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https://jungyong.com/2012/08/29/%ea%b5%ac%ea%b8%80-%ec%84%9c%eb%b9%84%ec%8a%a4%ec%97%90-%eb%8c%80%ed%95%9c-%ec%a2%8b%ec%9d%80-%ec%b0%b8%ea%b3%a0%ec%9e%90%eb%a3%8c-%ea%b5%ac%ea%b8%80-%ec%8b%a4%ec%a0%84-%eb%b0%94%ec%9d%b4%eb%b8%94/
구글 서비스에 대한 좋은 참고자료 - "구글 실전 바이블"