/**
  * {@inheritdoc}
  *
  * @param string $command
  * @return void
  */
 public function start($command)
 {
     $this->startWorkers($command);
     while ($this->isWorking()) {
         $worker = $this->getAvailableWorker();
         if ($worker && !empty($this->pending)) {
             $worker->run(array_shift($this->pending));
         }
         $this->broker->read();
     }
 }