Explain different between functional and non-functional requirements.
Functional vs Non-Functional Requirements​
Software requirements are broadly categorized into functional and non-functional requirements. While both are essential for successful software development, they serve different purposes and focus on different aspects of the system.
Functional Requirements​
Functional requirements specify what the system should do - the specific functions, features, or services that the system must provide.
Key Characteristics of Functional Requirements:​
- Behavior-Oriented: Describe specific behaviors, functions, or capabilities
- Feature-Specific: Define particular features that must be implemented
- Input-Output Related: Specify inputs, processes, and expected outputs
- User-Focused: Directly visible to users and fulfill user needs
- Task-Oriented: Define tasks the system must perform
Examples of Functional Requirements:​
- A user shall be able to search for products by name, category, or description
- The system shall send email notifications when an order is placed
- An administrator shall be able to create, update, and delete user accounts
- The system shall perform automatic backups of data every 24 hours
- The application shall verify user credentials before granting access
Non-Functional Requirements​
Non-functional requirements specify how the system should work - the quality attributes, constraints, and properties that define the manner in which the system delivers its functionality.
Key Characteristics of Non-Functional Requirements:​
- Quality-Focused: Define quality attributes and performance criteria
- Constraint-Oriented: Impose restrictions or conditions on the system design and implementation
- System-Wide: Apply across the entire system rather than to specific features
- Often Measurable: Typically include metrics or measurable criteria
- Environment-Related: Define how the system interacts with its operational environment
Categories of Non-Functional Requirements:​
- Performance: Response time, throughput, resource utilization
- Reliability: Availability, fault tolerance, recoverability
- Security: Authentication, authorization, data protection
- Usability: User interface, accessibility, learnability
- Scalability: Growth capacity, adaptability to increasing load
- Maintainability: Modifiability, testability, extensibility
- Portability: Adaptability to different environments
- Compliance: Adherence to standards and regulations
Examples of Non-Functional Requirements:​
- The web application shall load within 3 seconds for 95% of requests
- The system shall be available 99.9% of the time (maximum 8.76 hours of downtime per year)
- All sensitive data shall be encrypted using AES-256 encryption
- The user interface shall be accessible to users with visual impairments (WCAG 2.1 compliant)
- The system shall support up to 1 million records without performance degradation
Key Differences​
| Aspect | Functional Requirements | Non-Functional Requirements |
|---|---|---|
| Focus | What the system does | How the system behaves |
| Nature | Features and capabilities | Quality attributes and constraints |
| Verification | Verified through specific test cases | Measured through performance metrics |
| Expression | Expressed as functions or features | Expressed as quality attributes or constraints |
| Negotiability | Usually mandatory (must have) | Often negotiable based on priorities |
| User Visibility | Directly visible to users | Often invisible to users but affect experience |
| Scope | Feature-specific | System-wide |
| Failure Impact | Feature doesn't work | System works poorly or is hard to use |
Importance of Balancing Both Types​
A well-designed system must balance both functional and non-functional requirements. A system may implement all functional requirements perfectly but still fail to satisfy users if non-functional requirements such as performance, usability, and reliability are neglected.
Conversely, excellent non-functional characteristics cannot compensate for missing core functionality. Successful software development requires careful consideration of both types of requirements and their interdependencies throughout the development lifecycle.