When an element contained by a subsystem uses some behavior of an element contained by another subsystem, a dependency is created between the enclosing subsystems. To improve reuse and reduce maintenance dependencies, we want to express this in terms of a dependency on a particular Interface of the subsystem, not upon the subsystem itself nor on the element contained in the subsystem.

The reason for this is two-fold:

• We want to be able to substitute one model element (including subsystems) for one another as long as they offer the same behavior. We specify the required behavior in terms of interfaces, so any behavioral requirements one model element has on another should be expressed in terms of interfaces.
• We want to allow the designer total freedom in designing the internal behavior of the subsystem so long as it provides the correct external behavior. If a model element in one subsystem references a model element in another subsystem, the designer is no longer free to remove that model element or redistribute the behavior of that model element to other elements. As a result, the system is more brittle.

In creating dependencies, ensure that there are no direct dependencies or associations between model elements contained by the subsystem and model elements contained by other subsystems. Also ensure that there are no circular dependencies between subsystems and interfaces; a subsystem cannot both realize an interface and be dependent on it as well.

Dependencies between subsystems, and between subsystems and packages, can be drawn directly as shown below. When shown this way, the dependency states that one subsystem (Invoice Management, for example) is directly dependent on another subsystem (Payment Scheduling Management).

Example of Subsystem Layering using direct dependencies

When there is a potential for substitution of one subsystem for another (where they have the same interfaces), the dependency can be drawn to an Interface realized by the subsystem, rather than to the subsystem itself. This allows any other model element (subsystem or class) which realizes the same interface to be used. Using interface dependencies allows flexible frameworks to be designed using replaceable design elements.

Example of Subsystem Layering using Interface dependencies