Resolving the conflict between Autoptimize and WP Super Cache’s Expert delivery method

Just before I launched this website, I began setting up WP Super Cache and Autoptimize, the former caching all of the site’s pages, and the latter aggregating the site’s JavaScript and CSS. All of which went well until I was doing final testing and noticed that browser caching had effectively been disabled for the aggregated files created by Autoptimize.

When I’ve run into plugin conflicts in the past, it was often pretty obvious which plugin was at fault. In this case, it was more ambiguous: After debugging, I found that it was WP Super Cache’s .htaccess file in the root of the wp-content/cache directory that was adding a max-age of three seconds to all files in subdirectories of that directory—including wp-content/cache/autoptimize, where Autoptimize creates its aggregated files.

If you’d like to duplicate the issue, the steps are as follows:

1. Install WordPress on an Apache server.
2. Install and activate WP Super Cache and Autoptimize.
3. Set Autoptimize to aggregate JavaScript and CSS.
4. Turn on WP Super Cache and set its delivery method to Expert.

WP Super Cache’s Expert delivery method uses mod_rewrite instead of PHP to serve files, so the plugin generates a .htaccess file when that delivery method is activated. Inside the file is this line:

Header set Cache-Control 'max-age=3, must-revalidate'

Applied to all files in the cache directory’s subdirectories, it reduces cache time to three seconds. (So if you’re refreshing very quickly, cached versions of your Autoptimize files might be served by your browser the first one or two times before requiring another trip to the server for validation.)

I think ideally WP Super Cache would create its .htaccess file directly inside each of its own subdirectories, but I suspect that when the decision was made to create that file, no other plugins were creating subdirectories in wp-content/cache. (And if another plugin was doing so, there was probably an assumption that users wouldn’t run competing caching plugins simultaneously.)

The easiest solution for end users encountering this issue is to change Autoptimize’s cache directory:

define( 'AUTOPTIMIZE_CACHE_CHILD_DIR', '/resources/' );

(That line is taken from Autoptimize’s FAQ and should be added to wp-config.php; the directory would then be created at /wp-content/resources/, though you could also use values like /assets/ or /aggregated/ depending on what you think is appropriate.)