Demystifying maintainability

Due to its economic impact "maintainability" is broadly accepted as an important quality attribute of software systems. But in contrast to attributes such as performance and correctness, there is no common understanding of what maintainability actually is, how it can be achieved, measured, or assessed. In fact, every software organization of significant size seems to have its own definition of maintainability. We address this problem by defining an unique two-dimensional quality model that associates maintenance activities with system properties including the capabilities of the organization. The separation of activities and properties facilitates the identification of sound quality criteria and allows to reason about their interdependencies. The resulting quality controlling process enforces these criteria through toolsupported measurements as well as manual inspections. We report on our experiences with the incremental development of the quality model and its application to large scale commercial software projects. Among the positive effects are a slowdown of decay and a significantly increased awareness for long-term quality aspects.