Taehyeong Lee Thank you for the response. I also plan to develop a habit of doing so. By establishing a solid foundation, it seems possible to preemptively prevent deadlocks that may occur in unexpected areas.

Thanks to you, I was able to think deeply about lock and isolation. Thank you! You have set the @Transactional annotation on the Repository interface. I usually prefer to declare it in the Class method of the Service layer... Is it a better practice to declare it in the Repository?

Thanks to you, I was able to contemplate about lock and isolation. In source code, you implemented the @Transactional annotation in the Repository Interface, but according to Spring documentation, they recommend setting it at the class level. how do you think about it? https://docs.spring.io/spring-framework/reference/data-access/transaction/declarative/annotations.html#:~:text=The%20Spring%20team,a%20rollback%20scenario. ---- I discovered this after making the comment ---- It seems that the Repository with Data JPA interface is an exception where it's permissible to use Transactional.

very useful article, thanks!