Sustainability Debt: A Portfolio-Based Approach for Evaluating Sustainability Requirements in Architectures

Architectural Sustainability refers to the ability of an architecture to achieve its goals while sustaining its value on dimensions related to environmental, social, economic, individual and/or technical during its operation and evolution. While the process of architectural design implies a fit between the requirements, system conditions and constraints; incomplete information and uncertainty may increase the cost of the architecture, introduce risks, alter its value and influence the extent to which it can evolve and sustain. We propose an economics-driven architectural evaluation method which extends the Cost Benefits Analysis Method (CBAM) and integrates principles of modern portfolio theory to control the risks when linking sustainability concern to architectural design decisions. The method aims at identifying portfolio(s) of architecture design decisions which are more promising for adding/delivering value while reducing risk on the sustainability dimensions. The method quantifies the sustainability debt of these decisions. The ultimate goal is to develop an objective decision-support framework for reasoning about sustainability requirements in relation architecture decisions in the presence of uncertainty. We evaluate the approach with an Emergency Deployment System (EDS). The results show that the method can make the value, cost and risks of architectural design decisions and sustainability requirements explicit.