Chapter 5. Achieving Qualities

with Felix Bachmann, Mark Klein, and Bill Wood

Note: Felix Bachmann, Mark Klein, and Bill Wood are senior members of the technical staff at the Software Engineering Institute.

Every good quality is noxious if unmixed.

-Ralph Waldo Emerson

Chapter 4 characterized a number of system quality attributes. That characterization was in terms of a collection of scenarios. Understanding what is meant by a quality attribute enables you to elicit the quality requirements but provides no help in understanding how to achieve them. In this chapter, we begin to provide that help. For each of the six system quality attributes that we elaborated in Chapter 4, we provide architectural guidance for their achievement. The tactics enumerated here do not cover all possible quality attributes, but we will see tactics for integrability in Chapter 8.

We are interested in how the architect achieves particular qualities. The quality requirements specify the responses of the software to realize business goals. Our interest is in the tactics used by the architect to create a design using design patterns, architectural patterns, or architectural strategies. For example, a business goal might be to create a product line. A means of achieving that goal is to allow variability in particular classes of functions.

Prior to deciding on a set of patterns to achieve the desired variation, the architect should consider what combination of tactics for modifiability should be applied, as the tactics chosen will guide the architectural decisions. An architectural pattern or strategy implements a collection of tactics. The connection between quality attribute requirements (discussed in Chapter 4) and architectural decisions is the subject of this chapter.