Non-Functional Requirement (NFR)
A Non-Functional Requirement (NFR) defines how well the system performs, rather than what it does. It specifies quality attributes, constraints, and operational characteristics of the application.
Non-functional requirements answer: “How should the system behave under certain conditions?”
1. Definition
A Non-Functional Requirement (NFR) defines how well the system performs, rather than what it does. It specifies quality attributes, constraints, and operational characteristics of the application.
Non-functional requirements answer: “How should the system behave under certain conditions?”
2. Purpose of Non-Functional Requirements
- Define quality standards
- Set user experience expectations
- Ensure system reliability and stability
- Reduce business and operational risk
- Provide acceptance benchmarks beyond functionality
3. Key Characteristics of Good NFRs
- Measurable and verifiable
- Clear and specific
- Testable (even if conceptually/manual)
- Aligned with business expectations
- Not implementation-specific
4. Common Types of Non-Functional Requirements
4.1 Performance
- Response time
- Throughput
- Resource usage
Example: Page should load within 3 seconds under normal load.
4.2 Security
- Authentication and authorization rules
- Data protection
- Session handling
Example: User session expires after 15 minutes of inactivity.
4.3 Usability
- Ease of use
- Error message clarity
- Navigation simplicity
Example: Error messages must be user-friendly and actionable.
4.4 Reliability
- System availability
- Failure handling
- Recovery behavior
Example: System should recover automatically after restart.
4.5 Compatibility
- Browser, OS, device support
Example: Application must work on Chrome, Edge, and Firefox.
4.6 Scalability
- Ability to handle growth
Example: System should support 2× user growth without degradation.
4.7 Accessibility
- Support for users with disabilities
Example: Application must support keyboard navigation.
5. Manual Tester’s Role in NFR Validation
- Review NFRs during requirement analysis
- Identify missing quality attributes
- Validate NFRs conceptually or through observation
- Raise risks where measurement is unclear
- Ensure NFRs are included in acceptance discussions
6. Non-Functional Requirement vs Functional Requirement
| Aspect | Functional Requirement | Non-Functional Requirement |
|---|---|---|
| Defines | What system does | How system behaves |
| Examples | Login, Checkout | Performance, Security |
| Failure impact | Feature broken | Poor user experience / risk |
7. Common Issues with NFRs
- Not documented explicitly
- Too vague (“fast”, “secure”)
- Assumed instead of specified
- Tested too late
8. Real-Time Example
Functional: User can submit a form.
Non-Functional:
- Form submission completes within 2 seconds
- Error message displayed clearly on failure
9. Interview-Ready Answers
Short answer:
Non-functional requirements define how well a system should perform rather than what it should do.
Detailed answer:
Non-functional requirements specify quality attributes such as performance, security, usability, and reliability, ensuring the system meets user and business expectations.
10. Key Takeaway
Non-Functional Requirements define quality expectations—a system can be functionally correct but still fail without strong NFRs.