Cache validators are defined inside response headers and HTTP requests and are used to determine if whether or not a request to retrieve files from the browser cache are valid.
These validators are important because they work out whether a request needs to be sent to the server, which uses more resources and takes longer, or if the required files can be got from the local cache, thus increasing loading speed and using less resources.
It is always going to be better to retrieve files from the browser cache because it doesn’t involve the round trips that requests to the server involve. Using a cache validator, like an etag header or Last-Modified, can help to ensure that the browser is using caching as efficiently as possible.
How to Specify a Cache Validator
When you use a we serve, such as Nginx or Apache, the Last-Modified header is included by default. When your page speed tool, like GTMetrix or Google Page Speed, come up with the suggestion that you specify a cache validator, you are required to use either Last-Modified header, Etag and Expires Header or Cache Control Max Age header.
Last Modified and Etag headers are used for assisting the browser to work out if the requested file has been altered in any way since it was requested last. Cache Control and Expires header are used for working out how long a file should be stored in the cache before a new copy is requested from the server.
If you don’t use the Expires or Cache control headers, the browser has no way of knowing how long a local cached copy should be stored before a new one is retrieved. These may already have been defined on the server you use but, if you need to define them manually and you use Nginx or Apache, you can use these code snippets:
Apache
<filesMatch “.(ico|pdf|flv|jpg|jpeg|png|gif|js|css|swf)$”>
Header set Cache-Control “max-age=84600, public”
Nginx
location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
expires 2d;
add_header Cache-Control “public, no-transform”;
}
If you wanted to add the Last Modified header straight into a dynamic file, you would need to use the gmstrftime from the PHP file, as in the next example:
header(‘Last-Modified: ‘ . gmdate(‘D, d M Y H:i:s’) . ‘ GMT’);
On occasion, you may be trying to load resources stored on another domain. In this case, your speed test tools would send you a notice that you are required to specify a cache validator for those resources.
You could ignore this notice in safety – just like assets that are loaded from fonts.googleapis.com, for example, you do not have control over setting any cache validator for the resources and there is always a chance that what was specified has been intentionally removed.
Browser caching is a very important factor in WordPress website speed and making sure that you specify the right cache validator, and that it works as it should do, is critical for ensuring that browser caching is being used efficiently and results in speedier page loading.