I was able to learn about the two important roles of testing. I was familiar with preventing regression bugs, but writing testable code is closely related to good design. The lecture showed that testability can be increased through dependency injection and dependency inversion. Testability, good design, and dependency are all intertwined. The example project explained the process of adding tests to a typical MVC project and refactoring the layered architecture into a hexagonal architecture in an easy way, so even a first-year developer like me could understand it. The project of the team I work for also follows a similar structure. At first, I didn't understand it well, but after taking the lecture, I learned that the project has this kind of architecture structure. However, I think I still don't understand the part about separating the domain and DB entities. I think the diagnosis for "Why is my TDD failing?" is that I didn't think about the domain when designing the project, so I didn't properly utilize dependency inversion and dependency injection. I've been trying to write a lot of test codes recently, and I think this is a really helpful lecture that allows me to think about dependencies and domains while thinking about how to write tests.