Nice article and very detailed example! Like others pointed out I also noticed a real domain object seems to be missing; there's only `BookDto`. There doesn't seem to be any actual business logic in your domain. In my post (https://medium.com/jeroen-rosenberg/why-ravioli-is-my-favourite-recipe-34fb16f2bc3c) I argue that in such cases hexagonal architecture might be too much overhead. I'm curious as to what your thoughts are about this!