function __construct($engineType = 'mongo')
 {
     switch ($engineType) {
         case 'mongo':
             try {
                 $this->conn = new MongoClient(Config::getDSN($engineType));
             } catch (Exception $e) {
                 throw new Exception($e->getMessage());
             }
             break;
         case 'mysql':
             $this->conn = 'driver engine for mysql';
             break;
     }
 }