Seeking Empirical Evidence for Self-Organized Criticality in Open Source Software Evolution

We examine eleven open source software systems and present empirical evidence for the existence of fractal structures in software evolution. In our study, fractal structures are measured as power laws through the lifetime of a software system. We describe two specific power law related phenomena: the probability distribution of software changes decreases as a power function of change sizes; and the time series of software change exhibits long range correlations with power law behavior. The existence of such spatial (across the system) and temporal (over the system lifetime) power laws suggests that Self-Organized Criticality (SOC) occurs in the evolution of open source software systems. As a consequence, SOC may be useful as a conceptual framework for understanding software evolution dynamics (the cause and mechanism of change or growth). We give a qualitative explanation of software evolution based on SOC. We also discuss some potential implications of SOC to current software practices.