Measuring the evolutionary stability of software systems: case studies of Linux and FreeBSD

In software evolution, stability is defined as the ability of a module to remain largely unchanged when faced with newer requirements and/or changes in the environment. Although stability is an important long-term design characteristic for hardware systems, it has not been studied deeply for software systems. Stability is directly related to software evolvability and maintainability; and it affects the software evolution process. A model based on software version differences is presented to measure the evolutionary stability of software modules. This model represents and normalises two distances: the source code and the structure distances, between two versions of an evolving software module. As a case study based on this model, the evolutionary stability of Linux and FreeBSD modules is compared. The results of the study shows that the evolutionary stability of a software module in Linux and FreeBSD depends more on its function type and less on the system environment.