protected function getSubscriptionForChannel($channel)
    {
        $topic = $this->getTopicForChannel($channel);
        $clientIdentifier = $this->clientIdentifier ? $this->clientIdentifier : 'default';
        $clientIdentifier .= '.' . $channel;
        $subscription = $topic->subscription($clientIdentifier);
        if ($this->autoCreateSubscriptions && !$subscription->exists()) {
            $subscription->create();
        }
        return $subscription;
    }