Introduction
In software engineering, requirements are classified into functional requirements (FRs) and non-functional requirements (NFRs). Both play a crucial role in defining how a system should work and what constraints it must adhere to.
1. Functional Requirements (FRs)
Definition:
Functional requirements specify what the system should do. These are the core functionalities, features, and operations that the software must support.
📌 Functional requirements focus on:
✔ Input and output behavior
✔ Business logic and rules
✔ User interactions
✔ System processes
Examples of Functional Requirements
| Functional Requirement | Description |
|---|---|
| User Login | Users must be able to log in using a username and password. |
| Search Feature | The system should allow users to search for products using keywords. |
| Order Processing | Customers must be able to add products to the cart and complete the purchase. |
| Report Generation | The system should generate monthly sales reports for admin users. |
| Notifications | Users should receive email/SMS alerts for transactions. |
Characteristics of Functional Requirements
✔ Clearly defined behavior – Describes specific actions the system should perform.
✔ Testable – Can be verified through testing methods like unit testing and integration testing.
✔ Directly linked to business needs – Helps achieve the core purpose of the software.
How to Represent Functional Requirements?
Functional requirements are usually represented using:
📌 Use Case Diagrams – Show interactions between users and the system.
📌 Data Flow Diagrams (DFD) – Illustrate how data moves through the system.
📌 User Stories – Describe features from a user’s perspective.
2. Non-Functional Requirements (NFRs)
Definition:
Non-functional requirements define the quality attributes, performance, and constraints of the system. They describe how the system should perform rather than what it should do.
📌 NFRs focus on:
✔ Performance and response time
✔ Security and access control
✔ Usability and accessibility
✔ Reliability and scalability
Examples of Non-Functional Requirements
| Category | Non-Functional Requirement Example |
|---|---|
| Performance | The system must respond to user requests within 2 seconds. |
| Security | All user passwords must be encrypted using AES-256. |
| Scalability | The system should support 10,000 concurrent users. |
| Availability | The application should have 99.9% uptime. |
| Usability | The UI should be accessible to visually impaired users. |
| Maintainability | System updates should not take more than 5 minutes. |
Characteristics of Non-Functional Requirements
✔ Measurable – Defined in quantitative terms (e.g., response time ≤ 2s).
✔ Testable – Verified using performance testing, security testing, usability testing, etc.
✔ Affect the user experience – Improve system efficiency, reliability, and security.
How to Represent Non-Functional Requirements?
NFRs are usually documented in:
📌 Requirement specifications (e.g., IEEE 830 SRS format).
📌 Quality attribute tables that specify performance metrics.
📌 System architecture diagrams to show scalability and security measures.
3. Differences Between Functional and Non-Functional Requirements
| Feature | Functional Requirements | Non-Functional Requirements |
|---|---|---|
| Definition | Specifies what the system should do. | Defines how the system should perform. |
| Purpose | Ensures system functionality. | Ensures system performance and user experience. |
| Example | “User must be able to reset their password.” | “System must encrypt passwords using AES-256.” |
| Testability | Verified via unit and integration testing. | Verified via performance, security, and usability testing. |
| Change Impact | Modifying functional requirements can change system behavior. | Modifying NFRs affects system quality but not core features. |
4. Importance of Both Requirements
📌 Functional Requirements ensure that the system meets business needs.
📌 Non-Functional Requirements ensure that the system is secure, scalable, and performs well.
📌 Ignoring NFRs can lead to poor performance, security issues, and user dissatisfaction.
Conclusion
Both functional and non-functional requirements are essential in software development. While functional requirements define the system’s core features, non-functional requirements determine its performance, security, and usability. A well-balanced SRS should include both types of requirements to ensure a successful and high-quality software system.