https://blog.gongquiz.com/2021/09/14/%eb%8b%a4%ec%9d%8c-%ec%a4%91-master-slave-%ed%94%8c%eb%a6%bd%ed%94%8c%eb%a1%ad%ec%9d%80-%ec%96%b4%eb%96%a0%ed%95%9c-%ed%98%84%ec%83%81%ec%9d%84-%ed%95%b4%ea%b2%b0%ed%95%98%ea%b8%b0-%ec%9c%84%ed%95%9c/