Page Cache Vary

Provide the ability to specify by which Request header the cache should vary. Implement the vary feature in the Internal Page Cache by extanding it.
Works on cached pages visited by the anonymous user.

Features

Provide a new interface for Cache Context allowing to specify a list of headers to vary the response by (using https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Vary).

The Vary header is populated accordingly.

Usage example

Simply implement \Drupal\page_cache_vary\VaryCacheContextInterface with your CacheContext class.

namespace Drupal\page_cache_vary_test\Cache\Context;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Cache\Context\CalculatedCacheContextInterface;
use Drupal\Core\Cache\Context\RequestStackCacheContextBase;
use Drupal\page_cache_vary\VaryCacheContextInterface;

/**
 * Varies the context depending on the x-country-code header.
 *
 * Cache context ID: 'country'.
 */
final class CountryCacheContext extends RequestStackCacheContextBase implements CalculatedCacheContextInterface, VaryCacheContextInterface {

  const COUNTRY_CODE_HEADER = 'x-country-code';

  /**
   * {@inheritdoc}
   */
  public static function getLabel(): string {
    return (string) t('Country');
  }

  /**
   * {@inheritdoc}
   */
  public function getContext($parameter = NULL): string {
    return $this->requestStack->getMainRequest()->headers->get(self::COUNTRY_CODE_HEADER, '');
  }

  /**
   * {@inheritdoc}
   */
  public function getCacheableMetadata($parameter = NULL): CacheableMetadata {
    return new CacheableMetadata();
  }

  /**
   * Return the list of headers the response vary by.
   */
  public function getVaryHeaders(): array {
    return [self::COUNTRY_CODE_HEADER];
  }

}

Additional Requirements

This module as a requirement on the Internal Page Cache.