/**
  * Get unchecked subscribers by state
  *
  * @param string $state subscriber status
  * @return array
  */
 public function getUncheckedSubscribers($state)
 {
     // get unchecked subscribers
     $key = md5(__FUNCTION__ . $state);
     $subscribers = Cache::exists($key) ? Cache::fetch($key) : $this->storageInstance->getSubscribers($state, 0);
     Cache::store($key, $subscribers);
     return $this->arrayPartition($subscribers, $this->cpuCoreCount());
 }