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Chapter 12. The CBAM: A Quantitative Approach to Architecture Design Decision Making

with Jai Asundi and Mark Klein

Note: Jai Asundi teaches at the University of Texas, Dallas; Mark Klein is on the technical staff at the Software Engineering Institute.

A billion here, a billion there, pretty soon it adds up to real money.

-U.S. Senator Everett Dirksen (1896-1969)

As we saw in Chapter 11, the Architecture Tradeoff Analysis Method (ATAM) provides software architects a means of evaluating the technical tradeoffs faced while designing or maintaining a software system. In the ATAM, we are primarily investigating how well the architecture-real or proposed-has been designed with respect to the quality attributes that its stakeholders have deemed important. We are also analyzing architectural tradeoffs-the places where a decision might have consequences for several quality attributes simultaneously.

However, the ATAM is missing an important consideration: The biggest tradeoffs in large, complex systems usually have to do with economics. How should an organization invest its resources in a manner that will maximize its gains and minimize its risk? In the past, this question primarily focused on costs, and even then these were primarily the costs of building the system in the first place and not the long-term costs through cycles of maintenance and upgrade. As important, or perhaps more important than costs, are the benefits that an architectural decision may bring to an organization.

Given that the resources for building and maintaining a system are finite, there must be a rational process that helps us choose among architectural options, during both an initial design phase and subsequent upgrade periods. These options will have different costs, will consume differing amounts of resources, will implement different features (each of which brings some benefit to the organization), and will have some inherent risk or uncertainty. To capture these aspects we need economic models of software that take into account costs, benefits, risks, and schedule implications.

To address this need for economic decision making, we have developed a method of economic modeling of software systems, centered on an analysis of their architectures. Called the Cost Benefit Analysis Method (CBAM), it builds on the ATAM to model the costs and the benefits of architectural design decisions and is a means of optimizing such decisions. The CBAM provides an assessment of the technical and economic issues and architectural decisions.

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