Functional vs Non Functional Requirements in Software Engineering

What Are Functional Requirements?

Functional requirements are core functionalities or features that a software system has to support, which tell exactly what the system will do and the behavior of the system under certain specified conditions. These are the specifications that support creating the qualities of the system and directly correlate with the activities or operations that the system should perform.

Functional requirements are usually expressed from the user's perspective; they define how the user will interact with the software and how the system will respond. These requirements become a reference point for the developers to create the features and functionalities of the system.

Examples of Functional Requirements

Functional requirements are what describe the specific functionalities that the system must perform. These include:

Login Functionality: users must be able to log into the system with a valid username and password.
Processing Data: the system must process orders and calculate total costs including tax and shipping fees.
Payment Gateway Integration: Users must interact with a third-party payment integration.
Registration for Users: Users need to register an account by providing necessary details, such as name, email address, and password.
Report Generation: Users should be able to generate the financial state of affairs in a report on a month-on-month basis in a PDF document.

Importance of Functional Requirements

Functional requirements are vital because they outline the specific operations that the system must perform. The functional requirements help:

In the process of guidance: Functional systems direct system features design and development by developers.
In supporting the testing: Functional requirements serve as the basis for the system-testing processes, ensuring that the system functions correctly.
For conforming to the expectations of stakeholders: Functional requirements clarify the features of a system, enabling the development team to align with the expectations of stakeholders.

Documenting Functional Requirements

Functional requirements are typically documented in one of the following formats:

Use cases: A use case explains the interaction between the user (or system) and the system to accomplish certain tasks.
User story: User stories describe how the user of the system envisions the functionality. They take the format, "As a [user], I want to [perform a task] so that [I could achieve my goal]."
System specifications: These are extremely detailed documents detailing all of the things the system is supposed to accomplish in regard to inputs, outputs, processes, and error management.

What Are Non-Functional Requirements?

Surprising as it may seem, non-functional requirements (NFRs) specify the criteria for how the system should be functioning. Unlike functional requirements that center on what the system should do, non-functional specifications address almost all important functional aspects of the system: how a certain series of functions or services should be executed. In any event, they specify the quality attributes of the system in terms of performance, security, scalability, usability, and reliability.

Non-functional requirements specify the characteristics of the operation of the system, ensuring that the system will run according to the expectations of users and stakeholders regarding the behavior and performance of that system.

Examples of Non-Functional Requirements

A good system's performance and quality are determined by the non-functional requirements. They include:

Performance: A system is able to support 1,000 concurrent users without its performance degrading considerably.
Scalability: Should the system be required to cater to larger and larger numbers of users and transactions, it must support horizontal scaling.
Security: All data shall be encrypted during transfer in accordance with standard practices (TLS 1.2).
Availability: The system must have at least 99.9% uptime yearly.
Usability: The interface provides an intuitive way for navigating, where at most three clicks should take a user to any major feature.
Compliance: The system should be compliant with the General Data Protection Regulation (GDPR) regarding personal data handling.
Maintainability: It enables easy updating and maintenance due to the code being written clearly and following a good design.

Importance of Non-Functional Requirements

Non-functional requirements are important because these help determine how the system delivers under various conditions. Thus, a system ensures it provides the user with a favorable experience and meets the expectations of the business and regulations.

Establish quality standard levels: NFRs define what standards the system must meet in performance, to ensure the smooth, secure, and reliable operation of your software.
Steer architectural decisions: Non-functional requirements impact major architectural and design choices made for the whole system, such as the choice of technologies and platforms.
Enhance user satisfaction: By addressing issues of usability, performance, and security, NFRs improve the overall user experience and user satisfaction.

Documenting Non-Functional Requirements

Non-functional requirements are typically documented within:

Service Level Agreements (SLAs): These define the levels of service the system is expected to deliver, including uptime, response times, or other performance metrics.
Quality Attribute Scenarios: These detail quality attributes of the system that state expected performance outputs in certain situations (for instance, while waiting for a surge in traffic).
Technical Architecture and Design Document: These documents will define the architectural and technical standards that the system must meet. Those standards include security, performance, and scalability.

Key Differences Between Functional vs Non Functional Requirements

Aspect	Functional Requirements	Non-Functional Requirements
Definition	Describes what the system must do	Describes how the system should perform
Focus	Focus on system functionality and features	Focus on system quality attributes (e.g., performance, security)
Examples	User authentication, data processing, reporting	Scalability, security, performance, availability
Measurement	Often measurable in terms of actions or operations	Measured by metrics like speed, uptime, load capacity
Impact	Direct impact on user functionality and business logic	Impact on user satisfaction, system efficiency, and reliability
Priority	High priority for defining system behavior	High priority for ensuring the system operates efficiently

Why Both Functional and Non-Functional Requirements Matter

Balanced Development

Only using functional requirements would produce a system that may function as expected but will not scale well, be secure, or perform within what an end-user would find acceptable. A system must fulfill both functional and non-functional requirements to meet users' and business expectations.

Effective System Design

Non-fulfillment of non-functional requirements could result in a system with functional correctness but with degraded performance or security (which is normally not acceptable). For example, a website might work perfectly when showing product information (functional requirement) but may crash when 1000 users access it simultaneously (non-functional requirement).

By considering both of these types of requirements, a developer could be able to come up with software that is robust, makes provisions for expansion, is secure, and has improved performance, thus leading to a user level of satisfaction that will further reduce the cases of incidents with the system.

Best Practices for Handling Functional vs Non Functional Requirements

1. Clear Communication with Stakeholders

The success of capturing both functional and non-functional requirements begins with good communication with the stakeholders. It is important to know what the user, business owner, and technical team want so that a complete request can be written.

2. Prioritize Requirements

Not all requirements are of equal importance. Functional and non-functional requirements should be prioritized based on business value, technical feasibility, and user needs so that they can guide development decisions and help ward off scope creep.

3. Use Structured Documentation

Clearly develop documentation to represent both types of requirements. Structured templates and review boards provide a better way of tracking changes and updates of the requirements throughout the lifecycle of the project.

4. Regular Testing and Validation

Regular testing of both functional and non-functional qualities is needed. Functional testing assures the performance of the system as expected, while performance testing will check whether the system meets the desired non-functional criteria of speed, reliability, and scalability.

5. Review and Adjust Requirements

Revisit the requirements as development progresses to confirm that they are still valid and achievable. Change may occur within changeable, user expectations, and business goals; hence continuous review and adjustment are needed.

Conclusion

In software engineering, functional and non-functional requirements are like two sides of the same coin that are both vital for any powerful, high-performance solutions.

A successful software project needs to find a balance between both requirement sets so that the product being delivered will turn out to be functional along with being efficient, reliable, and convincing.