Identifying functional and non-functional requirements hinges on understanding what a system must do versus how well it must perform.
How to Identify Functional and Nonfunctional Requirements?
Functional requirements define specific actions or behaviors a system must exhibit to work, while non-functional requirements detail the qualities and constraints governing how that system should operate and perform. Without functional requirements, a system simply will not work; without meeting at least some non-functional requirements to a certain level, users will quickly become frustrated.
What are Functional Requirements?
Functional requirements specify the core functionalities or features of a system. They describe what the system is supposed to do, directly supporting the user's tasks and business processes. These are typically expressed as actions, operations, or behaviors.
Key Characteristics:
- Action-oriented: Describe specific actions the system performs.
- Directly observable: Users can see and interact with these features.
- Essential for system operation: The system cannot fulfill its purpose without them.
- Often captured in use cases or user stories.
Examples of Functional Requirements:
- The system must allow users to log in with a valid username and password.
- The system shall enable customers to add items to a shopping cart.
- The system must generate a sales report weekly.
- The system shall verify user input for valid email format.
- The system should send an email confirmation upon successful order placement.
What are Non-Functional Requirements?
Non-functional requirements (NFRs), also known as quality attributes or "ilities," describe the qualities of a system or constraints on its design and implementation. They define criteria for judging the operation of a system, rather than specific behaviors. While not directly visible as features, they significantly impact user satisfaction and system success.
Key Characteristics:
- Quality-oriented: Focus on criteria like performance, security, usability, reliability, and scalability.
- Define "how well": How fast, how secure, how easy to use.
- Crucial for user experience: Impact user satisfaction, system adoption, and overall business value.
- Often measured and tested for compliance.
Examples of Non-Functional Requirements:
- Performance: The system must load a user's dashboard within 2 seconds.
- Security: User passwords shall be encrypted using AES-256 before storage.
- Usability: The system should provide an intuitive interface, allowing new users to complete basic tasks within 5 minutes.
- Reliability: The system must be available 99.9% of the time.
- Scalability: The system shall support up to 10,000 concurrent users without degradation in response time.
- Maintainability: The codebase should be modular to facilitate future updates.
- Portability: The application must be compatible with both Windows and macOS operating systems.
Key Differences at a Glance
| Feature | Functional Requirements | Non-Functional Requirements |
|---|---|---|
| What it describes | What the system does (specific actions/behaviors). | How well the system does it (qualities and constraints). |
| Importance | Essential for the system to work at all. | Essential for the system to be useful and satisfying to users. |
| Visibility | Directly observable by users. | Often not directly observable, but impact user experience. |
| Examples | Login, generate report, add to cart. | Performance, security, usability, reliability. |
| Nature | Feature-oriented. | Quality-oriented; constraints. |
| Consequences of omission | System failure, inability to perform core tasks. | User frustration, poor adoption, system instability, security risks. |
| Testing focus | Black-box testing, use case testing. | Performance testing, security testing, usability testing. |
Practical Tips for Identifying Requirements
Identifying both types of requirements is a collaborative process that involves various stakeholders.
1. Ask the Right Questions
-
For Functional Requirements (What):
- What does the user need to do with this system?
- What information does the system need to process?
- What outputs should the system produce?
- What operations will the system perform on the data?
- How will the system interact with other systems?
- What are the specific tasks a user will accomplish?
-
For Non-Functional Requirements (How Well):
- How fast does the system need to be? (Performance)
- How secure does the data need to be? (Security)
- How easy should the system be to learn and use? (Usability)
- How reliable does the system need to be (e.g., uptime)? (Reliability)
- How many users should the system support concurrently? (Scalability)
- How easily can the system be updated or maintained? (Maintainability)
- What environments must the system operate in? (Portability, Compatibility)
- What regulations or legal standards must the system comply with? (Compliance)
2. Utilize Diverse Elicitation Techniques
- Interviews and Workshops: Direct conversations with stakeholders to understand their needs and expectations.
- Use Cases and User Stories: Describe how a user will interact with the system to achieve a goal. Functional requirements naturally emerge from these. Non-functional aspects can be added as "acceptance criteria" or separate stories (e.g., "As a user, I want to log in quickly").
- Scenarios and Storyboarding: Walk through specific user interactions step-by-step, revealing both functional flows and potential non-functional pain points.
- Quality Attribute Workshops (QAWs): Dedicated sessions to brainstorm, prioritize, and define quality attributes with stakeholders.
- Prototyping and Wireframing: Visual representations can help stakeholders articulate what they need and how they expect the system to behave.
- Document Analysis: Reviewing existing system documentation, business process models, and regulatory documents can uncover hidden requirements.
- Brainstorming: Group sessions to generate a wide range of ideas for system capabilities and qualities.
3. Prioritize and Refine
Not all requirements are equally important. Prioritize them based on business value, technical feasibility, and impact. Continuously refine and clarify requirements with stakeholders to ensure they are clear, unambiguous, and testable.
By systematically applying these methods and understanding the fundamental distinction, teams can effectively identify both the "what" and the "how well" of a system, leading to more robust and user-satisfying products.
