Apache Web Server and Yunohost Core – Cache Invalidation Interoperability Question

Targeted Cache Invalidation for YunoHost Applications

To invalidate cache entries for a specific YunoHost application without performing a system-wide purge, you must target the specific cache keys generated by mod_cache_disk. Because YunoHost manages Apache virtual hosts, the cache keys are typically derived from the URL and the server's cache root directory.

Mechanism for Mapping and Targeting

Apache's mod_cache_disk stores content in a hashed directory structure based on the URL. There is no native "app-to-folder" mapping provided by YunoHost core; instead, Apache maps the full request URI to a specific file on disk. To programmatically target a specific app, you cannot simply delete a folder named after the app. You must either:

• Use mod_cache_purge: If installed, this allows sending an HTTP PURGE request to a specific URL, which instructs Apache to remove that specific entry.
• Manual Disk Cleanup: Identify the cache root (defined by CacheRoot in Apache config) and use a script to find and remove files associated with the app's domain.

Implementing a Lifecycle Hook

A custom hook can be integrated into the YunoHost app-update lifecycle. Since YunoHost uses a set of scripts to manage app installation and updates, you can trigger a purge script post-update. Assuming mod_cache_purge is enabled, the hook would execute a curl command targeting the application's primary URLs:

# Example: Purging the home page of a specific app
curl -X PURGE http://app.yourdomain.tld/

If mod_cache_purge is not available, the hook must trigger a shell script to clear the CacheRoot directory. Note that without mod_cache_purge, granular invalidation is difficult because the disk layout is hashed, not hierarchical by domain.

Interoperability with .htaccess and Reverse Proxies

The interaction between these layers depends on where the cache resides:

• .htaccess: Rules defining Cache-Control headers affect the TTL (how long Apache stores the item) but do not dictate how the cache is purged. A manual purge overrides these headers.
• External Proxies (Varnish): A purge sent to Apache will not invalidate a cache in Varnish. If a reverse proxy is present, the lifecycle hook must send a purge request to the Varnish IP/port specifically, otherwise, the proxy will continue serving stale content even if Apache is updated.

Verification and Diagnostics

To verify if your environment supports targeted purging, run the following command to check for the purge module:

apache2ctl -M | grep cache_purge

Missing Diagnostic Detail: Please specify if you are using a custom CacheRoot configuration or if you have a Varnish instance positioned in front of your YunoHost server, as this changes the required purge target.