"A common operation for applications is logging a user in. It's actually two sub-operations composed together: first get the email address and password from the

"A common operation for applications is logging a user in. It's actually two sub-operations composed together: first get the email address and password from the user, second load the "user" model from the database and check whether the password matches." Why do you need to have Operations when that code could be into the model? The view tells the model: login(username, password); and the model just do that and saves the state in it's model. So there will be models that contains state and models that contains data... Why add another layer?