SQLite Object Cache vs Proxy Cache Purge

A persistent object cache helps your site perform well. This one uses the widely available SQLite3 extension, and optionally the igbinary and APCu extensions to php. Many hosting services offer those extensions, and they are easy to install on a server you control. What is this about? It’s about making your site’s web server perform better. An object cache does that by reducing the workload on your MariaDB or MySQL database. This is not a page cache; these persistent objects go into a different kind of cache. These objects aren’t chunks of web pages ready for people to view in their browsers, they are data objects for use by the WordPress software. Caches are ubiquitous in computing, and WordPress has its own caching subsystem. Caches contain short-term copies of the results of expensive database lookups or computations, and allow software to use the copy rather than repeating the expensive operation. This plugin (like other object-caching plugins) extends WordPress’s caching subsystem to save those short-term copies from page view to page view. WordPress’s cache happens to be a memoization cache. Without a persistent object cache, every WordPress page view must use your MariaDB or MySQL database server to retrieve everything about your site. When a user requests a page, WordPress starts from scratch and loads everything it needs from your database server. Only then can it deliver content to your user. With a persistent object cache, WordPress immediately loads much of the information it needs. This lightens the load on your database server and delivers content to your users faster. Who should use this? If your site runs on a single web server machine, and that server provides the SQLite3 and igbinary extensions to php, this plugin will almost certainly make your site work faster. And if that server provides the APCu extension, this plugin uses it too. Some hosting providers offer redis cache servers. If your provider offers redis, it may be a good choice. You can use it via the Redis Object Cache plugin. Sites using redis have one SQL database and another non-SQL storage server: redis. Other hosting providers offer memcached, which has the Memcached Object Cache plugin. And some large multipurpose cache plugins, such as the LiteSpeed Cache, also offer object caching based on one of those cache server software packages. The cache-server approach to object caching comes into its own when you have multiple load-balanced web server machines handling your site. SQLite doesn’t work correctly in a multiple-web-server environment. But, for single-server site configurations, SQLite, possibly assisted by APCu, performs well. And the vast majority of sites are single-server. APCu APCu is an in-memory storage medium. It lets php programs, like WordPress, store data in shared memory so it’s very fast to retrieve when needed. If APCu is available on your host server, you can configure this plugin to use it. It reduces the typical cache lookup time to one-fifth or less of the SQLite lookup time, which is itself a few tens of microseconds. Performance counts, especially on busy web sites. Please look at Installation to learn how to configure this plugin to use APCu. The plugin works fast without it, and faster with it. WP-CLI: Even if APCu is in use, caching with SQLite is necessary when your web site uses WP-CLI, because WP-CLI programs do not have access to the APCu cache. This plugin writes all cached data both to APCu and to SQLite and makes sure the two are synchronized. WP-CLI You can control this plugin via WP-CLI once you activate it. Please type this command into your shell for details. wp help sqlite-object-cache Credits Thanks to Till Krüss. His Redis Object Cache plugin serves as a model for this one. And thanks to Ari Stathopoulos and Jonny Harris for reviewing this. Props to Matt Jones for finding and fixing a bug that appeared on a heavily loaded system. Thanks to Massimo Villa for testing help, and to nickchomey for a comprehensive code review. All defects are, of course, entirely the author’s responsibility. And thanks to Jetbrains for the use of their software development tools, especially PhpStorm. It’s hard to imagine how a plugin like this one could be developed without PhpStorm’s tools for exploring epic code bases like WordPress’s. How can I learn more about making my WordPress site more efficient? We offer several plugins to help with your site’s database efficiency. You can read about them here.

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.