Functional Requirements vs Non Functional Requirements Differences

A Comparison between Functional and Non Functional Requirements

Functional and Non Functional Reequirements

If you’re developing a new product or even a new version of an existing product, it’s useful to distinguish between what we might call functional and non functional requirements. In this blog, we will discuss the following points of functional and non functional Requirements:

  • What are Functional Requirements?
  • How to Write Functional Requirement ?
  • Types of Functional Requirements
  • Functional Requirements Examples
  • What are non-functional requirements?
  • Types of Non Functional Requirements
  • Non-functional requirements Examples
  • Functional Vs Non-Functional Requirements
  • Advantages of Functional Requirements
  • Advantages of Non-functional requirements

What are Functional Requirements?

Functional requirements are the requirements that have to be met for a system to work. The purpose of functional requirements is to describe what the software actually does, especially in terms of externally visible behavior.

  • A functional requirement is a description of how a system performs a particular process. Functional requirements are often called “specifications,” which is a good name because they are specifications of what the product will do, without specifying how it should be done.
  • The functional requirements for a software system are the things it has to do. These include both “external” requirements, like “the system shall produce reports,” and “internal” requirements like “the system shall use the database to store most data.”

Click Here to Read: 30 Best Requirements Management Tools in 2021

How to Write Functional Requirement ?

We can express functional requirements in four sentences:

  • First sentence: The system shall <action> the <goal>.
  • Second sentence: The <condition> that triggers the action are <fact>.
  • Third sentence: The <result> that results from triggering the action are <fact>.
  • Fourth sentence: Someone must be able to <fact>.

These sentences are short, but their brevity hides a lot of complexity. Each one has a number of important elements, which I’ll make clear by explaining how to fill in the blanks.

Types of Functional Requirements

Functional requirements can be categorized in many ways. I’ll discuss three types of functional requirements: primary, secondary, and derived.

  • Primary Functional Requirements: Primary functional requirements are what the system is designed to do – What does the system need to do? List primary requirements first and foremost. These requirements often include multiple levels of detail.
  • Secondary Functional Requirements: Secondary functional requirements are not as critical as the primary ones but still must be considered. 
  • Derived Functional Requirements: Derived functional requirements were developed from the primary and secondary functional requirements and these derived functional requirements will ultimately drive the solution design.

Click Here to Read: What is Requirement Lifecycle Management?

Functional Requirements Examples

Following are the Functional Requirements Examples:

  • Word-Processing System Example: A functional requirement for a word-processing system might be: Users can create documents containing text and graphics. But a better one would say: Users can create documents containing text and graphics of different colors, fonts, sizes, and styles. The first requirement says that users type into something that then produces a document; the second says that they can directly manipulate what goes into the document. The first requirement is enough to let you build something that could be a word processor. The second is enough to let you build something that could be a paint program or a page layout program or a typesetting program. 
  • Shopping Cart Example: A list of functional requirements for a shopping cart might be: (1) A customer can view the details of an item in her shopping cart. (2)A customer can remove an item from her shopping cart. (3) A customer with no items in her shopping cart will be informed that she needs to select some items before proceeding.
  • Smartphone App Example: If you have a set of requirements for a smartphone application, one functional requirement might state that the application should have a “search” feature. This means that users can enter search criteria and find matching results.

What are non-functional requirements?

A non-functional requirement in software engineering is an intangible quality or characteristic of functionality that is not directly related to the operation of that functionality. Nonfunctional requirements are less widely understood, but even more important. They are quality attributes that don’t directly affect the user, the kind of thing that in manufacturing would be called quality control.

Types of Non-Functional Requirements

Following are the types of Non-Functional Requirements:

  • Performance
  • Security
  • Usability
  • Reliability
  • Maintainability
  • Compatibility
  • Portability
  • Correctness
  • Scalability

Non Functional Requirements Examples 

  Following are the Non Functional Requirements Examples:

  • Non-functional requirement in software engineering are generally features that one group or another would really like to have in the product, but there is no firm business case for their inclusion. These features might include:
  1. Reliability/durability/mean time between failure requirements
  2. Interoperability with other systems (“plug and play”)
  3. Hardware independence (“write once, run anywhere”)
  4. Standards compliance
  5. Scalability (thus being able to handle a large volume of data)
  6. Security requirements
  • If the software is meant to help an organization manage its inventory, non-functional requirements would include fast response time, consistency in data integrity, and accuracy in recording information
  • A requirement that a software program must be able to run on a Windows operating system, or that it must be able to print ten thousand pages per month, is a non-functional requirement.

Click Here to Read: What is a Non-Functional Requirement in Software Engineering? Types and Examples

Difference Between Functional and Non Functional Requirements 

  • A functional requirement of a website is a description of how users can interact with it, what it does. A non-functional requirement defines how it behaves, what it does well or badly.
  • A functional requirement is often referred to as workflows or scenarios. A non-functional requirement is often referred to as a quality attribute.
  • Functional requirements are requirements that an end-user should have. Non-functional requirements are requirements that a developer should have but do not have to be provided by the end-user.
  • Functional requirements specify concrete inputs, outputs, and processes. Non-functional requirements specify characteristics that a system will have to have but don’t involve direct user interaction.
  • Functional requirements deal with the logic behind a business process, while non-functional requirements can be described as qualities of a product or service.
  • Difference By Example 1: Functional requirements: The system shall allow users to book a flight online. Non-functional requirement: The system shall allow users to book a flight online in less than five minutes.
  • Difference By Example 2:Functional requirement: The system shall be able to hold at least 100,000 books. Non-functional requirement: The system shall be able to hold at least 100,000 ebooks and 50,000 hardcopies without losing any data for more than three days when the power is down and all network connections are lost.

Click Here to Read: What is a Non-Functional Requirement in Software Engineering? Types and Examples

Advantages of Functional Requirements

  • Functional requirements are independent of any implementation technique or technology.
  • Defining the functional requirements first helps make sure that the system is adequately defined before proceeding with detailed design
  • The use cases are written in the user’s terms, not the programmer’s terms.
  • Functional requirements make sure that user gets provision in terms that they will be familiar with.

Advantages of Non-Functional Requirements

  • These requirements are used to measure the quality of the product
  • It documents business policies and practices
  • Non-functional requirements can be prioritized
  • It assists customers to get more understanding about the product
  • Helps developers to test the product and its functionality.
  • Helps to monitor and control the project and team progress

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