That is an example of a service interface. Most architectures now, on some level or another, are using a [service oriented architecture|en.wikipedia.org/wiki/Service-oriented_architecture]. This is what your controller will typically invoke (whether a HTTP controller or a web service). Your service tier is typically stateless. It accepts requests for model functionality and returns model results. Although not required, when you combine SOA with AOP (enough acronyms for you), the service tier becomes the logical place in many cases for cross-cutting concerns such as transactions, validations, security, error handling, etc.