Chapter 4. Understanding Quality Attributes

with Felix Bachmann and Mark Klein

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

"Cheshire-Puss," [Alice] began, rather timidly … "Would you tell me, please, which way I ought to go from here?" "That depends a good deal on where you want to go to," said the Cat. "Oh, I don't much care where-" said Alice. Then it doesn't matter which way you go," said the Cat. "-so long as I get somewhere," said Alice. "Oh, you're sure to do that," said the Cat, "if only you walk long enough."

-Lewis Carroll, Alice's Adventures in Wonderland.

As we have seen in the Architecture Business Cycle, business considerations determine qualities that must be accommodated in a system's architecture. These qualities are over and above that of functionality, which is the basic statement of the system's capabilities, services, and behavior. Although functionality and other qualities are closely related, as you will see, functionality often takes not only the front seat in the development scheme but the only seat. This is short-sighted, however. Systems are frequently redesigned not because they are functionally deficient-the replacements are often functionally identical-but because they are difficult to maintain, port, or scale, or are too slow, or have been compromised by network hackers. In Chapter 2, we said that architecture was the first stage in software creation in which quality requirements could be addressed. It is the mapping of a system's functionality onto software structures that determines the architecture's support for qualities. In Chapter 5 we discuss how the qualities are supported by architectural design decisions, and in Chapter 7 we discuss how the architect can manage the tradeoffs inherent in any design.

Here our focus is on understanding how to express the qualities we want our architecture to provide to the system or systems we are building from it. We begin the discussion of the relationship between quality attributes and software architecture by looking closely at quality attributes. What does it mean to say that a system is modifiable or reliable or secure? This chapter characterizes such attributes and discusses how this characterization can be used to express the quality requirements for a system.