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

Introduction

• A good caching strategy significantly improves 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.

Configuration of cache duration with Expires

The Expires header allows you to specify 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 for the expires module:

   <IfModule mod_expires.c>

3. Set 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 files will be kept for a month.

   Adjust these durations according to the frequency of updates to 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 redownload 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 a 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.