protected function getConnection($node_type)
    {
        switch ($node_type) {
        case self::CONN_TYPE_MASTER:
            $this->write_conn = new \PDO($this->_master_dsn, $this->_master_username, $this->_master_password, $this->_master_options);
            break;
        case self::CONN_TYPE_SLAVE:
            $this->read_conn = new \PDO($this->_slave_dsn, $this->_slave_username, $this->_slave_password, $this->_slave_options);
            break;
        default:
            if (in_array($node_type, $this->extraConfigs) && in_array($node_type, $this->extraDsns)) {
                $extraConfig = $this->extraConfigs[$node_type];
                $this->extraConns[$node_type] = new \PDO($this->extraDsns[$node_type], $extraConfig['_username'], $extraConfig['_password'], $extraConfig['_options']);
            } else {
                $this->write_conn = new \PDO($this->_master_dsn, $this->_master_username, $this->_master_password, $this->_master_options);
            }
        }
    }