Recovering Maintainability Effort in the Presence of Global Data Usage∗

As the useful life expectancy of software continues to increase, the task of maintaining the source code has become the dominant phase of the software life-cycle. In order to improve the ability of software to age and successfully evolve over time, it is important to identify system design and programming practices which may result in increased difficulty for maintaining the code base. This study attempts to correlate the use of global variables to the maintainability of several widely deployed, large scale software projects as they evolve over time. Two measures are proposed to quantify the maintenance effort of a project. The first measure compares the number of CVS revisions for all source files in a release to the number of revisions applied to files where the usage of global data is most prevalent. A second degree of change is characterized by contrasting the amount of source code that was changed overall to the changes made to those source files which contain the majority of the references to global data. We observed a strong correlation between the number of revisions to global variables references and lines of code to global variable references. In all cases the correlation between the number of revisions and global variable references was stronger. This provides evidence that a strong relationship exists between the usage of global variables and both the number and scope of changes applied to files between product releases.