Functional and Nonfunctional Requirements in Software Engineering

Requirements engineering is a critical phase in software development, as it establishes a formal agreement between clients and providers regarding the goals and objectives of the project. Well-defined requirements mitigate financial risks, ensure project timelines are met, and lay the foundation for successful project outcomes. This article explores the concepts of functional and nonfunctional requirements in software engineering, drawing insights from reputable sources such as GeeksforGeeks, AltexSoft, and Enkonix.

Functional Requirements

Functional requirements define the specific functionalities or features that a software system must provide to meet the needs of the end user. They are stated in terms of inputs, operations, and expected outputs and are directly visible in the final product. Functional requirements ensure that the system behaves as intended under various conditions.

Examples of Functional Requirements

• Defining the features that need to be designed for the system.
• Considering edge cases in the design.
• Ensuring the system sends a confirmation email when a new user account is created.
• Allowing users to create accounts and log in using credentials or social media integration.

Nonfunctional Requirements

Nonfunctional requirements are quality constraints that the software system must satisfy according to the project contract. Also known as non-behavioral requirements, they deal with issues such as portability, security, maintainability, reliability, scalability, performance, reusability, and flexibility. Nonfunctional requirements are not directly visible in the final product but are essential for ensuring the overall quality and performance of the system.

Examples of Nonfunctional Requirements

• Specifying the maximum latency for processing each request.
• Ensuring the system’s high availability.
• Ensuring the system is compliant with PCI DSS (Payment Card Industry Data Security Standard).
• Ensuring the system can handle 20 million users without performance deterioration.

Difference between Functional and Nonfunctional Requirements

The primary difference between functional and nonfunctional requirements lies in their focus. Functional requirements define what the software system should do, while nonfunctional requirements define how the system should fulfill the functional requirements. Additionally, functional requirements are specified by the end user, while nonfunctional requirements are specified by technical experts such as architects, technical leaders, and software developers. Functional requirements are mandatory and captured in use cases, while nonfunctional requirements are not mandatory and are captured as quality attributes. Finally, functional testing is done to verify the functionality of the software, while nonfunctional testing is done to verify the performance, usability, security, and other nonfunctional aspects of the software.

Conclusion

Functional and nonfunctional requirements play complementary roles in software engineering. Functional requirements define the desired behavior of the system, while nonfunctional requirements ensure that the system meets the desired quality attributes. By clearly defining and documenting both types of requirements, stakeholders can ensure that the developed software system meets their expectations and delivers the intended value.

References

• Functional vs Non Functional Requirements – GeeksforGeeks
• Functional and Non-functional Requirements: Specification and Types
• Functional Requirements vs Non Functional Requirements

FAQs

What are functional requirements in software engineering?

Functional requirements define the specific functionalities or features that a software system must provide to meet the needs of the end user. They are stated in terms of inputs, operations, and expected outputs and are directly visible in the final product.

What are nonfunctional requirements in software engineering?

Nonfunctional requirements are quality constraints that the software system must satisfy according to the project contract. They deal with issues such as portability, security, maintainability, reliability, scalability, performance, reusability, and flexibility. Nonfunctional requirements are not directly visible in the final product but are essential for ensuring the overall quality and performance of the system.

What is the difference between functional and nonfunctional requirements?

Functional requirements define what the software system should do, while nonfunctional requirements define how the system should fulfill the functional requirements. Functional requirements are specified by the end user, while nonfunctional requirements are specified by technical experts. Functional requirements are mandatory and captured in use cases, while nonfunctional requirements are not mandatory and are captured as quality attributes.

Why is it important to document both functional and nonfunctional requirements?

Documenting both functional and nonfunctional requirements is important because it ensures that all stakeholders have a clear understanding of what the software system is expected to do and how it should perform. This reduces the risk of misunderstandings, errors, and rework during the development process.

What are some examples of functional requirements?

Examples of functional requirements include:

• The system shall allow users to create and manage their accounts.
• The system shall provide a search feature to find products and services.
• The system shall allow users to purchase products and services online.

What are some examples of nonfunctional requirements?

Examples of nonfunctional requirements include:

• The system shall be available 99.9% of the time.
• The system shall be able to handle 10,000 concurrent users.
• The system shall be secure against unauthorized access and data breaches.

How can functional and nonfunctional requirements be documented?

Functional and nonfunctional requirements can be documented using various techniques, including:

• Use cases
• User stories
• System requirements specifications (SRS)
• Quality attribute workshops
• Non-functional requirements matrices

How are functional and nonfunctional requirements tested?

Functional and nonfunctional requirements are tested using different types of testing, including:

• Functional testing: Verifies that the system performs the intended functions correctly.
• Nonfunctional testing: Verifies that the system meets the nonfunctional requirements, such as performance, security, and reliability.