Message Processor which listen to Contact's email changes and call API

ContactEmailChangeProcessor.php

<?php

namespace ClientCustomBundle\Async;

use Oro\Component\MessageQueue\Client\TopicSubscriberInterface;
use Oro\Component\MessageQueue\Consumption\MessageProcessorInterface;
use Oro\Component\MessageQueue\Transport\MessageInterface;
use Oro\Component\MessageQueue\Transport\SessionInterface;
use Oro\Bundle\DataAuditBundle\Async\Topics;

class ContactEmailChangeProcessor implements MessageProcessorInterface, TopicSubscriberInterface
{
    /**
     * {@inheritdoc}
     */
    public function process(MessageInterface $message, SessionInterface $session)
    {
        // check if entity == Contact and field == email
        // if yes -> call frontend's API with that data
    }

    /**
     * {@inheritdoc}
     */
    public static function getSubscribedTopics()
    {
        return [Topics::ENTITIES_CHANGED];
    }
}