Layering is probably the simplest way to discuss software architecture. It first got serious attention when John Lakos published his book about Large-scale C++ systems. Lakos argued that software consists of layers. The book introduced the concept of layering. The method is this. For each software component, count the number of other components it relies on. That is the metric of how complex the component is.
Lakos contended a good software follows the shape of a pyramid; i.e., there's a progressive increase in the cummulative complexity of each component, but not in the immediate complexity. Put differently, a good software system consists of small, reusable building blocks, each carrying its own responsibility. In a good system, no cyclic dependencies between components are present and the whole system is a stack of layers of functionality, forming a pyramid.
Lakos's work was a precursor to many developments in software engineering, most notably Refactoring. The idea behind refactoring is continuously sculpting the software to ensure it'is structurally sound and flexible. Another major contribution was by Dr Robert Martin from Object Mentor, who wrote about dependecies and acyclic architectures
Among tools that help engineers deal with system architecture are Structure 101 developed by Headway software, and SA4J developed by my former company, Information Laboratory, and now available from IBM.
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