On the outliers of file-structure evolution: a mining study of GitHub software repositories

Context Modern software systems change continuously. Larger changes like architectural redesigns, feature additions, or system-wide refactorings regularly impact the file structures within a software repository (i.e., developers adding, deleting, or moving files). Objective While a normal evolutionary phenomenon, file-structure changes in software repositories have received little attention in past research. An important question that arises is whether outliers (i.e., changes increasing or decreasing file structures more strongly) are potential signs of quality issues in repository management and tooling. Method In this article, we contribute the first large-scale study on outliers of file-structure changes. To this end, we investigated more than 12.2 million file-structure changes from 94,247 GitHub repositories that span ten programming languages. We first performed a quantitative analysis of all these changes to establish a baseline regarding the depths of file structures and changes to these. Using this baseline, we identified and manually inspected 3,049 outliers. Results Our quantitative data shows that file-structure changes are pervasive in the evolution of software repositories, representing 17.9% of all commits across the studied projects. Via our manual inspection, we found that outliers are often associated with programming errors, initial setups, and misconfigurations of package managers. Thus, they are an indicator of potential mistakes and quality problems. Conclusions Our findings demonstrate that current practices and tools for managing software repositories should take file-structure changes into account. This could help practitioners monitor for and mitigate erroneous or unintended structural changes. Researchers can use our methodology and findings to design follow-up studies and new techniques for more robust repository management.