public function setMainDB(INabuDBConnector $connector)
    {
        if ($this->isInstallMode()) {
            if ($this->main_database === null) {
                $this->main_database = $connector;
            } else {
                throw new ENabuCoreException(ENabuCoreException::ERROR_MAIN_DB_ALREADY_EXISTS);
            }
        } else {
            throw new ENabuCoreException(ENabuCoreException::ERROR_INSTALL_MODE_REQUIRED);
        }
    }