Varnish cache tag purger

default.vcl

sub vcl_recv {


  if (req.request == "BAN") {
    if (!client.ip ~ internal) {
      error 405 "Method not allowed";
    }
    ban("obj.http.X-Drupal-Cache-Tags ~ " + req.http.X-Drupal-Cache-Tag-Clears);
    error 200 "Banned" ;
  }
}

TagDeletionListener.php

<?php

/**
 * @file
 * Contains \Drupal\varnish_cache_tag_purger\TagDeletionListener.
 */

namespace Drupal\varnish_cache_tag_purger;

use Drupal\Core\Cache\NullBackend;
use Drupal\Core\Site\Settings;
use GuzzleHttp\ClientInterface;

/**
 * A fake cache backend that sends purge requests to varnish.
 */
class TagDeletionListener extends NullBackend {

  /**
   * @var \GuzzleHttp\ClientInterface
   */
  protected $httpClient;

  /**
   * {@inheritdoc}
   */
  public function __construct(ClientInterface $http_client) {
    parent::__construct('fake');
    $this->httpClient = $http_client;
  }

  /**
   * {@inheritdoc}
   */
  public function deleteTags(array $tags) {
    $this->purgeTags($tags);
  }

  /**
   * {@inheritdoc}
   */
  public function invalidateTags(array $tags) {
    $this->purgeTags($tags);
  }

  /**
   * Purges caches with those tags from Varnish.
   *
   * @param array $tags
   *   Array of cache tags.
   */
  protected function purgeTags(array $tags) {
    if ($varnish_url = Settings::get('varnish_url')) {
      $request = $this->httpClient->createRequest('BAN', $varnish_url)
        ->addHeader('X-Drupal-Cache-Tag-Clears', $this->convertCacheTagsToClearRegex($tags));
      $this->httpClient->send($request);
    }
  }

  /**
   * Converts the cache tag array structure to a varnish compatible regex.
   *
   * @param array $tags
   *   Cache tags.
   *
   * @return string
   *   String in the form of "(\Dnode:1\D|\Dnodes\D)", taken from
   *
   * @see http://www.smashingmagazine.com/2014/04/23/cache-invalidation-strategies-with-varnish-cache/
   */
  public function convertCacheTagsToClearRegex(array $tags) {
    $flat_tags = array();
    foreach ($tags as $namespace => $values) {
      if (is_array($values)) {
        foreach ($values as $value) {
          $flat_tags[] = "$namespace:$value";
        }
      }
      else {
        $flat_tags[] = "$namespace:$values";
      }
    }
    return '(\D' . implode('\D|\D', $flat_tags) . '\D)';
  }

}

varnish_cache_tag_purger.services.yml

services:
  # Define a fake cache bin by tagging it with cache.bin. This only implements
  # methods to clear cache tags.
  varnish_cache_tag_purger.tag_deletion_listener:
    class: Drupal\varnish_cache_tag_purger\TagDeletionListener
    arguments: ['@http_client']
    tags:
      - { name: cache.bin }

This is very, very nice and proves what we were all thinking already. Thanks Berdir!

So, as I'm working today (and next week on this), I'll translate and incorporate this into a VarnishPocPurger that'll do exactly what you are doing here. I was thinking of taking the same approach as you did by overloading the page cache, which will become a second (lightweight) module called 'purge_cachetags'. The module will be part of the purge project but allowing a separation of (API) concerns.