Functional and Non-Functional Requirements in Software Engineering
In software development and systems engineering, functional requirements describe the specific tasks or operations that a program or system is expected to perform. These systems can be software-only, or a combination of software-driven hardware and electronics. The discipline that focuses on defining, analyzing, and managing these requirements is known as requirements engineering.
Functional requirements ensure that a system is designed to deliver its intended results under normal conditions. They act as a bridge between the design stage and the final product, making sure the system meets both technical goals and user expectations.
In this guide, we’ll explore functional requirements and non-functional requirements, compare them, and understand why both are critical for delivering a product that truly satisfies client needs.
Functional Requirements
Functional requirements outline what the system should do. They define the expected behavior and operations of the system, typically in a way that is easy to understand.
Common examples include:
- User authentication and authorization
- Business rules and logic
- Audit logging and tracking
- Compliance and certification requirements
- Transaction processing and corrections
These requirements form the basis for verifying whether the application delivers all the necessary features and supports user goals effectively.
Why Functional Requirements Matter
Functional requirements play a key role at every stage of development:
- Guidance for developers – They clearly describe the features to be built.
- Foundation for testing – Test cases are designed based on these requirements to ensure the system works as intended.
- Meeting stakeholder expectations – Well-defined requirements make the product’s capabilities clear, reducing misunderstandings.
Documenting Functional Requirements
There are several ways to document functional requirements:
- Use Cases – Describe step-by-step interactions between the user (or another system) and the software to complete a task.
- User Stories – Simple, goal-focused descriptions of system behavior, usually following the format: “As a [user], I want to [do something] so that [I can achieve a goal].”
- System Specifications – Detailed documents outlining the system’s inputs, outputs, processes, and error handling.
- Models, prototypes, and diagrams – Visual tools that complement text-based documentation.
By clearly defining functional requirements, development teams can ensure the final product delivers the exact capabilities that were promised.
Non-Functional Requirements: What Are They?
Non-functional requirements (NFRs) define how a system should operate, rather than what it should do. While functional requirements focus on specific tasks and behaviors, NFRs set the quality standards for those operations—covering aspects like performance, usability, security, scalability, and reliability.
These requirements ensure the system not only works but works well, meeting the performance and behavioral expectations of stakeholders and users under real-world conditions.
Examples of Non-Functional Requirements
NFRs typically address system quality and performance. Common examples include:
- Performance – The system must support 1,000 concurrent users without noticeable slowdowns.
- Scalability – It should allow horizontal scaling to handle growing traffic and data volumes.
- Security – All data in transit must be encrypted using industry standards (e.g., TLS 1.2).
- Availability – Maintain at least 99.9% uptime annually.
- Usability – Key features should be accessible in no more than three clicks.
- Compliance – Must handle personal data in line with GDPR regulations.
- Maintainability – Well-structured code should allow for easy upgrades and troubleshooting.
Why Non-Functional Requirements Matter
NFRs are critical because they directly affect system reliability, performance, and the overall user experience. They:
- Set quality benchmarks – Define the minimum acceptable performance, security, and reliability levels.
- Guide architecture and design – Influence technology stack choices, infrastructure setup, and scalability strategies.
- Enhance user satisfaction – Address performance, usability, and security, which improve the end-user experience.
Documenting Non-Functional Requirements
NFRs are often captured in:
- Service Level Agreements (SLAs) – Define measurable service expectations such as uptime, response time, and throughput.
- Quality Attribute Scenarios – Describe system behavior under specific conditions (e.g., heavy load or peak traffic).
- Technical Architecture and Design Documents – Detail the technical standards for performance, security, and scalability.
Key Differences: Functional vs. Non-Functional Requirements
Why Both Types of Requirements Are Essential
- Balanced Development – A system built with only functional requirements might “work,” but could be slow, insecure, or hard to maintain.
- Reliable Performance – Ignoring NFRs can lead to poor scalability or downtime under high load, even if functional features work correctly.
- Example – A website may display product details flawlessly (functional) but crash when 1,000 users visit at once (non-functional issue).
By addressing both types, developers can create software that is functional, secure, scalable, and pleasant to use—resulting in fewer issues and higher customer satisfaction.
Best Practices for Managing Requirements
- Communicate with Stakeholders – Gather both functional and non-functional needs early through open discussions.
- Prioritize Requirements – Rank based on user value, technical feasibility, and business impact.
- Use Structured Documentation – Maintain clear, consistent templates for both types of requirements.
- Validate and Test Frequently – Run functional tests for behavior and performance tests for NFRs like speed and scalability.
- Review and Update Regularly – Adapt requirements as business needs and user expectations evolve.
