Non-Functional Requirement

Non-Functional Requirements (NFRs) define the quality attributes and performance standards of a system. Unlike functional requirements, they focus on how the system should operate, ensuring it is secure, reliable, efficient, and user-friendly.

• Define performance, reliability, usability, security, and maintainability standards.
• Specify operational requirements such as scalability, availability, and response time.

Example: An online banking system should process user requests within 2 seconds and maintain 99.9% uptime. It should also protect customer data through encryption and secure authentication mechanisms.

Characteristics

Non-functional requirements describe the quality attributes and operational standards that a system must meet. They focus on how the system performs rather than the specific functions it provides.

• Define performance, reliability, security, and usability standards.
• Specify system constraints and operational requirements.
• Can be measured and validated through testing.
• Focus on how the system should perform.

Importance of Documenting Non-Functional Requirements

Documenting non-functional requirements helps ensure that the system meets expected quality standards and performs efficiently under different conditions.

• Ensuring Quality: Defines standards for performance, reliability, security, and usability.
• Supporting System Design: Helps architects and developers design systems that meet operational expectations.
• Improving User Experience: Ensures the system is responsive, secure, and easy to use.
• Managing Performance Expectations: Establishes measurable criteria for evaluating system quality.

Types of Non-Functional Requirements

Below are the common types of non-functional requirements that define how a system should perform and operate. These requirements focus on the quality, efficiency, security, and reliability of the system rather than specific features.

Performance: Performance requirements define how fast the system should respond and how efficiently it handles user requests. It ensures the system can process operations quickly even when many users are using it at the same time.

Example: The system should display book search results within 2 seconds for up to 1,000 concurrent users.

Scalability: Scalability describes the system’s ability to handle increasing numbers of users, transactions, or data without slowing down. It ensures the system can grow as the business or user base expands.

Example: The system should support 10,000 concurrent users during peak traffic without performance degradation.

Availability: Availability ensures that the system is accessible and operational whenever users need it. High availability means the system has minimal downtime and remains accessible most of the time.

Example: The online bookstore should maintain 99.9% uptime and be available 24/7.

Reliability: Reliability refers to the system’s ability to operate correctly without failures for a specific period. It ensures the system consistently performs its functions accurately.

Example: The payment and order system should successfully process 99.95% of transactions without failure.

Security: Security requirements protect the system and user data from unauthorized access, data breaches, and cyber attacks. It includes authentication, authorization, and data encryption mechanisms.

Example: User passwords should be encrypted, and users must log in with authentication before placing orders.

Usability: Usability ensures the system is easy to learn, understand, and use for all users. A user-friendly interface helps users complete tasks quickly without confusion.

Example: A new user should be able to search and purchase a book in less than 4 steps.

Maintainability: Maintainability refers to how easily developers can update, fix bugs, or improve the system. A well-maintained system reduces downtime and makes updates easier.

Example: Developers should be able to deploy bug fixes or updates within 1 hour without affecting other system functions.

Portability: Portability means the system can run on different platforms, devices, or environments without major changes. It ensures flexibility in where the system can be used.

Example: The bookstore website should work properly on desktop, tablet, and mobile browsers like Chrome, Safari, and Firefox.

Example

Consider a simple example of Non-Functional Requirements for an online bookstore system.

Performance Requirements

The following requirements define the expected performance of the online bookstore system.

• The system should load web pages within 2 seconds under normal conditions.
• The system should support at least 1,000 concurrent users without significant performance degradation.
• Search results should be displayed within 3 seconds of a user query.

Security Requirements

The following requirements ensure the protection of user and business data.

• User passwords should be stored in encrypted form.
• The system should provide secure authentication and authorization mechanisms.
• All payment transactions should be processed through secure encrypted connections.

Reliability Requirements

The following requirements define the system's availability and dependability.

• The system should maintain at least 99.9% uptime.
• Data backups should be performed regularly to prevent data loss.
• The system should recover automatically from minor failures whenever possible.

Usability Requirements

The following requirements focus on user experience and ease of use.

• The user interface should be simple and easy to navigate.
• Users should be able to complete a purchase with minimal steps.
• The system should provide clear error messages and guidance when issues occur.

Scalability Requirements

The following requirements ensure the system can handle future growth.

• The system should support increasing numbers of users and transactions.
• Additional servers should be able to be added without major system modifications.
• Performance should remain stable as the user base grows.

Functional Requirements vs Non-Functional Requirements