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Abstract. The conventional rationale for using COTS (commercial offthe-shelf) components is that the more a software system is built from COTS products, the lower the cost of initial development. Less understood is that during the long term sustainment phase -from deployment through retirement -the cost of maintenance of a COTS-based system generally increases as the number of COTS products used increases. There exists then a tension between the imperative to maximize the use of COTS components to ease CBS development yet minimize the use of COTS components to ease CBS maintenance. A heuristic called the "CBS Functional Density Rule" is proposed to reconcile these two conflicting views. A corresponding metric for characterizing the "efficiency" of a given CBS design relative to another called the "COTS Functional Density" is then suggested. The paper concludes with suggestions for additional research to further validate the empirical foundations of the proposed heuristic and associated metric.

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