This guide explains how to optimize web resource caching using HTTP headers.

Preamble

• A good caching strategy can significantly improve your site's performance by avoiding the unnecessary retransmission of unchanged files.
• Web caching relies on two complementary mechanisms:
  1. The cache validity duration (via the Expires header) which indicates how long a resource can be reused without contacting the server.
  2. Conditional validation (via the Last-Modified/If-Modified-Since headers) which allows checking if a resource has changed before re-downloading it.

Configuring cache duration with Expires

The Expires header allows specifying a duration during which the browser can directly reuse resources from its local cache. Here's how to configure it in your .htaccess file:

1. Create or open the .htaccess file at the root of your site (usually in /web or /sites/domain.xyz).

2. Add the configuration of the expires module:

   <IfModule mod_expires.c>

3. Define the appropriate cache durations for each type of resource:

   ExpiresActive On
   ExpiresByType text/html "access plus 1 week"
   ExpiresByType image/jpeg "access plus 1 month"
   ExpiresByType text/css "access plus 1 month"
   ExpiresByType application/javascript "access plus 1 month"

   These directives mean that:

   • HTML pages will be cached for one week.
   • JPEG images, CSS files, and JavaScript will be kept for one month.

   Adjust these durations according to the update frequency of your resources.

4. Close the configuration section:

   </IfModule>

Conditional validation with Last-Modified

Even when a resource has expired in the cache, it is not always necessary to completely re-download it. The conditional validation mechanism allows the browser to check if its cached version is still up-to-date. This process works as follows:

1. The server automatically sends a Last-Modified header with each resource, indicating its last modification date.
   • Apache handles this natively for static files - no additional configuration is required.

2. When the browser requests the resource again, it sends an If-Modified-Since header containing the date it has in cache:

   GET /resource HTTP/1.1
   Host: www.example.com
   If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT

3. The server compares this date with the actual file modification date:
   • If the file has not changed, it simply returns a 304 Not Modified code, thus saving bandwidth.
   • If the file has been modified, it returns the new version with a 200 OK code.