-Controller for requesting fronend and response for Service -Service 做資料處理 -DAO- data access object -@Autowired -->Spring Boot 啟動時便會給該變數傳入物件-->「 依賴注入」（dependency injection） -Review https://www.youtube.com/watch?v=r-6BwGW4Sr8&ab_channel=Amigoscode
#10/21 add exception Before adding the mongodDB