public function subscribersByTopic(string $topic)
    {
        $key = self::TOPIC_KEY.".{$topic}";

        if (! $this->cache->has($key)) {
            return new Collection;
        }

        $channels = json_decode($this->cache->get($key), true);

        return (new Collection($channels))
            ->map(function (string $channel): ?Subscriber {
                return $this->subscriberByChannel($channel);
            })
            ->filter()
            ->values();
    }