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- 215.82.12.78 - - [02/Nov/2007:10:20:33 +0100] "GET /images/logo.gif HTTP/1.1" 304 -
To get rid of this unnecessary load the Apache web server is providing the module mod_expires [2] which enables you to stamp each delivered file with a kind of "valid until" mark. Therefore and according to the HTTP/1.1 specifications ([3], subsection 14.21), one line like the following is added to the http response header:
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- Expires: Thu, 01 Mar 2007 09:30:00 GMT
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- ExpiresActive On
- ExpiresDefault "access plus 5 minutes"
- ExpiresByType image/gif "access plus 2 days"
- ExpiresByType text/html "modification plus 5 minutes"
ExpiresByType is giving you the possibility to control the expiration based on mime types. In the example gif images won't be reloaded from the server for 24 hours from of the point of downloading - based on the assumption that the gif images aren't changing but rather would be replaced by new files with new names. If your site is using gif buttons with roll over effects this takes lots of requests off your apache. On the other hand: If the layout of the web page and the gif images are changed while the names of the images remain the same, this would lead to rather strange looking pages in browsers which firstly accessed the page less than 24 hours ago. So this directive, though it offers a lot of reduction of traffic, is also to be handled with care.
In the example html pages won't be reloaded if their content is younger than five minutes. Imagine a highly frequented front page of a web portal which is generated every five minutes from dynamic content but saved as static html file for reasons of performance. If this was managed by the "access" alternative, a client that just loaded the 4:59 minutes old page would miss the newer version for five minutes. Based on "modification" it's ensured that nobody would miss the latest news.
In short, use the access rule for content that doesn't change (or at least not frequently). If modifications are necessary, try to use a new name. Use the modification rule for content that often is modified or for what reasons ever has a short lifetime.
Last but not least, this way you don't only reduce the load on your apache but also speed up your web application on client side: images and javascripts that don't have to be asked for every time they are to be used maybe displayed just some fractions of a secong faster. But in the end (and at least subjectively) it all sums up.
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[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
[2] http://httpd.apache.org/docs/2.2/mod/mod_expires.html
[3] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html