Difference between Functional and Non Functional Requirements

In this article we will discuss about functional and non-functional requirements in the software development life cycle (SDLC). There are various requirements in the software development life cycle like business requirements, user requirements, transition requirements and solution requirements. There are two types of solution requirements: functional requirements and non functional requirements. Non-functional requirements are also known as quality attributes. 

What are Functional Requirements?

Functional requirements are features and functions of the system. These features define what software must do to satisfy user conditions. We develop these features as a software. For example, if you want to build a app then these are functional requirements according to user view -

• Create user profile
• Login
• Display details
• Log out

These are function requirement according to developer view for above app - 

• Authentication
• Input
  
• Output
  
• Processing
• Error handle
  
• Admin tools
  

Functional requirement describes what function should do, i.e., specific functionality or tasks. 

Functional requirements defines function of the system. We document these requirement properly to give developer, so developer can develop application according to given functional requirement. Hence, functional requirements should be wrritten in simple language to understand. Developer develop the application to meet user requirements. 

What are Non-Functional requirements? 

Non functional requirements are related to functionality of the system. We focus on how the system should be implemented rather then what it should be. We focus on reliability, usability and efficiency of the system. Non-functional requirements are also known as quality attributes. For example, for above app these are non-functional requirements -

• How easy it is to use
• How reliable it is
• How well it runs

Non-functional requirement describes how the system should perform, i.e., system attributes or quality. 

There are two types of non functional requirements: execution qualities and evolution qualities. 

• Execution qualities are observable at run time, i.e., security and usability.
• Evolution qualities are embedded in the static structure of the software system, i.e., testability, maintainability, extensibility, and scalability.

We define quality attributes of the software in the non-functional requirements. We define general characteristics and behaviour of the system. We minimize the cost factor for better user experience. Non-functional requirements also follow legal rules given my user. 

Balancing Both Functional and Non-Functional Requirements

We should always balance the functional and non-functional requirements because of various advantages as given below -

• Balancing improves user experience because if you only focus on one type of requirements then there can be issues like slow and unresponsive systems and difficult to use.
• Balancing improves performance of the system because we can balance both the type of requirements when one type of requirements causes failure.
• Balancing reduces cost because we can prevent costly changes and re-architecting early.
• Balancing support system evolution. 

Common Challenges in Defining these Requirements

We sometimes face some challenges while defining functional and non-functional requirements as given in the table. 

Challenge	Description	Impact
Ambiguity in Requirements	Sometimes requirements are not clear so we can not differentiate between functional and non-functional requirements.	Developers may develop incomplete or incorrect applications.
Conflicting Requirements	Sometimes requirements are overlapping, for example, security features	This can lead to trade-offs and difficult decisions.
Changing Requirements	Sometime requirements change because of business goals, trends, or user needs.	It can increase development cost.
Difficulty in Prioritization	Sometimes it is challenging to decidewhich requirements are most important.	Critical non-functional aspects may be missed without prioritizing correctly.
Measuring Non-Functional Requirements	Functional requirements are easier to test, but non-functional ones like usability, scalability, and reliability are harder to measure.	Developers may find it hard to confirm if the system meets expectations for performance, security and scalability.

Differences between Functional Requirements and Non-Functional Requirements

These are various different between functional and non-functional requirements - 

Feature	Functional requirements	Non-functional requirements
Definition	Functional requirements define what the system should do, i.e., specific functionality or tasks. It focuses on what part.	Non-functional requirements define how the system should perform, i.e., system attributes or quality. It focuses on how part
Purpose	It is behaviour and functions of the system.	It is based on performance, usability, and other quality attributes.
Scope	We define the action and operations of the system.	We define constraints and conditions under which the system must operate.
Examples	Some of the examples of the functional requirements: data input, output, processing, authentication, etc.	Some of the examples of the non-functional requirements: reliability, scalability, security, etc.
Measurement	We can measure it easily.	It is difficult to measure.
Impact on Development	It drives the core design and functionality of the system.	It can affect the architecture and overall performance of the system.
Focus on User Needs	It is directly related to business and user requirements.	It is related to system performance and user experience.
Documentation	We document its functional specifications, use cases, etc.	We document technical specifications, performance criteria, etc.
Evaluation	We can test it using functional testing like unit or integration tests.	We test it using usability testing, security testing, performance testing, etc.
Dependency	It depends on what the system does.	It depends on how the system does.

Conclusion

In this article, we have discussed functional and non-functional requirements in the software development life cycle (SDLC). We have discussed how functional requirements depend on what part of the system. We have also discussed how functional requirement depend on how part of the system. Functional requirements are user perspective and non-functional requirements are developer perspective. We have discussed various differences between these functional and non-functional requirements in the table.

Frequently Ask Questions (FAQs)

Que-1: Can non-functional requirements impact user satisfaction?

Ans: Yes. Though non-functional requirements are developer perspective, but these requirements can also impact user satisfaction, like performance and usability. 

Que-2: How do you decide unclear requirements?

Ans: We document these requirements by conducting workshops, interviews. We write them in simple language then divide them into functional and non-functional requirements.

Que-3: What tools do you use to manage and track functional and non-functional requirements?

Ans: We use tools, like JIRA, Confluence and Trello. These tools prioritize, update and collaborate on requirements throughout the development process.