In software development, requirements engineering is a necessary step that defines what a system should do and how it should perform; the gathering of requirements is mostly done by system analysts or developers based on what the organisation wants. While functional requirements describe the specific behaviours or functions of a system, non-functional requirements (NFRs) define the qualities and constraints that govern how the system operates. These requirements are critical because even if a system meets all functional requirements, failure to satisfy its non-functional requirements can result in poor user experience, security vulnerabilities, or system failures; this leads to failure in achieving organization objectives


Understanding Non-Functional Requirements


Non-functional requirements are the criteria that judge the operation of a system rather than specific behaviours. They often address "how" the system performs tasks rather than "what" tasks it performs. Examples include performance, reliability, security, scalability, maintainability, and usability. While functional requirements can often be tested with straightforward scenarios, NFRs frequently involve performance metrics, quality standards, and compliance considerations.

Organizations must always learn to test how their systems perform in order to make patches


Why Non-Functional Requirements Are Important


Neglecting NFRs can have serious consequences. For instance, a web application may provide all required features (functional), but if it cannot handle high traffic, it may crash under load and this sometimes means stepping back in the development process. Similarly, a system may be highly functional, but poor security measures can leave it vulnerable to cyber attacks such as 


  1. denial of service
  2. SQL injections
  3. Man in the middle attacks
  4. Ransomware attacks
  5. Password attacks
  6. Zero day exploits

Non-functional requirements ensure that the system is robust, reliable, and usable, ultimately determining the success and adoption of the software.


Common Categories of Non-Functional Requirements

  1. Performance: Measures how efficiently the system responds to inputs or executes tasks. Examples include response time, throughput, and resource utilization.

  2. Security: Protects data integrity, confidentiality, and availability. Security requirements may include encryption, authentication, authorization, and compliance with data protection regulations.

  3. Usability: Ensures the system is easy to learn and use. This includes user interface design, accessibility features, and user satisfaction.

  4. Reliability: Indicates how consistently the system performs under expected conditions. Reliability encompasses fault tolerance, error handling, and recovery procedures.

  5. Maintainability: Defines how easily the system can be updated, fixed, or enhanced. Maintainable systems reduce long-term costs and improve adaptability to changing requirements.

  6. Scalability: Determines how well the system can handle growth in users, data, or workload. Scalable systems accommodate expansion without significant degradation in performance.

  7. Availability: Specifies the system's operational uptime. High-availability systems are crucial for mission-critical applications where downtime can lead to substantial losses.



Examples of Non-Functional Requirements in Real Systems

  • An e-commerce website must handle at least 10,000 concurrent users without noticeable slowdown (performance and scalability).

  • A banking application must encrypt sensitive user data both in transit and at rest (security).

  • A mobile app should allow users to complete core tasks in less than three steps (usability).

  • A cloud service should maintain 99.99% uptime annually (availability).


Conclusion

Non-functional requirements are fundamental to the success of any system. They ensure that the software not only performs the intended functions but also meets standards for quality, security, and user satisfaction. By properly identifying, documenting, and validating NFRs during requirements engineering, organizations can build systems that are robust, reliable, and adaptable to future needs. Ignoring them can result in systems that are technically functional but practically ineffective or unsafe, emphasizing that NFRs are not optional but essential for software excellence.