Functional Requirements
Functional requirements define what a software system must do. They specify the system's behaviors, functions, and features, describing how it should respond to specific inputs and user actions. They are the 'what' of a system, as opposed to non-functional requirements, which describe the 'how'.
First used·1970s
Definitions·1
Synonyms·3
Category·Software Engineering
Also known as
Definitions
What it means.
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Functional Requirements in Software Engineering
A Functional Requirement defines a specific behavior, function, or service that a software system must be capable of performing. It describes what the system should do in response to inputs and how it should behave in particular situations. These requirements are the fundamental building blocks that dictate the system's features and capabilities from the user's perspective.
They are often contrasted with Non-Functional Requirements, which describe how the system should perform its functions (e.g., performance, security, reliability).
Key Concepts
- Specificity: They must be clear, concise, and unambiguous to avoid misinterpretation by developers and testers.
- Testability: Each requirement should be verifiable. It must be possible to write a test case that can determine whether the requirement has been successfully implemented.
- Completeness: The set of functional requirements should ideally capture all the necessary functions of the system.
Common Formats for Documentation
- Software Requirements Specification (SRS) Document: A traditional format where requirements are listed with unique identifiers, often using language like "The system shall..."
- Use Cases: These describe interactions between a user (or 'actor') and the system to achieve a specific goal. They provide more context than a simple list of requirements.
- User Stories: A common format in Agile development, typically written as: "As a [type of user], I want [some goal] so that [some reason]." These are often supplemented with acceptance criteria.
Examples
- User Authentication: "The system shall allow registered users to log in with their username and password."
- Search Functionality: "The system shall enable users to search for products by name, category, or brand."
- Reporting: "The system shall generate a monthly sales report in PDF format."
Origin
Where it comes from.
Etymology
The term is a compound of "Functional" and "Requirement." "Functional" originates from the Latin "functio," meaning "performance" or "execution." "Requirement" comes from the Old French "requerre," meaning "to seek to obtain." Combined, the term literally means "a required performance or execution," which accurately describes its role in defining what a system must do.
Historical context
The concept of specifying system functionality has existed since the dawn of computing. However, the formalization of Functional Requirements gained prominence in the 1970s with the rise of structured software development methodologies like the Waterfall model.
During this era, comprehensive upfront documentation was critical. The IEEE Standard 830-1984, "Guide for Software Requirements Specifications," was a landmark document that standardized how requirements, including functional ones, should be documented. This approach emphasized creating a detailed and frozen set of requirements before design and development began.
With the advent of Agile methodologies in the late 1990s and early 2000s, the handling of functional requirements evolved. Instead of exhaustive upfront documents, Agile practices favor expressing requirements in more lightweight formats like User Stories. These are gathered and refined iteratively throughout the development lifecycle, allowing for greater flexibility and adaptation to change.
Usage
In context.
The business analyst is responsible for gathering and documenting all the functional requirements before the development sprint begins.
A critical functional requirement for the new application is the ability for users to reset their passwords via email, which is a core system behavior.
During the review, the team realized the functional specifications were incomplete and did not account for user roles and permissions.
The QA team writes test cases directly based on the functional requirements to ensure every feature works as expected.
FAQ
Common questions.
The primary difference lies in what they describe:
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Functional Requirements define what the system does. They specify behaviors, features, and functions. For example, "The system shall allow a user to upload a profile picture."
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Non-Functional Requirements define how the system performs its functions. They specify qualities, constraints, and characteristics like performance, security, or usability. For example, "The profile picture upload must complete within 3 seconds," or "The system must be available 99.9% of the time."
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