Drupal Cache Debugging

Systematic approaches to identify and resolve caching issues.

When to Use

Pages not caching for anonymous users
X-Drupal-Dynamic-Cache shows UNCACHEABLE
Content is stale after edits
Need to find source of max-age:0
Performance profiling cache effectiveness

Debug Headers Setup

// settings.local.php $settings['http.response.debug_cacheability_headers'] = TRUE;

Warning: Disable in production - headers can exceed 16KB.

Headers Reference

Header Values Meaning

X-Drupal-Cache

HIT/MISS Page Cache (anonymous)

X-Drupal-Dynamic-Cache

HIT/MISS/UNCACHEABLE Dynamic Page Cache

X-Drupal-Cache-Tags

Space-separated tags All cache tags

X-Drupal-Cache-Contexts

Space-separated contexts Applied contexts

X-Drupal-Cache-Max-Age

Seconds or -1 Effective max-age

Examples

Example 1: Quick Health Check

Input: "How do I quickly check if caching is working?"

Output:

Anonymous user - should hit Page Cache

curl -sI https://site.com/ | grep -iE 'x-drupal|cache-control'

Expected output:

X-Drupal-Cache: HIT

Cache-Control: max-age=3600, public

Authenticated user - should hit Dynamic Cache

curl -sI -H "Cookie: SESSabc=xyz" https://site.com/ | grep X-Drupal-Dynamic-Cache

Expected output:

X-Drupal-Dynamic-Cache: HIT

Example 2: Debug Page Not Caching (Anonymous)

Input: "Anonymous pages always show X-Drupal-Cache: MISS"

Output:

Step 1: Check for session cookies being set

curl -sI https://site.com/ | grep -i set-cookie

If you see SESS* or SSESS* → Something creates session for anonymous

Step 2: Check if Page Cache module is enabled

drush pm:list | grep page_cache

Step 3: Search for kill switch usage

grep -r "page_cache_kill_switch" web/modules/custom/

Step 4: Check for session-creating code

grep -r "getSession|session_start|$_SESSION" web/modules/custom/

Example 3: Debug UNCACHEABLE Response

Input: "X-Drupal-Dynamic-Cache shows UNCACHEABLE, how do I fix it?"

Output:

Step 1: Enable debug headers

settings.local.php: $settings['http.response.debug_cacheability_headers'] = TRUE;

Step 2: Check max-age

curl -sI https://site.com/ | grep X-Drupal-Cache-Max-Age

If 0 → Something set max-age:0

Step 3: Find the source with Xdebug

Set conditional breakpoint in core/lib/Drupal/Core/Cache/Cache.php

Method: mergeMaxAges()

Condition: $a === 0 || $b === 0

Check stack trace when it hits

Example 4: Debug Stale Content After Edit

Input: "I edited a node but old content still shows"

Output:

Step 1: Clear caches and verify still stale

drush cr curl -sI https://site.com/node/1 | grep X-Drupal-Cache

If still HIT with old content → Tags not invalidating

Step 2: Check cache tags on response

curl -sI https://site.com/node/1 | grep X-Drupal-Cache-Tags

Should include: node:1

Step 3: Manually invalidate and test

drush cache-tag-invalidate node:1 curl https://site.com/node/1 | grep "expected new content"

Step 4: If using Varnish/CDN, check purge queue

drush p:queue-stats

Example 5: Find max-age:0 Source with Temporary Logging

Input: "I can't use Xdebug, how else can I find max-age:0?"

Output:

// Temporarily modify core/lib/Drupal/Core/Cache/Cache.php // Add to mergeMaxAges() method:

public static function mergeMaxAges($a, $b) { // TEMPORARY DEBUG - REMOVE AFTER FINDING ISSUE if ($a === 0 || $b === 0) { $trace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 10); $simplified = array_map(function($t) { return ($t['class'] ?? '') . '::' . ($t['function'] ?? '') . ' in ' . ($t['file'] ?? ''); }, $trace); \Drupal::logger('cache_debug')->warning('max-age:0 merge: @trace', [ '@trace' => implode("\n", $simplified), ]); } // END TEMPORARY DEBUG

return min($a, $b); }

// Then check logs: // drush watchdog:show --type=cache_debug

Example 6: Enable Render Debug Comments

Input: "How do I see cache info per render element in HTML?"

Output:

development.services.yml

parameters: twig.config: debug: true renderer.config: debug: true

Example 7: WebProfiler Analysis

Input: "How do I use WebProfiler for cache debugging?"

Output:

Install Devel and WebProfiler

composer require drupal/devel drush en devel webprofiler

Access the profiler toolbar at bottom of page (admin users)

Click "Cache" panel to see:

- Cache hit/miss counts per bin

- All collected cache tags

- Applied cache contexts

- Timeline of cache operations

Example 8: Database Cache Inspection

Input: "How do I inspect cache entries directly in database?"

Output:

-- Check render cache entries for a node SELECT cid, expire, tags FROM cache_render WHERE cid LIKE '%node%' LIMIT 10;

-- Find entries with specific tag SELECT cid, created, expire FROM cache_render WHERE tags LIKE '%node:123%';

-- Count entries per cache bin SELECT 'cache_render' as bin, COUNT() as entries FROM cache_render UNION ALL SELECT 'cache_page', COUNT() FROM cache_page UNION ALL SELECT 'cache_dynamic_page_cache', COUNT(*) FROM cache_dynamic_page_cache;

Example 9: Drush Cache Commands

Input: "What drush commands help with cache debugging?"

Output:

Full cache rebuild

drush cr

Clear specific bin

drush cache:clear render drush cache:clear page drush cache:clear dynamic_page_cache drush cache:clear discovery

Invalidate specific tag

drush cache-tag-invalidate node:1 drush cache-tag-invalidate "config:system.site"

Get cache item programmatically

drush php:eval "print_r(\Drupal::cache('render')->get('entity_view:node:1:full'));"

List all cache bins

drush php:eval "print_r(array_keys(\Drupal::getContainer()->getParameter('cache_bins')));"

Debugging Decision Tree

Page not caching? ├── Anonymous user? │ ├── X-Drupal-Cache: MISS always? │ │ └── Check for session cookies, kill switch │ └── X-Drupal-Cache: HIT but stale? │ └── Check cache tags, invalidation └── Authenticated user? ├── X-Drupal-Dynamic-Cache: UNCACHEABLE? │ └── Find max-age:0 source ├── X-Drupal-Dynamic-Cache: MISS always? │ └── Check if module enabled, cache bin working └── Dynamic Cache working but slow? └── Check for missing lazy builders on personalized content

Common Issues Quick Reference

Symptom Likely Cause First Check

Always MISS (anonymous) Session created curl -I for Set-Cookie

Always UNCACHEABLE max-age:0 X-Drupal-Cache-Max-Age header

Stale after edit Missing tags X-Drupal-Cache-Tags header

Per-user cache explosion user context X-Drupal-Cache-Contexts header

BigPipe not streaming Server buffering Check Nginx/Apache config

drupal-cache-debugging

Safety Notice

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Anonymous user - should hit Page Cache

Expected output:

X-Drupal-Cache: HIT

Cache-Control: max-age=3600, public

Authenticated user - should hit Dynamic Cache

Expected output:

X-Drupal-Dynamic-Cache: HIT

Step 1: Check for session cookies being set

If you see SESS* or SSESS* → Something creates session for anonymous

Step 2: Check if Page Cache module is enabled

Step 3: Search for kill switch usage

Step 4: Check for session-creating code

Step 1: Enable debug headers

settings.local.php: $settings['http.response.debug_cacheability_headers'] = TRUE;

Step 2: Check max-age

If 0 → Something set max-age:0

Step 3: Find the source with Xdebug

Set conditional breakpoint in core/lib/Drupal/Core/Cache/Cache.php

Method: mergeMaxAges()

Condition: $a === 0 || $b === 0

Check stack trace when it hits

Step 1: Clear caches and verify still stale

If still HIT with old content → Tags not invalidating

Step 2: Check cache tags on response

Should include: node:1

Step 3: Manually invalidate and test

Step 4: If using Varnish/CDN, check purge queue

development.services.yml

Install Devel and WebProfiler

Access the profiler toolbar at bottom of page (admin users)

Click "Cache" panel to see:

- Cache hit/miss counts per bin

- All collected cache tags

- Applied cache contexts

- Timeline of cache operations

Full cache rebuild

Clear specific bin

Invalidate specific tag

Get cache item programmatically

List all cache bins

Source Transparency

Related Skills

drupal-lazy-builders

drupal-cache-contexts

drupal-cache-maxage