I see that a lot of the models in Magento 2 implement Magento\Framework\DataObject\IdentityInterface.
This interface has a single method called getIdentities
The implementations of the method usually return return [self::CACHE_TAG . '_' . $this->getId()];.

An example can be found here
What is this used for?

  • I don't really know if that is used but the class is present in the dev/tests/static/testsuite/Magento/Test/Legacy/_files/obsolete_classes.php file so maybe it is intended to be removed later. – Matthéo Geoffray Apr 22 '16 at 12:05
  • 2
    Judging by the class comment it is used for cache and make a unique entity ID and used in ESI headers vendor/magento/module-page-cache/Controller/Block/Esi.php line 28 – Matthéo Geoffray Apr 22 '16 at 12:09
  • @MatthéoGeoffray yes you're right but the getIdentities method is called on a block class in this case, the OP is asking about the models classes – Raphael at Digital Pianism Apr 22 '16 at 12:10
  • Oh yes right my bad ;) – Matthéo Geoffray Apr 22 '16 at 12:14
  • @MatthéoGeoffray. I think you are right. You might want to put your comment as an answer. – Marius Apr 22 '16 at 12:15

From what I understood, this is kind of the equivalence of the getCacheTags method of Magento 1.

The getIdentities from model classes is then used in every block class referencing this model.

Ok let's take the /Magento/Catalog/Model/Category.php:

public function getIdentities()
    $identities = [
        self::CACHE_TAG . '_' . $this->getId(),
    if ($this->hasDataChanges() || $this->isDeleted()) {
        $identities[] = Product::CACHE_PRODUCT_CATEGORY_TAG . '_' . $this->getId();
    return $identities;

This method is then referenced in /Magento/Catalog/Block/Category/View.php:

public function getIdentities()
    return $this->getCurrentCategory()->getIdentities();

In M2, you now have to declare the cache tag with the getIdentities method at the model level, then you can use it in blocks referencing those models.

If you check every block implementing the getIdentities method, they all reference a corresponding model getIdentities method or a corresponding model cache tag such as \Magento\Catalog\Model\Product::CACHE_TAG

Then those block getIdentities methods are used in Varnish for caching reasons as mentioned by Matthéo to set the X-Magento-Tags header.

This header is then used in Magento/Framework/App/PageCache/Kernel.php in the process() method to save the cache:

$tagsHeader = $response->getHeader('X-Magento-Tags');
$tags = $tagsHeader ? explode(',', $tagsHeader->getFieldValue()) : [];

if (!headers_sent()) {
$this->cache->save(serialize($response), $this->identifier->getValue(), $tags, $maxAge);
| improve this answer | |
  • 5
    Be careful - identities are just for the full page cache. For the block cache you still need to implement cache_tags and cache_lifetime! It's an addition, not a replacement. – Robert Egginton Jan 20 '17 at 15:52
  • I wanted to keep FPC in a page with a dynamic block so I had to implement getIdentities in the block, but the model (Slider in my case) depends also on changes in child Model (Banners), should I add both in the identities array ? or the child change means only the parent is needed ? thank you @RobertEgginton – medmek Nov 12 '19 at 18:51

Judging by the class Magento\Framework\DataObject\IdentityInterface comment it is used for cache and make a unique entity ID that is used in Varnish ESI headers in vendor/magento/module-page-cache/Controller/Block/Esi.phpline 28.

if ($blockInstance instanceof \Magento\Framework\DataObject\IdentityInterface) {
   $response->setHeader('X-Magento-Tags', implode(',', $blockInstance->getIdentities()));
| improve this answer | |

The  IdentityInterface  will  force  Model  class  to  define  the  ​getIdentities()  method which  will return a unique id for the model. You must only use this interface if your model required cache refresh after database operation and render information to the frontend page.

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