Let's assume I have two service classes with the following methods: GroupService createGroup() deleteGroup() updateGroup() findGroup() UserService createUser() deleteUser() updateUser() findUser() Now, I am thinking about the aesthetics of theses classes. Imagine we want to implement a method which search for all user of a specific group. Which service class is responsible for such a method? I mean, the return value is a user (or maybe a collection of users) but the parameter (which means the name of the group) is a group. So which service class is the better place to put this method in?