# Implementing Caching for New Caching Identities

ScandiPWA controls cache in a different way than [default Magento 2](https://devdocs.magento.com/guides/v2.3/graphql/develop/create-graphqls-file.html#query-caching) does. We are still using the caching identities, but, instead of specifying them on GraphQL queries we use events.

## General rule

To make some of your model work as cache identity manager:

1. Make sure it implements the `Magento\Framework\DataObject\IdentityInterface`

```php
use Magento\Framework\DataObject\IdentityInterface;
use Magento\Framework\Model\AbstractModel;

class Slide extends AbstractModel implements IdentityInterface {
    public function getIdentities() {
        // TODO: implement
    }
}
```

2\. Specify the caching tag constant, it should be short and unique:

```php
class Slide extends AbstractModel implements IdentityInterface {
    const CACHE_TAG = 'sw_sld';

    protected $_cacheTag = self::CACHE_TAG;
}
```

3\. Implement the `getIdentities` method, specify all involved cache identities. In our example, on slide save, the slider model should also be invalidate:

```php
class Slide extends AbstractModel implements IdentityInterface {
    const CACHE_TAG = 'sw_sld';

    public function getIdentities() {
        return [
            self::CACHE_TAG . '_' . $this->getId(),
            Slider::CACHE_TAG . '_' . $this->getSliderId()
        ];
    }
}
```

4\. Add the names for events, prefer unique, descriptive names:

```php
class Slide extends AbstractModel implements IdentityInterface {
    protected $_eventPrefix = 'scandiweb_slider_slide';
}
```

5\. In case you have a resource model, i.e. the `Collection`, add the event after collection save:

```php
use Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;

class Collection extends AbstractCollection {
    protected function _afterLoadData() {
        parent::_afterLoadData();

        $collection = clone $this;

        if (count($collection)) {
            $this->_eventManager->dispatch(
                'scandiweb_slider_slider_collection_load_after',
                ['collection' => $collection]
            );
        }

        return $this;
    }
}
```

6\. It is finally time to connect the events, to granular cache management classes. Create, or modify the `etc/events.xml` with following content:

```php
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="scandiweb_slider_slider_collection_load_after">
        <observer name="pq_cc_slider" instance="ScandiPWA\Cache\Observer\Response\TagEntityResponse"/>
    </event>
    <event name="scandiweb_slider_slider_save_after">
        <observer name="pq_cc_slider" instance="ScandiPWA\Cache\Observer\FlushVarnishObserver"/>
    </event>
    <event name="scandiweb_slider_slide_collection_load_after">
        <observer name="pq_cc_slide" instance="ScandiPWA\Cache\Observer\Response\TagEntityResponse"/>
    </event>
    <event name="scandiweb_slider_slide_save_after">
        <observer name="pq_cc_slide" instance="ScandiPWA\Cache\Observer\FlushVarnishObserver"/>
    </event>
</config>
```

Note, there are two classes used as observers:

* `ScandiPWA\Cache\Observer\FlushVarnishObserver` - responsible for flushing, must be triggered on save of the model.
* `ScandiPWA\Cache\Observer\Response\TagEntityResponse` - responsible for tagging, must be triggered after load of the collection / model.