public function getDriver(/*# string */ $tag = '')/*# : DriverInterface */
    {
        // match drivers
        $matched = $this->driverMatcher($tag);

        if (count($matched) > 0) {
            return $this->drivers[$matched[rand(1, count($matched)) - 1]];
        } else {
            throw new NotFoundException(
                Message::get(Message::DB_DRIVER_NOTFOUND),
                Message::DB_DRIVER_NOTFOUND
            );
        }
    }