April 01, 2022

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples

Yulya Glamazdina

Marketing Manager

8 minutes

If you’re ordering custom software, you and your development team need to hash out its requirements — otherwise, you risk getting a final product that is unreliable, poor performing, and non-secure. In the software development world, there are two types of requirements: functional and non-functional. Read on to learn the distinction between the two and why they’re crucial to your project’s success.

Content
What Are Functional Requirements?
What Are NFRs or Non-Functional Requirements?
Comparison Table
Conclusion

What Are Functional Requirements.jpg

What Are Functional Requirements?

Functional requirements specify which functions your system (or system component) must have. In other words, they define what your custom software needs to do. If these requirements are not met, the software fails; it’s non-negotiable. The most common functional requirements examples are (1) use cases and (2) document descriptions.

Use Case

Use cases can be numbered lists with text or diagrams that describe user actions. Each use case illustrates behavioral scenarios through one or more functional requirements. First, a set of use cases is selected — and then an analyst can derive the functional requirements that need to be implemented in order for each use case to be fulfilled. In the diagram below, a navigational use-case model for a conference management system is proposed. Each main functionality is associated with user roles: navigational use-case model for a conference management system is proposed.png And here’s an example of a use case numbered list for a subway ticket machine: example of a use case numbered list for a subway ticket machine.png

Written Document Descriptions

A typical written functional requirement will contain a unique name and number, a brief summary, and a rationale. This information is used to help the reader understand why the requirement is necessary and develop a system to track it. Some examples of written functional requirements include:

  • Interface requirements
  • Data manipulation and processing requirements
  • Verification requirements
  • Documentation requirements
  • Security requirements
  • Acceptance criteria

During the development stage, functional requirements are used to describe whether or not the system is working as intended. That means that there might be many more requirement types for you to write down until you create a complete picture of a correctly working system.

Who Is Responsible for Preparing Functional Requirements?

Functional requirements are typically collaborated on by a business analyst, system analysts, and an implementation team. The project manager is responsible for making sure the functional requirements are delivered on time and they’re signed off by the System Owner and QA. What Are NFRs or Non-Functional Requirements.jpg

What Are NFRs or Non-Functional Requirements?

Non-functional requirements define how the software fulfills its functional goals - not what the system will do but how the system will do it. These requirements define criteria that are used to evaluate the system’s performance, reliability, and maintainability. They define and impose external constraints on the software being developed, as well as on the development process. Non-functional requirements are presented as a specific aspect or general property of the system as a whole — as opposed to an exact function. Aspect = a feature linked to many parts of the system but not related to its primary function Properties = characteristics of software, including integrity, reliability, accuracy, etc. Non-functional requirements can be divided into two main categories: performance and evolutionary qualities.

Performance QualitiesEvolutionary Qualities
ReliabilityTestability
SurvivabilityMaintainability
EfficiencyExtensibility
IntegrityScalability

Performance qualities are observed at runtime, whereas evolutionary qualities are embodied in the system’s static structure. Let’s take a closer look at some non-functional requirements examples presented in a paper published by the IEEE.

Performance

Efficiency: How well does it utilize resources?

Integrity/Security: How vulnerable is it to cyberattacks?

Reliability: What level of confidence can be placed in its functions?

Survivability: How well does the system perform under poor conditions?

Design

Correctness: How well does the system conform to requirements?

Verifiability: How easy is it to verify the system’s performance?

Maintainability: How easy is it to repair the software?

Adaptation

Expandability: How easy is it to upgrade or expand its capability/performance?

Interoperability: How easily does it interact with other systems?

Flexibility: How easy is it to change?

Portability: How easily transportable is the system?

Reusability: How easily can it be converted for use with another system?

Who Is Responsible for Preparing Non-Functional Requirements?

While functional requirements can be presented in a Functional Requirements Document (FRD), a Product Requirements Document (PRD) combines functional and non-functional ones. Non-functional requirements are usually defined by developers, software architects, and other technical experts, and the PRD is typically prepared by the product manager.

Comparison Table

To see the differences between functional and non-requirement functions at a glance, just check out the table below:

Functional RequirementsNon-Functional Requirements
Helps the reader understand what functions the system will haveHelps the reader understand how the system will perform
Explains the system’s necessary featuresExplains the way the system should behave
Identifies what the system must (and mustn’t) doIdentifies how the software should do it
The system won’t work if these requirements aren’t fulfilledThe system will still work if these requirements aren’t fulfilled
Ensures the software meets user requirementsEnsures the software will meet user expectations
Straightforward to defineHarder to define
Can be documented through a use caseCan be documented as a quality attribute

Conclusion

Your software needs to fulfill both functional and non-functional requirements for optimal performance. The former enables it to run, while the latter enhances the user experience. It’s important to work with a custom software development team that meticulously documents both kinds of requirements. Otherwise, you could end up with a disappointing product or a decent one that exceeds budget and time constraints. At Brocoders, we strive to create successful software that streamlines workflows and an enhanced customer experience — and our deep requirements-gathering process makes this possible. Want to know how Brocoders can develop an outstanding product for your business? Get in touch, and we’d be happy to give you more info!

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