Chapter Outline Architecture and Requirements Functionality Quality Attribute Considerations Specifying Quality Attribute Requirements Achieving Quality Attributes through Tactics Guiding Quality Design Decisions Summary

System requirements can be categorized as: Functional requirements. These requirements state what the system must do, how it must behave or react to run-time stimuli. Quality attribute requirements. These requirements annotate (qualify) functional requirements. Qualification might be how fast the function must be performed, how resilient it must be to erroneous input, how easy the function is to learn, etc. Constraints. A constraint is a design decision with zero degrees of freedom. That is, it’s a design decision that has already been made for you. Functionality Functionality is the ability of the system to do the work for which it was intended. Quality Attribute Considerations If a functional requirement is "when the user presses the green button the Options dialog appears”: a performance QA annotation might describe how quickly the dialog will appear; an availability QA annotation might describe how often this function will fail, and how quickly it will be repaired; a usability QA annotation might describe how easy it is to learn this function. Quality Attribute Considerations There are three problems with previous discussions of quality attributes: 1. The definitions provided for an attribute are not testable. It is meaningless to say that a system will be “modifiable”. 2. Endless time is wasted on arguing over which quality a concern belongs to. Is a system failure due to a denial of service attack an aspect of availability, performance, security, or usability? 3. Each attribute community has developed its own vocabulary. Quality Attribute Considerations A solution to the first two of these problems (untestable definitions and overlapping concerns) is to use quality attribute scenarios as a means of characterizing quality attributes. A solution to the third problem is to provide a discussion of each attribute—concentrating on its underlying concerns—to illustrate the concepts that are fundamental to that attribute community