Why, sometimes the full content is returned unchanged, while If-Modified-Since is set?

Question

my php-generated thumbnail files and combined javascript and css files (all having their expiration dates anc cache=public set etc) return the full content sometimes, other times they load from proxy cache!

This is the comment that redbot.org gives, sometimes:

An If-Modified-Since conditional request returned the full content unchanged.

*HTTP allows clients to make conditional requests to see if a copy that they hold is still valid. Since this response has a Last-Modified header, clients should be able to use an If-Modified-Since request header for validation. RED has done this and found that the resource sends a full response even though it hadn't changed, indicating that it doesn't support Last-Modified validation.**

the PHP thumbnail generator header

header ("Content-type: image/jpeg");
header ("Last-Modified: " . gmdate("D, d M Y H:i:s", time() - 404800000)." GMT");
header ("Expires: " . gmdate("D, d M Y H:i:s", time() + 2419200) . " GMT");
header ("Cache-Control: public, max-age=2419200");

htaccess

<IfModule mod_headers.c>
ExpiresActive On
ExpiresDefault M172800

##### DEFAULT EXPIRES
<FilesMatch "\\.(ico|jpg|png|gif|svg|swf|css|js|fon|xml|pdf|flv)$">
    ExpiresDefault M1209600
    Header set Cache-Control "max-age=1209600, public"
</FilesMatch>

##### DYNAMIC PAGES
<FilesMatch "\\.(php|cgi|pl)$">
    ExpiresDefault M7200
    Header set Cache-Control "public, max-age=7200"
</FilesMatch>

Header unset Pragma
Header unset ETag
Header unset Last-Modified
FileETag None
</IfModule>

Update: New Info:

google SpeedTest gives 91/100 score on the top it says: and list all the files except the php generated files that have a explicit expire header set ((which still return full content!??) it says:

The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources.

Sameer · Answer 1 · 2011-03-03T16:39:02.050

Can you use Firebug and paste the HTTP response headers. I have a feeling that even though in your code you are setting cache control to Public, Apache is overriding that because for Php File type you are setting Cache-Control to private.

One thing you can do it remove the cache settings for Dynamic Pages from Apache configuration. That should fix the problem because a Proxy doesn't cache a response without the correct headers.

EDIT

Hi Sam, Revisiting you question I found the solution to the problem. The following code snippet appears to be a problem. In the output of your Php the Last-Modified header always changes and when a browser sends a 304 If modified request it see a change and therefore re-requests that content.

header ("Last-Modified: " . gmdate("D, d M Y H:i:s", time() - 404800000)." GMT");

Unset Last-Modified and ETags from your content to speed up the website. This site provides some excellent tips too.
http://www.askapache.com/htaccess/apache-speed-last-modified.html

score 0 · Answer 2 · edited Nov 15 '12 at 00:16

I added the following code in my htaccess file when http://pagespeed.googlelabs.com/ in chrome I got Leverage browser caching which is now solved but now it gives error of Specify a cache validator, how to do it else I have already added cache controls, below is the code

<"ifModule mod_gzip.c>

  mod_gzip_on Yes

  mod_gzip_dechunk Yes

  mod_gzip_item_include file \\.(html?|txt|css|js|php|pl)$

  mod_gzip_item_include handler ^cgi-script$

  mod_gzip_item_include mime ^text/.*

  mod_gzip_item_include mime ^application/x-javascript.*

  mod_gzip_item_exclude mime ^image/.*

  mod_gzip_item_exclude rspheader ^Content-Encoding:.*gzip.*

<"/ifModule>

<"ifModule mod_expires.c>

  ExpiresActive On

  ExpiresDefault "access plus 1 seconds"

  ExpiresByType text/html "access plus 1 seconds"

  ExpiresByType image/gif "access plus 2592000 seconds"

  ExpiresByType image/jpeg "access plus 2592000 seconds"

  ExpiresByType image/png "access plus 2592000 seconds"

  ExpiresByType text/css "access plus 604800 seconds"

  ExpiresByType application/x-javascript "access plus 216000 seconds"

<"/ifModule>

<"ifModule mod_headers.c>

  <"filesMatch "\\\\.(ico|pdf|flv|jpg|jpeg|png|gif|swf)$">

    Header set Cache-Control "max-age=2592000, public"

  <"/filesMatch>

  <"filesMatch "\\\\.(css)$">

    Header set Cache-Control "max-age=604800, public"
  <"/filesMatch>
  <"filesMatch "\\\\.(js)$">

    Header set Cache-Control "max-age=216000, private"

  <"/filesMatch>

  <"filesMatch "\\\\.(xml|txt)$">

    Header set Cache-Control "max-age=216000, public, must-revalidate"

  <"/filesMatch>

  <"filesMatch "\\\\.(html|htm|php)$">

    Header set Cache-Control "max-age=1, private, must-revalidate"

  <"/filesMatch>

<"/ifModule>

<"ifModule mod_headers.c>

  Header unset ETag

<"/ifModule>

FileETag None

<"ifModule mod_headers.c>

  Header unset Last-Modified

<"/ifModule>

Why, sometimes the full content is returned unchanged, while If-Modified-Since is set?

An If-Modified-Since conditional request returned the full content unchanged.

Update: New Info:

2 Answers2