.htaccess: How to “Specify a cache validator”?

Question

I'm running Google PageSpeed on my site and it's tell me that I need to
"Specify a cache validator."

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:

... then it lists images, CSS, JS, etc.

According to http://code.google.com/speed/page-speed/docs/caching.html#LeverageBrowserCaching:

Set the Last-Modified date to the last time the resource was changed. If the Last-Modified date is sufficiently far enough in the past, chances are the browser won't refetch it.

I have the following in my .htaccess:

<IfModule mod_headers.c>
    <FilesMatch "\.(bmp|css|flv|gif|ico|jpg|jpeg|js|pdf|png|svg|swf|tif|tiff)$">
        Header set Last-Modified "Tue, 31 Aug 2010 00:00:00 GMT"
    </FilesMatch>
</IfModule>

What am I doing wrong?

Answer 1

I think the problem you are having is with Expire: and not with Last-Modified:. Apache would by default send the file Last-Modified: header based on the file date. I suggest removing the upper code and replacing it with the following:

<IfModule mod_expires.c>
    ExpiresActive On
    ExpiresDefault "access plus 1 year"
</IfModule>

Try with that, if it didn't work try adding this as well:

<IfModule mod_headers.c>
    <FilesMatch "\.(bmp|css|flv|gif|ico|jpg|jpeg|js|pdf|png|svg|swf|tif|tiff)$">
        Header set Last-Modified "Mon, 31 Aug 2009 00:00:00 GMT"
    </FilesMatch>
</IfModule>

Answer 2

In order to "Set A Cache Validator" you need to send the following in your headers:

Expires or Cache-Control: max-age

AND

Last-Modified or ETag

So, for example, in PHP you could add the following for CSS and JS files:

<filesMatch "\.(js|css)$">
    Header set Expires "Thu, 21 May 2013 20:00:00 GMT"
    Header set Last-Modified "Thu, 21 May 2012 20:00:00 GMT"
</filesMatch>

This will satisfy Google's Pagespeed calculator.

Answer 3

I tested all of above codes, But see no change in gtmetrix rank. Using this improved Cache-Control (Specify a cache validator) rank for my wordpress site:

## EXPIRES CACHING ##
<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType image/jpg "access 1 year"
ExpiresByType image/jpeg "access 1 year"
ExpiresByType image/gif "access 1 year"
ExpiresByType image/png "access 1 year"
ExpiresByType text/css "access 1 month"
ExpiresByType application/pdf "access 1 month"
ExpiresByType text/x-javascript "access 1 month"
ExpiresByType application/x-shockwave-flash "access 1 month"
ExpiresByType image/x-icon "access 1 year"
ExpiresDefault "access plus 1 year"
</IfModule>
## EXPIRES CACHING ##

<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=2592000, 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>

I recommend you to customize max-age values for your site and it's files yourself.