U Layered Architecture M There are up-calls and down-calls. L

Program to Interfaces U I1 I2 Allows any layer to be re-implemented in a different way. M Does this structure lead to upward dependencies in the layered structure? It depends on who “owns” the interfaces (i.e., which packages you put the interfaces in). I3 I4 L

Pure Dependency Inversion Callers own the interfaces. M This results in upward dependencies, but maybe we can live with it. I3 I4 L

Pure Layering U I1 I2 Lower layer owns the interfaces. M All dependencies flow downward. I3 I4 L

Separate Interface Packages U Separate Interface Packages I1 I2 Create separate packages to contain interfaces. M Enhances reusability of layers (makes it easier to take the interfaces you need to a new system). I3 I4 L