Using Quality Attribute Workshops to Evaluate Early-Stage Architecture Design Decisions
MARIO BARBACCI

The architectural decisions made in the beginning of the design process, by and large, determine the software's quality attributes in the end. These architectural design decisions are the hardest to change after the system has been implemented. Therefore, they are the most important to get right. To help architects and other stakeholders evaluate the implications of early-stage design decisions, the Software Engineering Institute has developed the Quality Attribute Workshop (QAW) method. This approach uses test cases to examine an architecture's ability to achieve desired attributes. It represents a cost-effective and efficient means of exploring the impact of architectural decisions before they are made.

In our March 2000 column we presented our ideas for the QAW method and our early experiences from conducting one workshop. Since then we have modified the method based on our findings from several workshops. This column presents a more refined view of the QAW method.

Quality attributes are interdependent: performance affects modifiability, availability affects safety, security affects performance, and everything affects cost. Design evaluations can help architects explore the impact of decisions on the quality attributes of their software systems. Ideally, these assessments should take place as early in the architecture-development stage as possible. The SEI has developed the Quality Attribute Workshop (QAW) method, which uses test cases to assess an architecture for quality attributes early in the design process. (For a more complete definition of the QAW method, and how it differs from the Architecture Tradeoff Analysis Method, see the sidebar QAW and ATAM: How They Differ.)

The QAW process can be applied before the architecture has been completely designed. It can be used on a wide range of architectural representations. It also requires relatively little time and effort on the part of QAW participants. Yet the QAW method can help designers ensure that the resulting architecture will deliver the quality attributes that the system and its stakeholders require.

 

The QAW Process

The QAW process is composed of four steps. As shown in Figure 1, these steps are: (1) scenario generation, where we generate, prioritize, and refine scenarios; (2) test case development; (3) test case analysis, in which test cases are analyzed against the architecture; and (4) results presentation, in which results are presented to other stakeholders.

Figure 1: The QAW Process

To understand the power and the potential of the QAW, let's examine each step in detail.