Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (2024)

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (3)

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.

CONTENTWhat Are Functional Requirements?What Are NFRs or Non-Functional Requirements?Comparison TableConclusion

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (4)

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 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:

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (5)

And here’s an example of a use case numbered list for a subway ticket machine:

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (6)

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.

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.

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (7)

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 QUALITIESReliabilityTestabilitySurvivabilityMaintainabilityEfficiencyExtensibilityIntegrityScalability

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.

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?

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?

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?

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.

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

FUNCTIONAL REQUIREMENTSNON-FUNCTIONAL REQUIREMENTSHelps the reader understand what functions the system will haveHelps the reader understand how the system will performExplains the system’s necessary featuresExplains the way the system should behaveIdentifies what the system must (and mustn’t) doIdentifies how the software should do itThe system won’t work if these requirements aren’t fulfilledThe system will still work if these requirements aren’t fulfilledEnsures the software meets user requirementsEnsures the software will meet user expectationsStraightforward to defineHarder to defineCan be documented through a use caseCan be documented as a quality attribute

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!

Functional vs. Non-Functional Requirements: The Full Guide, Definitions & Technical Examples (2024)

FAQs

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

Functional requirements define what a product must do and what its features and functions are. Nonfunctional requirements describe the general properties of a system. They are also known as quality attributes.

What are functional and non-functional requirements with examples? ›

Examples of functional requirements include user authentication, data management, workflow and business logic, reporting and analytics, and integration with external systems. Non-Functional Requirements define the qualities and constraints that the system should meet.

What is the difference between functional non-functional and technical requirements? ›

The Bottom Line

Functional requirements define how the system must work and non functional requirements detail how it should perform. Without functional requirements the system will not work. Without at least some non functional requirements being met to a certain level, users will become frustrated.

What are non-functional requirements in BRD? ›

Non-functional requirements are the criteria that define how a system should behave, rather than what it is supposed to do. Unlike functional requirements, which describe specific system functions, non-functional requirements define aspects like performance, security, usability, reliability, and scalability.

How to identify functional and non-functional? ›

Non-functional testing includes performance, reliability, usability, and stress tests to check the system's properties. Functional testing includes API, system, applications, features, end-to-end, and integration tests to check the system's processes.

What are 5 other nonfunctional requirements? ›

What are the different types of nonfunctional requirements?
  • Performance and scalability. ...
  • Operating constraints. ...
  • Platform constraints. ...
  • Modifiability. ...
  • Portability requirements and capability. ...
  • Reliability. ...
  • Security. ...
  • Usability.

What are functional and technical requirements? ›

Functional specs are based on the business requirements and contain the details of end user expectations of the product functionality. Software will be developed based on the functional specs. Technical specs contain the details of the how this is/can be achieved and the final product functionality details.

What is functional vs non-functional metrics? ›

Functional requirements focus on what the project should do, and non-functional requirements focus on how it should be.

What is an example of a functional specification? ›

Example of functional specification. The following is an example of a functional specification: Use case diagram – this helps depict the interaction between the system and its users. Every user role is called an “actor” and the different functions or processes are represented in the diagram.

Does a BRD include functional requirements? ›

A BRD deals with what an organization hopes to achieve through a vendor partnership. On the other hand, a functional requirements document (FRD) deals with how they expect to achieve it.

How to prepare BRD and FRD? ›

Seven steps to create a BRD
  1. Define the company's needs. ...
  2. Define the BRD objectives. ...
  3. Get others involved. ...
  4. Identify the phases of the project. ...
  5. Establish standards for all requirements. ...
  6. Develop a process scheduling and measurement. ...
  7. Use an appropriate template. ...
  8. Executive summary.
Aug 18, 2024

What are Frs and NFRs? ›

The two most common types of requirements that you will likely encounter are Functional Requirements (FRs) and Non-Functional Requirements (NFRs). Many teams struggle to gain a clear understanding of these two types of requirements, which can often lead to less-than-desirable performance and output.

What are examples functional vs non-functional requirements? ›

Let's take an example of a functional requirement. A system loads a webpage when someone clicks on a button. The related non-functional requirement specifies how fast the webpage must load.

What is an example of a functional requirement? ›

A functional requirement is a statement of what a product (system, subsystem, device, or software program) must do. Example: The control system shall prevent engine overspeed.

Is SLA a non-functional requirement? ›

Usually, Service Level Agreements (as known as SLAs) and Non-Functional Requirements (as known as NFRs) both can be used to provide criteria to assess performance testing, but they are different, so it is worth knowing the difference and how both can be applied to validate performance testing.

What are the types of functional and non-functional testing with examples? ›

Comparison between Functional and Non-functional Testing
CategoryFunctional TestingNon-Functional Testing
When to performBefore non-functional testingAfter functional testing
Testing types exampleSmoke testing, Sanity testing, Regression testingLoad testing, Stress testing, Accessibility testing
4 more rows
May 1, 2024

What are the four types of requirements? ›

The types of requirements are usually classified into four types: business requirements, stakeholder requirements, solution requirements, and transition requirements.

Is login a functional requirement? ›

Having user login is a functional requirement of multi-user systems, and is essential for security in single-user systems.

References

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