How to set up caching in Apache web server

Caching in Apache improves web server performance by storing frequently requested resources. This lowers server load and shortens response times by serving content from a dedicated storage layer, making it essential in high-traffic environments.

Various caching options, such as disk-based caching with mod_cache_disk and memory-based caching with mod_socache_shmcb, address different performance goals. The right choice depends on the type of content served and the hardware resources available.

Careful configuration of Cache-Control directives determines how long content remains stored and when it is invalidated. Proper planning ensures that static and dynamic resources are cached appropriately, improving the user experience and reducing server overhead. In production environments, verifying correct caching behavior is vital for maintaining reliable service.

1. Enable the necessary caching modules in Apache.

   $ sudo a2enmod socache_shmcb expires
   Enabling module socache_shmcb.
   Enabling module expires.
   To activate the new configuration, you need to run:
     systemctl restart apache2

   Distribution with a2enmod support can simply run the command above without having to manually enable the required modules.

   Related: How to enable or disable Apache modules

2. Open the Apache configuration file using your preferred text editor.

   $ sudo vi /etc/apache2/apache2.conf

   Related: Location for Apache configuration

3. Add the required directives to enable and configure caching.

   <IfModule mod_cache_socache.c>
       CacheEnable socache /
       CacheSocache shmcb
       CacheSocacheMaxSize 512000
       CacheDefaultExpire 3600
       # Other directives as necessary
   </IfModule>

   Directive	Description
   CacheEnable	This directive defines the storage type and the URL space for the cache. Example: CacheEnable socache / enables memory-based caching (using shared object cache) for all URLs.
   CacheSocache	Defines the shared object cache provider to use for caching. In this case, shmcb indicates using a shared memory mechanism for storage.
   CacheSocacheMaxSize	Sets the maximum amount of shared memory storage in bytes. In this example, 512000 bytes (or 0.5MB) is allocated for the cache storage.
   CacheDefaultExpire	Determines the default expiration time in seconds for cached content. If no expiration time is specified in the HTTP headers, this value is used. Example: 3600 sets a default expiration time of one hour.

4. Set up cache control headers in your htaccess or virtual host file for more granular control.

   <FilesMatch ".(jpg|jpeg|png|gif|js|css)$">
     ExpiresActive On
     ExpiresDefault "access plus 1 month"
   </FilesMatch>

5. Restart Apache to apply the changes.

   $ sudo systemctl restart apache2 # Ubuntu and Debian
   $ sudo systemctl restart httpd # CentOS and Red Hat

   Related: How to manage the Apache web server service

6. Verify that caching is working by inspecting the HTTP headers of a cached resource.

   $ curl -I https://www.example.com/image.png
   HTTP/1.1 200 OK
   Date: Fri, 25 Aug 2023 09:13:40 GMT
   Server: Apache/2.4.55 (Ubuntu)
   Last-Modified: Fri, 25 Aug 2023 08:57:56 GMT
   ETag: "1f61-603bb8a696d78"
   Accept-Ranges: bytes
   Content-Length: 8033
   Cache-Control: max-age=2592000
   Expires: Sun, 24 Sep 2023 09:13:40 GMT
   Content-Type: image/png

   Look for headers like Cache-Control and ETag in the response to confirm caching.