Functional vs Non Functional Requirements
By Requirements are the foundation of software development. They define what the software system should do and how it should do it. There are two main types of requirements: functional and non-functional requirements. Functional requirements define the features and functions that the software should have, while non-functional requirements define the quality attributes of the software system. In this blog post, we will discuss the differences between functional and non-functional requirements, their importance, and how they are used in software development.
Functional Requirements
Functional requirements define what the software system should do. They are typically expressed as use cases, which describe the interaction between the user and the system. Functional requirements can be categorized into different types:
1.1. Business Requirements
Business requirements define the objectives of the software system. They are typically defined by the stakeholders, such as the product owner, the business analyst, or the customer. Business requirements are usually expressed in natural language and are often documented in a requirements document or a user story.
1.2. User Requirements
User requirements describe the features and functions that the users of the software system expect. They are usually expressed as use cases or user stories. User requirements are typically documented in a requirements document, a user story map, or a product backlog.
1.3. System Requirements
System requirements describe the features and functions that the software system should have. They are usually expressed as functional specifications or system requirements specifications. System requirements are typically documented in a requirements document, a functional specification, or a system requirements specification.
Non-Functional Requirements
Non-functional requirements define the quality attributes of the software system. They describe how the system should perform, rather than what it should do. Non-functional requirements can be categorized into different types:
2.1. Performance Requirements
Performance requirements describe how the software system should perform in terms of speed, capacity, and scalability. They are usually expressed as performance goals or performance requirements. Performance requirements are typically documented in a performance requirements specification.
2.2. Security Requirements
Security requirements describe how the software system should protect the data and resources from unauthorized access or attacks. They are usually expressed as security goals or security requirements. Security requirements are typically documented in a security requirements specification.
2.3. Usability Requirements
Usability requirements describe how easy it is for users to use the software system. They are usually expressed as usability goals or usability requirements. Usability requirements are typically documented in a usability requirements specification.
2.4. Reliability Requirements
Reliability requirements describe how the software system should perform in terms of availability, fault tolerance, and recoverability. They are usually expressed as reliability goals or reliability requirements. Reliability requirements are typically documented in a reliability requirements specification.
2.5. Maintainability Requirements
Maintainability requirements describe how easy it is to maintain the software system. They are usually expressed as maintainability goals or maintainability requirements. Maintainability requirements are typically documented in a maintainability requirements specification.
Importance of Functional and Non-Functional Requirements
Functional and non-functional requirements are essential for successful software development. They help to ensure that the software system meets the needs of the stakeholders, performs as expected, and is of high quality.
3.1. Importance of Functional Requirements
Functional requirements are important because they:
- Define what the software system should do.
- Help to ensure that the software system meets the needs of the stakeholders.
- Provide a basis for testing the software system.
- Help to ensure that the software system is of high quality.
- Help to ensure that the software system is delivered on time and within budget.
3.2. Importance of Non-Functional Requirements
Non-functional requirements are important because they:
- Define how the software system should perform.
- Help to ensure that the software system is of high quality.
- Help to ensure that the software system is secure and reliable.
- Provide a basis for testing the software system.
- Help to ensure that the software system meets the needs of the stakeholders.
- Help to ensure that the software system is delivered on time and within budget.
How Functional and Non-Functional Requirements are Used in Software Development
Functional and non-functional requirements are used throughout the software development life cycle to ensure that the software system meets the needs of the stakeholders and is of high quality.
4.1. Requirements Gathering
During requirements gathering, the business analyst or product owner works with the stakeholders to identify the functional and non-functional requirements of the software system. This is usually done through interviews, surveys, and workshops. The requirements are then documented in a requirements document, a user story map, or a product backlog.
4.2. Requirements Analysis
During requirements analysis, the requirements are analyzed to ensure that they are complete, consistent, and unambiguous. The business analyst or product owner works with the stakeholders to clarify any uncertainties or inconsistencies in the requirements.
4.3. Design and Development
During design and development, the functional and non-functional requirements are used to guide the design and development of the software system. The software developers use the requirements to create use cases, functional specifications, and system requirements specifications. They also use the non-functional requirements to design the architecture and infrastructure of the software system.
4.4. Testing
During testing, the functional and non-functional requirements are used to guide the testing of the software system. The testers use the requirements to create test cases, test plans, and test scripts. They also use the non-functional requirements to test the performance, security, usability, reliability, and maintainability of the software system.
4.5. Maintenance
During maintenance, the functional and non-functional requirements are used to guide the maintenance of the software system. The software developers use the requirements to identify any changes or enhancements that need to be made to the software system. They also use the non-functional requirements to ensure that the software system remains secure, reliable, and maintainable.
Examples of Functional and Non-Functional Requirements
5.1. Example of Functional Requirements
Functional Requirement: The user should be able to create an account.
Description: The software system should allow users to create an account by providing their name, email address, and password. The user should be able to verify their email address by clicking on a link in the verification email sent by the software system. Once the user has verified their email address, they should be able to log in to the software system.
5.2. Example of Non-Functional Requirements
Non-Functional Requirement: The software system should be able to handle 1000 concurrent users.
Description: The software system should be able to handle 1000 concurrent users without experiencing any significant decrease in performance. The response time of the software system should not exceed 2 seconds for any user request. The software system should be able to scale up or down to accommodate changes in the number of users.
In conclusion, functional and non-functional requirements are essential for successful software development. Functional requirements define what the software system should do, while non-functional requirements define how the software system should perform. Both types of requirements are used throughout the software development life cycle to ensure that the software system meets the needs of the stakeholders and is of high quality.
Frequently Asked Questions
What is the difference between functional and non-functional requirements?
Functional requirements define what the software system should do, while non-functional requirements define how the software system should perform.
Why are functional and non-functional requirements important?
Functional and non-functional requirements are important because they help to ensure that the software system meets the needs of the stakeholders, performs as expected, and is of high quality.
How are functional and non-functional requirements used in software development?
Functional and non-functional requirements are used throughout the software development life cycle to guide the design, development, testing, and maintenance of the software system.
What are some examples of functional requirements?
Examples of functional requirements include creating an account, logging in, and searching for a product.
What are some examples of non-functional requirements?
Examples of non-functional requirements include performance, security, usability, reliability, and maintainability.
Who is responsible for defining the functional and non-functional requirements of a software system?
The business analyst or product owner is responsible for defining the functional and non-functional requirements of a software system.
How are functional and non-functional requirements documented?
Functional and non-functional requirements are typically documented in a requirements document, a user story map, a product backlog, a functional specification, a system requirements specification, a performance requirements specification, a security requirements specification, a usability requirements specification, or a reliability requirements specification.
Can functional and non-functional requirements change during the software development life cycle?
Yes, functional and non-functional requirements can change during the software development life cycle as stakeholders’ needs and priorities change.
How are functional and non-functional requirements prioritized?
Functional and non-functional requirements are prioritized based on their importance to the stakeholders and the software system.
What happens if functional or non-functional requirements are not met?
If functional or non-functional requirements are not met, the software system may not meet the needs of the stakeholders, may not perform as expected, or may not be of high quality. This can lead to user dissatisfaction, decreased productivity, and increased costs.
