Prioritizing Unit Test Creation for Test-Driven Maintenance of Legacy Systems

Test-Driven Development (TDD) is a software development practice that prescribes writing unit tests before writing implementation code. Recent studies have shown that TDD practices can significantly reduce the number of pre-release defects. However, most TDD research thus far has focused on new development. We investigate the adaptation of TDD-like practices for already implemented code, in particular legacy systems. We call this adaptation of TDD-like practices for already implemented code ``Test-Driven Maintenance´´ (TDM). In this paper, we present an approach that assists software development and testing managers, who employ TDM, utilize the limited resources they have for testing legacy systems efficiently. The approach leverages the development history of the project to generate a prioritized list of functions that managers should focus their unit test writing resources on. The list is updated dynamically as the development of the legacy system progresses. To evaluate our approach, we conduct a case study on a large commercial legacy software system. Our findings suggest that heuristics based on the function size, modification frequency and bug fixing frequency should be used to prioritize the unit test writing of legacy systems.