Improving Design Decomposition

Decomposing a system into subsystems is essential to the design of large software systems. Traditionally, it is performed intuitively without rigorously analyzing the system model. This makes it difficult to check the decomposition correctness, and risks creating subsystems that are either too tightly coupled or not cohesive enough. An aggravating factor is that traditionally classes are the atomic design units. In many cases, however, the same classes play a role in more than one subsystem, and partitioning them unbroken among the subsystems may increase coupling and reduce cohesion. We present an analytical approach that enables reasoning about early exploration of decomposition alternatives. In addition, we describe a visual notation for diagramming the composition of subsystems, and an automatic technique for suggesting good decompositions. A key to our approach is that individual relations, not classes, are the atomic design units. We illustrate the approach with examples and demonstrate its effectiveness on a commercial system.