Functional vs. Non-Functional Requirements in Software Engineering
Software requirements are the foundation of development, defining the features, behavior, and performance standards of the system. They cover a wide variety of factors which all contribute to a final product that meets user needs and stakeholder expectations.
Types of Requirements
There are many different types of requirements for software development, ranging from performance and business requirements to user and interface requirements. Most fall into two broad categories — functional and non-functional requirements. Like we explored in our functional vs. non-functional testing article, these group types by their primary focus on either actions and outputs or performance and efficiency.
What are Functional Requirements?
Functional requirements work to identify and describe the features, capabilities, and behavior of software. In other words, they specify how the system should respond to inputs by defining what outputs or results should be produced. These requirements typically revolve around the core functionality and purpose of your software and are often informed by and based on business goals and user needs.
Attributes
- Inputs and outputs
- How users will interact with the system
- What the system should do
- Data handling and management
Examples
- Login feature allows access when provided a valid username and password combination
- If login fails, an error message should be returned within 2 seconds
- Checkout system allows users to use valid Visa, Mastercard, or PayPal information to complete a transaction
Business Requirements vs. Functional Requirements
While they are similar (and related), functional requirements are separate from business requirements. Business requirements are often the precursor, identifying the overall goal or problem that needs to be solved (the “what” and “why”), while functional requirements clarify the “how” with specific system functionalities to fulfill these.
What are Non-Functional Requirements?
The other side of this equation are non-functional requirements (NFRs), which define the qualities and attributes of how the system actually performs its functional requirements. These establish expectations and goals for the software’s reliability and overall performance. Unlike functional requirements, NFRs aren’t tied to a specific feature or action and instead pertain to the overall application or system.
Attributes
- Performance
- Scalability
- Security and access controls
- User experience
- Uptime
Examples
- Able to handle 1,000 concurrent users without performance slowdowns
- Response time for search results less than 2 seconds
- Greater than 99.9% uptime over a year
Functional vs. Non-Functional Requirements
To summarize the differences we’ve covered so far, functional requirements outline what the system does, while non-functional requirements detail how it should perform. Functional requirements also correspond with functional testing (such as unit or integration testing), while non-functional requirements are evaluated with non-functional test methods (like performance or security testing).
However, both are necessary considerations for any software development project, as excluding or neglecting one could result in poor user experience. By gathering, documenting, and testing functional requirements, you can confidently create a system that includes all necessary features to meet business and user needs. By doing the same for non-functional requirements, you’re bolstering the performance and consistent experience of that system, which will enhance user adoption and reduce the risk of long-term pitfalls like security breaches.
Best Practices for Documenting Requirements
When it comes to recording these, both functional and non-functional requirements are typically listed in documents like the Software Requirements Specification (SRS) and Product Requirements Document (PRD). They’re also further clarified in individual documents like use cases and user stories that further explore how users will interact with the system. But what considerations should be kept in mind for documentation and writing of your project’s requirements?
- Clear and Specific Language: Many different people with varying levels of technical experience may use your requirement documentation, so avoid vagueness and emphasize measurable outcomes.
- Include Acceptance Criteria: Building on the point above, everyone should agree upon the same definition for a requirement’s “success,” whether the system has to meet a percentage threshold or perform a resultant action.
- Distinct Functional and Non-Functional Sections: To avoid confusion during the development and testing process, clearly separate your functional requirements from non-functional.
- Prioritize Requirements: Depending on resources and scope, you may not be able to test every single requirement — identify must-have vs. nice-to-have requirements to manage work efficiency.
- Use an RTM: A Requirements Traceability Matrix links individual requirements to business goals, test cases, and specific deliverables so nothing is overlooked or lost in the shuffle of a complex project.
Don’t Cut Corners on Requirements Management
Requirements form the foundation of your projects and can make or break a successful, high-quality final product. While it may be tedious to come up with, write, and manage them by hand, requirements management tools like SpiraTeam help automate, collaborate on, and organize all of your requirements in a single, unified place. It doesn’t have to create headaches for your team and stakeholders trying to keep track of everything.
SpiraTeam’s cutting-edge features make it easy to not only manage your project requirements, but expand them beyond what was possible with manual management for even more polished final products. Hear from our partners why SpiraTeam is an invaluable tool for modern software development, or try a free 30-day trial to see for yourself.