Surrogate-Key header support for cache tag purging

What's New

Acquia Purge now automatically generates Surrogate-Key headers when the Acquia Platform One CDN purger is enabled. These headers contain hashed representations of Drupal cache tags, enabling efficient CDN-level cache invalidation.

New functionality:

• Surrogate-Key header generation - Automatically added to all page responses
• Hashed cache tags - Cache tags converted to surrogate keys
• CDN-level purging - More efficient cache invalidation at the CDN edge

How It Works

When the Acquia Platform One CDN purger is enabled:

• Drupal cache tags are collected for each page response
• Cache tags are hashed and converted to surrogate keys
• Surrogate-Key header is automatically added to the HTTP response
• CDN uses these keys for efficient, targeted cache purging

Example response header:
Surrogate-Key: abc123def456 xyz789ghi012 mno345pqr678
Each surrogate key represents one or more Drupal cache tags.

Benefits

More efficient purging:

• CDN can purge specific cached items by surrogate key
• No need to purge entire cache or use URL-based purging
• Faster cache invalidation across the CDN

Better performance:

• Targeted purging reduces unnecessary cache clears
• Content stays cached longer when only specific items are invalidated
• Improved cache hit rates

Configuration

No configuration required! When you enable the Acquia Platform One CDN purger:

• Navigate to /admin/config/development/performance/purge
• Ensure "Acquia Platform One CDN" purger is enabled
• Surrogate-Key headers are automatically generated

Verification:Check HTTP response headers to see Surrogate-Key header in page responses.

For Module Developers

Drupal cache tags automatically work with surrogate keys:

// Standard Drupal cache tags
$build['#cache']['tags'][] = 'node:123';
$build['#cache']['tags'][] = 'user:456';

// Automatically converted to Surrogate-Key headers
// No additional code needed

Benefits

✅ Automatic - No configuration or code changes needed
✅ Efficient purging - Targeted CDN cache invalidation
✅ Better performance - Higher cache hit rates
✅ Standards-based - Uses industry-standard Surrogate-Key headers