Quality Impacts of Clandestine Common Coupling

The increase in maintenance of software and the increased amounts of reuse are having major positive impacts on the quality of software, but are also introducing some rather subtle negative impacts on the quality. Instead of talking about existing problems (faults), developers now discuss potential problems, that is, aspects of the program that do not affect the quality initially, but could have deleterious consequences when the software goes through some maintenance or reuse. One type of potential problem is that of common coupling, which unlike other types of coupling can be clandestine. That is, the number of instances of common coupling between a module M and the other modules can be changed without any explicit change to M. This paper presents results from a study of clandestine common coupling in 391 versions of Linux. Specifically, the common coupling between each of 5332 kernel modules and the rest of the product as a whole was measured. In more than half of the new versions, a change in common coupling was observed, even though none of the modules themselves was changed. In most cases where this clandestine common coupling was observed, the number of instances of common coupling increased. These results provide yet another reason for discouraging the use of common coupling in software products.