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You’ve probably heard of the Fat Model/Thin Controller vs. Thin Model/Fat Controller distinction. I recently heard that you can have something in between where some of the logic from the model goes into a service layer. How common is this? and do you know of (or can think of) any real examples that illustrate it?
Martin Fowler describes the Service Layer pattern of his great book Patterns of Enterprise Application Architecture. If you care about questions like the one you asked, you should read this book.
One use that comes to my mind is managing database transactions. Some people try to encapsulate starting and committing transactions in their domain models. But then they get confused when domain models invoke other domain models that also try to start and commit db transactions. So which model really gets to decide if a transaction is committed or rolled back? And what do you do if a given model is used in different ways by different clients?
The Service Layer is a solution for this, because this is the layer in which you can start and commit work that involves multiple domain models.
As for how common this is, I don’t think it’s common at all. Most people using Zend Framework (or any other PHP or Ruby framework) have just barely moved from “Active Record solves everything” to the new shiny, “Data Mapper solves everything.” It seems this community learns only one new pattern every five years. They won’t get to Service Layer for a while.
Re comment from @ktutnik:
No, the Service Layer pattern is different from Repository pattern. Repository is about abstracting database access so you can use a database like a Collection. Service Layer is about encapsulating complex application operations.
Another way of thinking about them is their relationship to the Domain Model. The Repository is used between the Domain Model and the database. Whereas the Service Layer uses one or more Domain Models.