Development and Application of an Automated Source Code Maintainability Index

When software maintenance results in changing the source code, the change frequently occurs in an undisciplined manner that results in source code that is inherently more difficult to maintain. The long-term effect may be thought of as a downward spiral that culminates in virtually ‘unmaintainable’ code where it is more cost effective to just start again. Too often, software personnel and managers, aware that the code is becoming less maintainable, have been unable to estimate with useful accuracy the degree to which maintainability has diminished. The result is that they either continue to waste time and money maintaining code which is effectively unmaintainable, or they opt to live with what seems to be bad code, not touching it for fear of breaking it. To avoid this state of affairs, software developers need to be able to quanitify both the current level of software maintainability and the impact on it of any given change. This paper discusses the application of software metrics as a tool for quantifying code maintainability and, by extension, for making decisions. The first part of the discussion focuses on deriving a minimal set of easily calculated metrics which, when taken together, can produce a single-valued quantification (or index) of code maintainability. Case studies are then presented which serve to illustrate not only the degree to which software can degrade over time, but how this ‘maintainability index’ (MI) can be used to quantify maintainability and aid decision making. Finally, a methodology for making metric assessments readily available to software personnel is presented so that they can now easily integrate maintainability metrics into maintenance (and development) processes. As a result, the downward spiral of code degradation becomes at least recognizable, hopefully less precipitous, and perhaps avoidable entirely. © 1997 by John Wiley & Sons, Ltd.