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Study after study has shown poor requirements management is the leading cause of failure for traditional software development teams. When it comes to requirements, agile software developers typically focus on functional ones that describe something of value to end users—a screen, report, feature, or business rule. Most often these functional requirements are captured in the form of user stories, although use cases or usage scenarios are also common, and more advanced teams will iteratively capture the details as customer acceptance tests. Over the years, agilists have developed many strategies for dealing with functional requirements effectively, likely one of the factors leading to the higher success rates enjoyed by agile teams. Disciplined agile teams go even further, realizing that there is far more to requirements than just this, that we also need to consider nonfunctional requirements and constraints.
Nonfunctional requirements (NFRs), also known as "technical requirements" or "quality of service" (QoS) requirements, focus on aspects that typically cross-cut functional requirements. Common NFRs include accuracy, availability, concurrency, consumability (a superset of usability), environmental/green concerns, internationalization, operations issues, performance, regulatory concerns, reliability, security, serviceability, support, and timeliness.
A constraint defines a restriction on your solution, such as being required to store all corporate data in DB2 per your enterprise architecture, or only being allowed to use open source software (OSS), which conforms to a certain level of OSS license. Constraints can often impact your technical choices by restricting specific aspects of your architecture, defining suggested opportunities for reuse, and even architectural customization points. Although many developers will bridle at this, the reality is that constraints often make things much easier for your team because some technical decisions have already been made for you. I like to think of it like this—agilists will have the courage to make tomorrow's decisions tomorrow, disciplined agilists have the humility to respect yesterday's decisions as well.
Although agile teams have pretty much figured out how to effectively address functional requirements, most are still struggling with NFRs and constraints.
Author: Scott Ambler
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