Is 100% Test Coverage a Reasonable Requirement? Lessons Learned from a Space Software Project

To ensure the dependability and safety of spaceflight devices, rigorous standards are defined. Among others, one requirement from the European Cooperation for Space Standardization (ECSS) standards is 100% test coverage at software unit level. Different stakeholders need to have a good knowledge of the implications of such a requirement to avoid risks for the project that this requirement might entail. In this paper, we study if such a 100% test coverage requirement is a reasonable one. For this, we interviewed the industrial developers who ran a project that had the sole goal of achieving 100% unit test coverage in a spaceflight software. We discuss costs, benefits, risks, effects on quality, interplay with surrounding conditions, and project management implications. We distill lessons learned with which we hope to support other developers and decision makers when considering a 100% unit test coverage requirement.