WP Super Cache vs Proxy Cache Purge

This plugin does not install nor configure a cache proxy. It acts as an interface with such services. One common method of caching content for websites is via the use of reverse proxy caching. Common examples of this are Varnish and NGINX. These systems allow a website to update content and have the visitor’s experience cached without the need for complex plugins storing the files locally and using up a user’s disk space. A reverse proxy cache is installed in front of a server and reviews requests. If the page being requested is already cached, it delivers the cached content. Otherwise it generates the page and the cache on demand. The Proxy Cache Purge plugin sends a request to delete (aka flush) the cached data of a page or post every time it’s modified. How It Works When content on a site is updated by WordPress, the plugin reaches out to the proxy cache service with the URL of the page, requesting the cache be deleted. Not all pages are deleted from the cache on every change. For example, when a post, page, or custom post type is edited, or a new comment is added, only the following pages will purge: The front page The post/page edited Any categories, tags, and/or custom taxonomies associated with the page Related feeds Associated JSON API pages In addition, your entire cache will be deleted on the following actions: Changing themes Pressing the Empty Cache button on the toolbar Plugins can hook into the purge actions as well, to filter their own events to trigger a purge. On a multisite network using subfolders, only network admins can purge the main site. Keep Your Cache Warm Purging is only half the story. Once Proxy Cache Purge empties a page, the cache for that URL is gone, so the next visitor triggers a full uncached render and waits for it. On busy sites this “cache stampede” shows up as slow pages right after every update. Cacheability Pro is a companion plugin that closes that gap: it automatically re-warms purged URLs in the background so visitors keep hitting fast, cached responses, and it tunes your cache headers so more of your pages stay cacheable in the first place. Proxy Cache Purge handles invalidation; Cacheability Pro handles everything around it. Monitor Your Cache Proxy Cache Purge keeps your cache fresh, but how often is it actually being hit? If you run your own Varnish or NGINX server, GetPageSpeed Amplify shows your cache hit rate, request volume, and backend health over time, with a free tier. Proxy Cache Purge handles invalidation; Amplify shows you whether the cache is doing its job. Development Mode If you’re working on a site and need to turn off caching in one of two ways: Add define( 'VHP_DEVMODE', true ); to your wp-config.php file Go to Proxy Cache -> Settings and enable Debug Mode for 24 hours at a time That will break cache on page loads. It is not recommended for production! Cache Tags (BETA) As of version 5.4.0, Proxy Cache Purge includes an optional Cache Tags / Surrogate Keys purge mode. This feature is marked as BETA and is disabled by default. When enabled, the plugin: Adds cache-tag headers to WordPress responses (for example, tagging pages by post ID, post type, taxonomy terms, author, and archives). Uses tag-based purges instead of individual URL purges when content is updated, which can reduce purge traffic and improve consistency on complex sites. Requirements: A proxy cache that supports Cache Tags / Surrogate Keys and advertises this via standard Surrogate-Capability headers (for example, Surrogate-Capability: vhp="Surrogate/1.0 tags/1"). How to enable: Go to Proxy Cache → Settings → Purge Method and check “Use Cache Tags (Surrogate Keys)”. The checkbox is only enabled when your cache tells WordPress it supports tags (or when you explicitly enable it via a define). Alternatively, you can force-enable or force-disable detection via wp-config.php: define( ‘VHP_VARNISH_TAGS’, true ); // Force treat cache as tag-capable define( ‘VHP_VARNISH_TAGS’, false ); // Force treat cache as not tag-capable Because this feature depends on your cache configuration, it is recommended that you test it carefully in staging before enabling it on production. Background Purging with WP-Cron On busy sites, sending many PURGE requests directly from admin requests can slow things down. When you define DISABLE_WP_CRON as true in wp-config.php (because you are running a real system cron that calls wp-cron.php), Proxy Cache Purge automatically switches to an asynchronous mode: Purge requests (both URL-based and tag-based, when Cache Tags are enabled) are collected into a small per-site queue. The queue is processed by WP-Cron in the background, keeping your admin and content-editing actions responsive even when many URLs or tags must be invalidated. Object-cache purges (the “Purge Database Cache” option) remain synchronous and are not affected by this behaviour. The Proxy Cache settings page and Site Health integration expose basic queue status so you can verify that background purging is healthy; if the queue appears large or very old, check that your system cron is correctly invoking WordPress cron. Important: Cron Frequency and Cache Freshness When using background purging, the frequency of your system cron determines how quickly cache invalidations are processed. The longer the interval between cron runs, the longer visitors may see stale content after updates. For minimal stale content, run your system cron every minute: * * * * * /usr/bin/php /var/www/html/wp-cron.php If you can tolerate slightly longer delays, every 2-5 minutes is also acceptable. However, running cron less frequently (e.g., every 15 minutes) means cache purges may be delayed by that amount after content changes. Note: Scheduled posts are handled specially. When a scheduled post is published via WP-Cron, the cache is purged synchronously within the same cron run, ensuring immediate cache invalidation without waiting for the next cron execution. For detailed instructions on setting up a proper Linux-based WordPress cron, see: WordPress Cron Optimization. Disabling Background Purging If you have DISABLE_WP_CRON defined but do not want background purging (for example, on low-traffic sites where immediate purges are preferred), you can force-disable cron-based purging by adding this to your wp-config.php: define( 'VHP_DISABLE_CRON_PURGING', true ); With this constant set, all cache purges will execute immediately during the request, regardless of the DISABLE_WP_CRON setting. WP-CLI Purge Purge commands let you empty the cache. wp varnish purge – Flush the entire site cache (equivalent to clicking “Empty Cache” in admin) wp varnish purge --all – Explicitly flush the entire site cache wp varnish purge – Flush cache for a specific URL and all content below it (wildcard) wp varnish purge --url-only – Flush cache for only the exact URL specified (no wildcard) wp varnish purge --tag= – Flush cache by tag (requires Cache Tags mode to be enabled) Examples: wp varnish purge – Purge entire site wp varnish purge --all – Same as above, more explicit wp varnish purge https://example.com/hello-world/ – Purge this URL and everything below it wp varnish purge https://example.com/hello-world/ --url-only – Purge only this exact URL wp varnish purge https://example.com/wp-content/themes/ --wildcard – Purge all theme files wp varnish purge --tag=p-123 – Purge all pages tagged with post ID 123 wp varnish purge --tag=pt-post – Purge all cached pages of post type “post” Debug Debugging can help you figure out why your cache isn’t working as well as it could. The default is for your home page, but you can pass any URL on your domain. wp varnish debug [ ] Available parameters: [--include-headers] — Include headers in debug check output [--include-grep] — Grep active theme and plugin directories for common issues DevMode Development mode allows you to disable the cache, temporarily. wp varnish devmode [ ] – Change development mode state Async purge queue (cron-mode) When you define DISABLE_WP_CRON as true and run a real system cron for WordPress, Proxy Cache Purge can move heavy purge work into a small background queue that is processed by WP‑Cron. You can inspect and manage that queue via WP‑CLI: wp varnish queue status – show whether cron-mode is active, if a full purge is queued, counts of queued URLs/tags, and the last queue run time. wp varnish queue process – process any items currently in the queue (useful to run after deploys or cache‑sensitive operations). wp varnish queue clear – clear the queue without sending any PURGE requests. These commands do not replace your normal WordPress cron (you still need a cron entry that calls wp cron event run --due-now or hits wp-cron.php), but they give you a simple operational handle when using cron‑mode. Understanding Purge Behavior There are different types of cache purges, and they behave differently: Manual Purges (Admin Bar) “Purge Cache (All Pages)” – Sends a single regex purge request to invalidate the entire cache. Always executes immediately. “Purge Cache (this page)” – Purges only the exact URL you’re viewing. Always executes immediately. Manual purges are always immediate, even when background cron-mode is enabled. This is intentional: when you click a button, you expect immediate results. Automatic Purges (Post Save/Update) When you save or update a post, the plugin automatically purges: The post’s URL The homepage Category archive pages Tag archive pages Author archive page Date-based archives RSS feeds Related REST API endpoints This can be 20-50+ URLs depending on your site structure. When cron-mode is enabled, these automatic purges are queued and processed in the background to avoid slowing down the post editor. Key Difference Action URLs Purged Uses Cron Queue? “Purge Cache (All Pages)” 1 (regex) No – always immediate “Purge Cache (this page)” 1 No – always immediate Post save/update 20-50+ Yes (if cron-mode enabled) If you need to immediately purge all URLs related to a specific post (not just the post URL), save the post – the automatic purge will handle all related URLs.

This plugin generates static html files from your dynamic WordPress blog. After a html file is generated your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts. The static html files will be served to the vast majority of your users: Users who are not logged in. Users who have not left a comment on your blog. Or users who have not viewed a password protected post. 99% of your visitors will be served static html files. One cached file can be served thousands of times. Other visitors will be served custom cached files tailored to their visit. If they are logged in, or have left comments those details will be displayed and cached for them. The plugin serves cached files in 3 ways (ranked by speed): Expert. The fastest method is by using Apache mod_rewrite (or whatever similar module your web server supports) to serve “supercached” static html files. This completely bypasses PHP and is extremely quick. If your server is hit by a deluge of traffic it is more likely to cope as the requests are “lighter”. This does require the Apache mod_rewrite module (which is probably installed if you have custom permalinks) and a modification of your .htaccess file which is risky and may take down your site if modified incorrectly. Simple. Supercached static files can be served by PHP and this is the recommended way of using the plugin. The plugin will serve a “supercached” file if it exists and it’s almost as fast as the mod_rewrite method. It’s easier to configure as the .htaccess file doesn’t need to be changed. You still need a custom permalink. You can keep portions of your page dynamic in this caching mode. WP-Cache caching. This is mainly used to cache pages for known users, URLs with parameters and feeds. Known users are logged in users, visitors who leave comments or those who should be shown custom per-user data. It’s the most flexible caching method and slightly slower. WP-Cache caching will also cache visits by unknown users if supercaching is disabled. You can have dynamic parts to your page in this mode too. This mode is always enabled but you can disable caching for known users, URLs with parameters, or feeds separately. Set the constant “DISABLE_SUPERCACHE” to 1 in your wp-config.php if you want to only use WP-Cache caching. If you’re not comfortable with editing PHP files then use simple mode. It’s easy to set up and very fast. Recommended Settings Simple caching. Compress pages. Don’t cache pages for known users. Cache rebuild. CDN support. Extra homepage checks. Garbage collection is the act of cleaning up cache files that are out of date and stale. There’s no correct value for the expiry time but a good starting point is 1800 seconds. Consider deleting the contents of the “Rejected User Agents” text box and allow search engines to cache files for you. Preload as many posts as you can and enable “Preload Mode”. Garbage collection of old cached files will be disabled. If you don’t care about sidebar widgets updating often set the preload interval to 2880 minutes (2 days) so all your posts aren’t recached very often. When the preload occurs the cache files for the post being refreshed is deleted and then regenerated. Afterwards a garbage collection of all old files is performed to clean out stale cache files. Even with preload mode enabled cached files will still be deleted when posts are modified or comments made. Development Active development of this plugin is handled on GitHub. Translation of the plugin into different languages is on the translation page. Documentation If you need more information than the following, you can have a look at the wiki or the Developer documentation. Preloading You can generate cached files for the posts, categories and tags of your site by preloading. Preloading will visit each page of your site generating a cached page as it goes along, just like any other visitor to the site. Due to the sequential nature of this function, it can take some time to preload a complete site if there are many posts. To make preloading more effective it can be useful to disable garbage collection so that older cache files are not deleted. This is done by enabling “Preload Mode” in the settings. Be aware however, that pages will go out of date eventually but that updates by submitting comments or editing posts will clear portions of the cache. Garbage Collection Your cache directory fills up over time, which takes up space on your server. If space is limited or billed by capacity, or if you worry that the cached pages of your site will go stale then garbage collection has to be done. Garbage collection happens on a regular basis and deletes old files in the cache directory. On the advanced settings page you can specify: 1. Cache timeout. How long cache files are considered fresh for. After this time they are stale and can be deleted. 2. Scheduler. Setup how often garbage collection should be done. 3. Notification emails. You can be informed on garbage collection job progress. There’s no right or wrong settings for garbage collection. It depends on your own site. If your site gets regular updates, or comments then set the timeout to 1800 seconds, and set the timer to 600 seconds. If your site is mostly static you can disable garbage collection by entering 0 as the timeout, or use a really large timeout value. The cache directory, usually wp-content/cache/ is only for temporary files. Do not ever put important files or symlinks to important files or directories in that directory. They will be deleted if the plugin has write access to them. CDN A Content Delivery Network (CDN) is usually a network of computers situated around the world that will serve the content of your website faster by using servers close to you. Static files like images, Javascript and CSS files can be served through these networks to speed up how fast your site loads. You can also create a “poor man’s CDN” by using a sub domain of your domain to serve static files too. OSSDL CDN off-linker has been integrated into WP Super Cache to provide basic CDN support. It works by rewriting the URLs of files (excluding .php files) in wp-content and wp-includes on your server so they point at a different hostname. Many CDNs support origin pull. This means the CDN will download the file automatically from your server when it’s first requested, and will continue to serve it for a configurable length of time before downloading it again from your server. Configure this on the “CDN” tab of the plugin settings page. This is an advanced technique and requires a basic understanding of how your webserver or CDNs work. Please be sure to clear the file cache after you configure the CDN. REST API There are now REST API endpoints for accessing the settings of this plugin. You’ll need to be authenticated as an admin user with permission to view the settings page to use it. This has not been documented yet but you can find all the code that deals with this in the “rest” directory. Custom Caching It is now possible to hook into the caching process using the add_cacheaction() function. Three hooks are available: ‘wp_cache_get_cookies_values’ – modify the key used by WP Cache. ‘add_cacheaction’ – runs in phase2. Allows a plugin to add WordPress hooks. ‘cache_admin_page’ – runs in the admin page. Use it to modify that page, perhaps by adding new configuration options. There is one regular WordPress filter too. Use the “do_createsupercache” filter to customize the checks made before caching. The filter accepts one parameter. The output of WP-Cache’s wp_cache_get_cookies_values() function. WP Super Cache has its own plugin system, loaded before most of WordPress. Add your own plugin either by putting it in the wp-content/plugins/wp-super-cache-plugins directory, or by calling wpsc_add_plugin( $name ) with the full path to the plugin. The cookies used to identify “known users” can be modified with wpsc_add_cookie( $name ) and wpsc_delete_cookie( $name ). See plugins/searchengine.php as an example. Troubleshooting If things don’t work when you installed the plugin here are a few things to check: Is wp-content writable by the web server? Is there a wp-content/wp-cache-config.php ? If not, copy the file wp-super-cache/wp-cache-config-sample.php to wp-content/wp-cache-config.php and make sure WPCACHEHOME points at the right place. Is there a wp-content/advanced-cache.php ? If not, then you must copy wp-super-cache/advanced-cache.php into wp-content/. You must edit the file and change the path so it points at the wp-super-cache folder. If pages are not cached at all, remove wp-content/advanced-cache.php and recreate it, following the advice above. Make sure the following line is in wp-config.php and it is ABOVE the “require_once(ABSPATH.’wp-settings.php’);” line: define( 'WP_CACHE', true ); Try the Settings->WP Super Cache page again and enable cache. Look in wp-content/cache/supercache/. Are there directories and files there? Anything in your php error_log? If your browser keeps asking you to save the file after the super cache is installed you must disable Super Cache compression. Go to the Settings->WP Super Cache page and disable it there. File locking errors such as “failed to acquire key 0x152b: Permission denied in…” or “Page not cached by WP Super Cache. Could not get mutex lock.” are a sign that you may have to use file locking. Edit wp-content/wp-cache-config.php and uncomment “$use_flock = true” or set $sem_id to a different value. You can also disable file locking from the Admin screen as a last resort. Make sure cache/wp_cache_mutex.lock is writable by the web server if using coarse file locking. The cache folder cannot be put on an NFS or Samba or NAS share. It has to be on a local disk. File locking and deleting expired files will not work properly unless the cache folder is on the local machine. Garbage collection of old cache files won’t work if WordPress can’t find wp-cron.php. Check your access_logs for wp-cron.php entries and that your hostname resolves to the external IP address other servers on the network/Internet use. If old pages are being served to your visitors via the supercache, you may be missing Apache modules (or their equivalents if you don’t use Apache). 3 modules are required: mod_mime, mod_headers and mod_expires. The last two are especially important for making sure browsers load new versions of existing pages on your site. The error message, “WP Super Cache is installed but broken. The path to wp-cache-phase1.php in wp-content/advanced-cache.php must be fixed!” appears at the end of every page. Open the file wp-content/advanced-cache.php in your favourite editor. Is the path to wp-cache-phase1.php correct? This file will normally be in wp-content/plugins/wp-super-cache/. If it is not correct the caching engine will not load. Caching doesn’t work. The timestamp on my blog keeps changing when I reload. Check that the path in your .htaccess rules matches where the supercache directory is. You may have to hardcode it. Try disabling supercache mode. If supercache cache files are generated but not served, check the permissions on all your wp-content/cache/supercache folders (and each of wp-content cache and supercache folders) and wp-content/cache/.htaccess. If your PHP runs as a different user to Apache and permissions are strict Apache may not be able to read the PHP generated cache files. To fix you must add the following line to your wp-config.php (Add it above the WP_CACHE define.) Then clear your cache. umask( 0022 ); If you see garbage in your browser after enabling compression in the plugin, compression may already be enabled in your web server. In Apache you must disable mod_deflate, or in PHP zlib compression may be enabled. You can disable that in three ways. If you have root access, edit your php.ini and find the zlib.output_compression setting and make sure it’s “Off” or add this line to your .htaccess: php_flag zlib.output_compression off If that doesn’t work, add this line to your wp-config.php: ini_set('zlib.output_compression', 0); After uninstalling, your permalinks may break if you remove the WordPress mod_rewrite rules too. Regenerate those rules by visiting the Settings->Permalink page and saving that form again. If your blog refuses to load make sure your wp-config.php is correct. Are you missing an opening or closing PHP tag? Your front page is ok but posts and pages give a 404? Go to Settings->permalinks and click “Save” once you’ve selected a custom permalink structure. You may need to manually update your .htaccess file. If certain characters do not appear correctly on your website your server may not be configured correctly. You need to tell visitors what character set is used. Go to Settings->Reading and copy the ‘Encoding for pages and feeds’ value. Edit the .htaccess file with all your Supercache and WordPress rewrite rules and add this at the top, replacing CHARSET with the copied value. (for example, ‘UTF-8’) AddDefaultCharset CHARSET The error message, “WP Super Cache is installed but broken. The constant WPCACHEHOME must be set in the file wp-config.php and point at the WP Super Cache plugin directory.” appears at the end of every page. You can delete wp-content/advanced-cache.php and reload the plugin settings page or edit wp-config.php and look for WPCACHEHOME and make sure it points at the wp-super-cache folder. This will normally be wp-content/plugins/wp-super-cache/ but you’ll likely need the full path to that file (so it’s easier to let the settings page fix it). If it is not correct the caching engine will not load. If your server is running into trouble because of the number of semaphores used by the plugin it’s because your users are using file locking which is not recommended (but is needed by a small number of users). You can globally disable file locking by defining the constant WPSC_DISABLE_LOCKING, or defining the constant WPSC_REMOVE_SEMAPHORE so that sem_remove() is called after every page is cached but that seems to cause problems for other processes requesting the same semaphore. Best to disable it. Set the variable $htaccess_path in wp-config.php or wp-cache-config.php to the path of your global .htaccess if the plugin is looking for that file in the wrong directory. This might happen if you have WordPress installed in an unusual way.