Understanding Quality Attributes in Software Architecture
- Between stimulus and response, there is a space. In that space is our power to choose our response. In our response lies our growth and our freedom.
- —Viktor E. Frankl, Man’s Search for Meaning
As we have seen in the Architecture Influence Cycle (in Chapter 3), many factors determine the qualities that must be provided for in a system’s architecture. These qualities go beyond 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 the front seat in the development scheme. This preference is shortsighted, 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 they are too slow; or they have been compromised by hackers. In Chapter 2, we said that architecture was the first place 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 Chapters 5–11 we discuss how various qualities are supported by architectural design decisions. In Chapter 17 we show how to integrate all of the quality attribute decisions into a single design.
We have been using the term “quality attribute” loosely, but now it is time to define it more carefully. A quality attribute (QA) is a measurable or testable property of a system that is used to indicate how well the system satisfies the needs of its stakeholders. You can think of a quality attribute as measuring the “goodness” of a product along some dimension of interest to a stakeholder.
In this chapter our focus is on understanding the following:
- How to express the qualities we want our architecture to provide to the system or systems we are building from it
- How to achieve those qualities
- How to determine the design decisions we might make with respect to those qualities
This chapter provides the context for the discussion of specific quality attributes in Chapters 5–11.
4.1. Architecture and Requirements
Requirements for a system come in a variety of forms: textual requirements, mockups, existing systems, use cases, user stories, and more. Chapter 16 discusses the concept of an architecturally significant requirement, the role such requirements play in architecture, and how to identify them. No matter the source, all requirements encompass the following categories:
- Functional requirements. These requirements state what the system must do, and how it must behave or react to runtime stimuli.
- Quality attribute requirements. These requirements are qualifications of the functional requirements or of the overall product. A qualification of a functional requirement is an item such as how fast the function must be performed, or how resilient it must be to erroneous input. A qualification of the overall product is an item such as the time to deploy the product or a limitation on operational costs.
- Constraints. A constraint is a design decision with zero degrees of freedom. That is, it’s a design decision that’s already been made. Examples include the requirement to use a certain programming language or to reuse a certain existing module, or a management fiat to make your system service oriented. These choices are arguably in the purview of the architect, but external factors (such as not being able to train the staff in a new language, or having a business agreement with a software supplier, or pushing business goals of service interoperability) have led those in power to dictate these design outcomes.
What is the “response” of architecture to each of these kinds of requirements?
- Functional requirements are satisfied by assigning an appropriate sequence of responsibilities throughout the design. As we will see later in this chapter, assigning responsibilities to architectural elements is a fundamental architectural design decision.
- Quality attribute requirements are satisfied by the various structures designed into the architecture, and the behaviors and interactions of the elements that populate those structures. Chapter 17 will show this approach in more detail.
- Constraints are satisfied by accepting the design decision and reconciling it with other affected design decisions.