cache_explain 1.0.x-dev

First release of Cache Explain. It adds a block that inspects the current page and shows, for each caching layer, whether it served the page and why. It's meant for developers and site
builders trying to work out which cache is holding stale content, or why a page isn't cached.

Features

• Browser cache: reads Cache-Control, Expires, ETag and Last-Modified, and explains no-store,
  no-cache, max-age, s-maxage, private, immutable and the rest.
• CDN, Cloudflare: explains every CF-Cache-Status value (HIT, MISS, EXPIRED, STALE, UPDATING, REVALIDATED, BYPASS, DYNAMIC). It flags pages whose
  Cache-Control allows edge caching while Cloudflare isn't caching them, shows where the edge TTL comes from, and shows the CF-Ray data centre.
• CDN, Akamai: sends Akamai's debug Pragma headers and reads X-Cache, X-Cache-Remote, X-Check-Cacheable and the cache
  keys.
• Reverse proxy (Varnish or nginx): X-Varnish, X-Cache, X-Cache-Hits, Age, Via. When the CDN served the
  page from its own cache, this layer reports "Not reached" instead of guessing.
• Drupal Internal Page Cache and Dynamic Page Cache: X-Drupal-Cache, X-Drupal-Dynamic-Cache, plus cache tags, contexts and max-age when
  cacheability debug headers are on.
• Render cache and cache bins: these aren't visible over HTTP, so the block explains what they do and how to inspect them.
• An equivalent curl command and a dump of every raw response header.
• The panel renders inside a shadow root, so it looks the same under any theme and doesn't affect the theme's styles.

Requirements

• Drupal 10.3 or later, or Drupal 11. So far it has been tested on Drupal 11.4.
• The core Block module.
• The server must be able to make HTTP requests to its own public URL.

Installation

composer require 'drupal/cache_explain:^1.0@alpha'
  drush en cache_explain

Then place the Cache Explain block under Structure → Block layout, and restrict it to the Administrator role under Visibility → Roles. There is no
configuration page.

Known limitations

• Development and troubleshooting tool. Each render makes the site send a HEAD request to its own URL, roughly doubling the work for that page. Keep the
  block admin-only, and remove it from production once you've finished investigating.
• TLS certificates are not verified on the probe request, so it also works with self-signed certificates on development sites. Making this configurable is planned before
  a stable release.
• The probe carries no session cookie, so Drupal page-cache results describe what an anonymous visitor gets, not your logged-in view.
• CDNs cache per data centre. CDN results reflect the data centre nearest the server, which may differ from the one a visitor reaches.
• The probe URL is built from the incoming request, so make sure trusted_host_patterns is set in settings.php.
• Requires JavaScript.
• Only Cloudflare and Akamai are recognised as CDNs. Fastly, CloudFront and others show as "No CDN detected" for now. Patches are welcome in the issue queue.