Here is the conventional layered application model.

Ignoring the infrastructure layer, which isn’t relevant to us here, there are three layers.

          The user interface is at the top in the Presentation Layer.

          All the business objects and domain state are in the Domain Layer at the bottom.

          The Application Layer in the middle mostly controls the use cases and doesn’t hold domain state.

Explicit control flows downward only; there is some control flow in the opposite direction, but it is hidden.

In the next few slides we’re going to evolve this model into a slightly different one, in which the focus is not on control but on data, and the data flows in the opposite direction.

This conventional layered model is a model of program execution.

But we are going to modify it into a requirements/design model.