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Non-Functional Requirements in Architecture

 

Introduction

The architecture of software systems is primarily shaped by functional requirements, quality attributes (also known as non-functional requirements), and constraints.

Software architects make critical design decisions to decompose a system based on these requirements. While functional requirements define what the system does, non-functional requirements (NFRs) dictate how well the system performs. Many of these decisions focus on achieving quality attributes while considering external constraints such as time, cost, and existing technology limitations.

Tactics and Their Role in Architecture

Tactics are specific design decisions that help achieve a quality attribute (Bass et al., 2003). These decisions influence both the structure and behavior of a system but usually have a smaller impact compared to architectural patterns. While patterns define high-level structures and interactions, tactics provide low-level solutions to enhance quality attributes. In some cases, a tactic may be a subset of a larger architectural pattern.

Architectural Methods for Handling Quality Attributes

Several architectural methodologies provide structured approaches to achieving NFRs:

  • Viewpoints and Perspectives – A method used to systematically address different architectural concerns.

  • Microsoft Software Architecture Method – A set of industry best practices to ensure software reliability and maintainability.

  • Process of Software Architecting (PSA) – An iterative method focusing on balancing functional and non-functional aspects.

  • ACDM (Architectural Conceptual Design Model) – A methodology for high-level system design.

  • RUP (Rational Unified Process) – A disciplined approach to software development with a focus on iterative design.

  • ADD (Attribute-Driven Design) – A methodology specifically focused on achieving quality attributes through structured decision-making.

Among these, ADD (Attribute-Driven Design) is particularly significant as it explicitly prioritizes non-functional requirements in architectural decisions.

Quality Attributes and Their Standards

Non-functional requirements are categorized under different standards, including ISO/IEC 25010:2011, CISQ, and FURPS.

ISO/IEC 25010:2011 (Current Standard)

This standard replaces ISO/IEC 9126 and provides a comprehensive model for assessing software quality:

Quality in Use:

  • Effectiveness

  • Efficiency

  • Satisfaction

  • Freedom from Risk

  • Context Coverage

Product Quality:

  • Functional Suitability

  • Reliability

  • Performance Efficiency

  • Usability

  • Security

  • Compatibility

  • Maintainability

  • Portability

Each of these attributes has further subcategories to define specific evaluation criteria.

CISQ (Consortium for IT Software Quality)

CISQ focuses on software quality at the code level to ensure robustness and maintainability:

  • Reliability – Ensuring system availability and fault tolerance.

  • Performance/Efficiency – Optimizing system responsiveness.

  • Security – Protecting against vulnerabilities and threats.

  • Maintainability – Ensuring ease of future modifications and bug fixes.

FURPS Model

FURPS is a widely used model introduced by HP to classify software requirements:

  • Functionality – The system’s ability to meet business requirements.

  • Usability – The ease with which users can interact with the system.

  • Reliability – The system’s ability to perform consistently under different conditions.

  • Performance – The efficiency of execution in terms of speed and resource usage.

  • Supportability – The ease of maintaining and extending the system.

Conclusion

Non-functional requirements play a crucial role in shaping software architecture. Effective architectural methodologies, such as ADD, help ensure that quality attributes are met alongside functional requirements. Standards like ISO/IEC 25010, CISQ, and FURPS provide structured frameworks to assess and enhance software quality. By strategically implementing tactics and patterns, architects can build systems that are not only functionally rich but also robust, scalable, and maintainable.

Interesting resources

1. How do architecture patterns and tactics interact? A model and annotation

http://www.cs.rug.nl/paris/papers/JSS10b.pdf

2. ADD 3.0: Rethinking Drivers and Decisions in the Design Process

https://resources.sei.cmu.edu/asset_files/Presentation/2015_017_101_438648.pdf

3. Decision Model for Software Architectural Tactics Selection Based on Quality Attributes Requirements

https://www.researchgate.net/publication/283953802_Decision_Model_for_Software_Architectural_Tactics_Selection_Based_on_Quality_Attributes_Requirements

4. Modeling Architectural Non Functional Requirements: From Use Case to Control Case

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.460.7189&rep=rep1&type=pdf

5. Rigorous Component-Based System Design Using the BIP Framework

http://www-verimag.imag.fr/~sifakis/RecentPublications/2011/ieee-software.pdf

6. Proposed Framework for Handling Architectural NFR’s within Scrum Methodology

http://worldcomp-proceedings.com/proc/p2015/SER3465.pdf

7. https://it-cisq.org/standards/code-quality-standards/

8. https://www.iso.org/obp/ui/#iso:std:iso-iec:25010:ed-1:v1:en

9. https://www.ibm.com/developerworks/rational/library/4706.html#N100A7

10. http://www.users.abo.fi/lpetre/SA11/SALecture2.pdf

11. https://resources.sei.cmu.edu/asset_files/TechnicalReport/2007_005_001_14858.pdf

12. https://resources.sei.cmu.edu/asset_files/TechnicalReport/2009_005_001_15101.pdf

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