/**
  * {@inheritDoc}
  */
 public function validateSubscription(ISubscription $subscription)
 {
     if ($subscription->getType() !== self::IDENTIFIER) {
         throw new BackendUtils\SubscriptionInvalidException('Subscription-type not supported');
     }
     $this->validateSubscriptionUrl($subscription);
     $this->sendTestRequest($subscription);
     return true;
 }
 /**
  * validate a subscription
  *
  * @param ISubscription $subscription
  * @throws Exception
  */
 private function validateSubscription(ISubscription &$subscription)
 {
     $backend = $this->backends->bySubscriptionType($subscription->getType());
     if ($backend === null) {
         throw new Exception('Subscription-type not supported');
     }
     try {
         $backend->getBackendAPI()->validateSubscription($subscription);
     } catch (BackendUtils\Exception $ex) {
         throw Exception::fromException($ex);
     }
 }