systems that do not fllow the documented rules as far as cache tags are concerned.  He is right, they are not complicated, sadly however, the rules are not observed."

I think we both may be saying something quite similar.  I agree with you that no matter what the "book" says you can never count on a browser/proxy to go and figure out that something needs refreshing and have it ALWAYS refresh - sometimes it will just dispaly the cached copy.  You can, however,  always count on a browser calling your script if you generate a unqiue URL.  That was my point.

....Sam
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Expert Comment

by:jhurst
ID: 9732366
but you can always prevent the cache by adding something unique to the url so that the cache believes that it is different.
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Expert Comment

by:elsamman
ID: 9732427
That is exactly what I am saying.
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Expert Comment

by:jmcg
ID: 10089853
Nothing has happened on this question in more than 8 weeks. It's time for cleanup!

My recommendation, which I will post in the Cleanup topic area, is to
split points between jhurst and elsamman.

PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!

jmcg
EE Cleanup Volunteer
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