- Many web sites have lists and examples of NFR's.
- Some try to define NFR's, few succeed.
- Others admit that NFR's are difficult to gather.
- Few apply NFR’s to systems (vs. software)
- FURPS+, ISO-9126, ISO-25010 and similar didn't treat security as a first class citizen, nor did they address legal requirements.
- Erik Simmons and John Terzakis (Intel) each have a fair bit of good information in various presentations that are readily searchable.
- Tom Gilb's 'Planguage' seemed like a valuable tool, and both Simmons and Terzakis describe how to use Planguage for requirements writing.
Specifying Effective Non-Functional Requirements, John Terzakis Intel Corporation June 24, 2012 ICCGI Conference Venice, Italy21st Century Requirements Engineering: A Pragmatic Guide to Best Practices, Erik Simmons, Intel Corporation
Into the Non-Functional Requirement Abyss
- Functional Requirements describe the intended behavior of the system (or software), or what a system should do.
- Non-functional Requirements describe how well the system does whatever it does and under what constraints the system must operate. NFR's describe operational characteristics, performance, availability, etc.
Specific: Requirements will be clear, concise, unambiguous, with consistent terminology, and with detail sufficient such that designs based on the requirements will meet operational goals.Measurable: A test can be devised that verifies the requirement using a bounded measurement.Attainable: The requirement is technically feasible within the constraints of current technology, and for which there is at least one design and implementation.Realizable: The requirement is fiscally and manageably implementable within the constraints of organizational budget and staffing.Unambiguous: The requirement will have a single, non-conflicting interpretation.Traceable: The source of a requirement will be traceable to stakeholder need. The requirement is traceable to business strategy or roadmap. The life cycle of the requirement is traceable from its conception to its current state.
Resiliency - The requirements that describe the ability of the system to continue to function during common failure modes. A resilient system continues to work after routine failures (disk, server, OS or process). Resiliency is necessary to meet availability requirements and usability requirements. A resilient system may use technologies such as redundancy, clustering, load balancing, error handling, and error recovery to function after component failure. Resiliency encompasses the concepts of availability, reliability, robustness, fault tolerance and exception handling as described by other authors.
Recoverability - The requirements that describe the ability to recover from failed states and return the system to its as-built condition. Using the example of a failed unit of hardware, a resilient system will continue to function after failure, a recoverable system will have a simple and predictable method for recovering from the hardware failure. Data backups, data replication, hot-swap hard drives, and automated operating system and application deployment tools may be technologies or techniques to recover a failed component.
Maintainability - The requirements that describe the ability to maintain the system over its operational life. Among other attributes, a maintainable system can have routine hardware upgrades and application deployments without user affecting outages, it will have monitoring, logging and auditing sufficient for routine troubleshooting, it will have a low operational cost. Maintainability encompasses manageability, upgradability, deployability and flexibility as describe by other authors.
Scalability -The requirements that describe the ability to add and remove capacity to the system without affecting the availability to the system, while maximizing maintainability and constraining costs.
Security - The ability to maintain the confidentiality and integrity of a system and the data contain in or controlled by the system. Requirements related to system access, system integrity, system confidentiality and system configuration.