There are few aspects of modern life that remain unaffected by software, and as our day-to-day challenges change, so too must our software. Software systems are complex, and as they grow larger and more interconnected, they become more difficult to modify due to excessive change propagation. This is known as the ripple effect. The primary strategies to mitigate it are modular design, and minimization of coupling, or between-module interaction. However, analysis of complex networks has shown that many are scale-free, which means that they contain some components that are highly connected. The presence of scale-free structure implies high coupling, which suggests that software systems may be hard to modify because they suffer from the ripple effect.
In this thesis, a large corpus of open-source software systems is analysed to determine whether software systems are scale-free, whether scale-free structure results in high coupling, and whether high coupling results in ripple effects that propagate change to a large proportion of classes.
The results show that all systems in the corpus are scale-free and that that property results in high coupling. However, analysis of system evolution reveals that existing code is modified infrequently and that there is rarely sufficient evidence to be confident that ripple effects involving a high proportion of classes have actually occurred. This thesis concludes first that while it is desirable to avoid excessive interconnectivity, it is difficult to completely eliminate high coupling; and second, that the presence of high coupling does not necessarily imply poor system design
