/**
  * Executes an asynchronous command.
  *
  * @param CommandInterface $command Command to execute.
  *
  * @return AwsDynamoDbResponse
  */
 protected function executeAsyncCommand(CommandInterface $command)
 {
     try {
         $result = $this->client->executeAsync($command);
     } catch (AwsException $e) {
         /* Return an error response. */
         return new AwsDynamoDbResponse(null, $e);
     }
     return new AwsDynamoDbResponse($result, null);
 }