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

2. 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

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

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

5. Separate Interface PackagesU
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