# How to override cache tags in a Drupal response object

While working on some improvements to the caching granularity in the Prosple platform, I found myself needing to override the cache tags being sent in a GraphQL response.

This is because Drupal has **a lot** of cache tags, and while they are all incredibly useful you might find yourself in a situation where they never get used and, for caching systems like CDNs, it can cause you to do over the allowed limits for header sizes.

So how to do this?

### Add an Event Subscriber to your custom module

The first thing we need to do is to create an event subscriber.

You can do this by creating a new module or adding it to an existing custom one, which is what I’ll do here. The module is called `example` .

First, make sure that you have an [`example.services`](http://example.services)`.yml` file in your module root where you define your Event Subscriber. Here is one for the `example` module:

```plaintext
services:
  example_event_subscriber:
    class: '\Drupal\example\EventSubscriber\ExampleEventSubscriber'
    tags:
      - { name: 'event_subscriber' }
```

### Create an Event Subscriber

Next, we are going to create the actual Event Subscriber:

```plaintext
<?php

namespace Drupal\example\EventSubscriber;

use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Class ExampleEventSubscriber.
 *
 * @package Drupal\example\EventSubscriber
 */
class ExampleEventSubscriber implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      KernelEvents::RESPONSE => ['onKernelResponse', 100],
    ];
  }

  /**
   * React to a response event
   *
   * @param FilterResponseEvent $event
   *   The response event.
   */
  public function onKernelResponse(FilterResponseEvent $event) {
    /** @var \Drupal\Core\Cache\CacheableJsonResponse $response */
    $response = $event->getResponse();
    if(is_a($response, '\Drupal\Core\Cache\CacheableJsonResponse')){
      $cache_tags = ['test'];
      $response->getCacheableMetadata()->setCacheTags($cache_tags);
    }
    else{
      return;
    }
  }

}
```

So let’s look at this closely.

First, we implement the getSubscribedEvents method. You can see more details on how to create Event Subscribers in [https://www.drupal.org/docs/8/creating-custom-modules/subscribe-to-and-dispatch-events](https://www.drupal.org/docs/8/creating-custom-modules/subscribe-to-and-dispatch-events)

The important part here is that priority argument (which I’m setting to 100). This is to ensure that this Event Subscriber runs before the `FinishResponseSubscriber` .

Now, within our `onKernelResponse` method we can get our Response object and override the cache tags.

In our case, we are checking that the response is a `CacheableJSONResponse` , which might not make sense to you.

What matters is what’s inside that conditional statement:

```plaintext
$cache_tags = ['test'];
$response->getCacheableMetadata()->setCacheTags($cache_tags);
```

I’m just setting a list of dummy tags here, you could get the ones in the response by using **$response-&gt;getCacheableMetadata()** or I can just set my own list as I’m doing above.

Then I call the method `setCacheTags` in the `CacheableMetadata` class and add the list of tags I want.

That’s it!