Functional and Non Functional Requirements

Requirements analysis is an essential process in software development. It helps to determine whether a system or project will meet its objectives and achieve success.
To make this analysis effective, requirements are generally divided into two categories:

What are Functional Requirements?

Functional requirements define the specific features and operations a system must perform to meet business and user needs. They describe what the system should do and how it should interact with users or other systems.

• Focus on system behavior and functionality.
• Represent the features that can be directly observed and tested in the final product.
• Common examples include user authentication, data processing, search, payment handling, and report generation.

Sample Questions

• What features should the system include?
• What edge cases must be considered in the design?

What are Non-Functional Requirements?

Non-functional requirements (NFRs) define how a system should operate, focusing on performance, reliability, and user experience rather than specific features. They ensure the system is efficient, secure, and maintainable over time.

• Performance – speed and responsiveness
• Security – protection against unauthorized access
• Usability – ease of use
• Reliability – system stability and availability
• Scalability – ability to handle growth
• Maintainability – ease of updates and fixes
• Portability – ability to run in different environments

Sample Questions

• How fast should the system respond to user actions?
• How secure should it be against unauthorized access?
• How available and reliable should the system be?
• Extended Requirements.

Examples of Functional and Non-functional Requirements

Let's consider a couple of examples to illustrate both types of requirements:

1. Online Banking System

Functional Requirements:

• Users should be able to log in with their username and password.
• Users should be able to check their account balance.
• Users should receive notifications after making a transaction.

Non-functional Requirements:

• The system should respond to user actions in less than 2 seconds.
• All transactions must be encrypted and comply with industry security standards.
• The system should be able to handle 100 million users with minimal downtime.

2. Food Delivery App

Functional Requirements

• Users can browse the menu and place an order.
• Users can make payments and track their orders in real time.

Non-functional Requirements:

• The app should load the restaurant menu in under 1 second.
• The system should support up to 50,000 concurrent orders during peak hours.
• The app should be easy to use for first-time users, with an intuitive interface.

Differences between Functional Requirements and Non-Functional Requirements:

Definition

• Functional requirements define what the system should do, the exact features, tasks, or operations.
• Non-functional requirements define how the system should perform, the qualities or attributes like speed, security, or usability.

Purpose

• Functional Requirements focuses on the behavior and features of the system.
• Non-Functional Requirements focuses on the performance, usability, and overall quality of the system.

Scope

• Functional Requirements defines the actions and operations the system must support.
• Non functional Requirements defines constraints or conditions under which those actions should occur.

Measurement

• Functional requirements are easily measured by verifying outputs or results.
• Non-functional requirements are harder to measure, often validated against benchmarks, metrics, or SLAs.

Impact on Development

• Functional requirements drive the core design and features of the system.
• Non-functional requirements influence the system architecture and performance optimization.

User Perspective

• Functional requirements are directly visible to users and tied to business needs.
• Non-functional requirements shape the user experience by ensuring efficiency, reliability, and smooth operation.

Documentation

• Functional requirements are documented in use cases, user stories, or functional specifications.
• Non-functional requirements are captured in performance criteria, technical specs, or design constraints.

Evaluation

• Functional requirements are validated through functional testing (unit, integration, or acceptance tests).
• Non-functional requirements are verified via performance, security, and usability testing.

Dependency

• Functional requirements define what must be built to meet user needs.
• Non-functional requirements define how well the system must operate once built.

Examples

• Functional requirements: login authentication, data input/output, transaction processing.
• Non-functional requirements: system scalability, security, response time, reliability, maintainability.

Importance of Balancing Both Functional and Non-Functional Requirements

Balancing functional and non-functional requirements ensures a system is both useful and reliable. Key benefits include:

• Improves User Experience: Functional features alone may work but feel slow or hard to use. Non-functional aspects like performance, usability, and availability shape how users interact with the system.
• Enhances System Performance: Scalability, reliability, and security ensure the system performs effectively under real-world conditions.
• Prevents Bottlenecks and Failures: Proper attention to non-functional needs reduces outages, security breaches, and downtime.
• Reduces Long-Term Costs: Addressing non-functional requirements early avoids expensive redesign, rework, or performance fixes later.
• Supports System Evolution: Maintainability and extensibility allow the system to scale and adapt smoothly as business and user needs grow.

Common Challenges in Defining these Requirements

Defining functional and non-functional requirements can be complex due to several challenges:

• Ambiguity in Requirements: Vague or incomplete requirements make it difficult to define what the system must do (functional) and how it should perform (non-functional).
• Changing Requirements: Evolving business goals, market trends, or user expectations can shift requirements, making it harder to maintain stable design.
• Difficulty in Prioritization: Functional requirements often take precedence, while critical non-functional aspects like security or scalability may be overlooked.
• Measuring Non-Functional Requirements: Functional needs are easier to test; non-functional attributes like usability, scalability, or reliability are harder to quantify and validate.
• Overlapping or Conflicting Requirements: Some requirements may conflict, such as stronger security potentially impacting system performance, requiring careful trade-offs.

How to Gather Functional and Non-functional Requirements

Gathering requirements involves multiple approaches and collaboration between the development team, stakeholders, and end-users:

1. Functional Requirements:

• Interviews: Talk to stakeholders or users to understand their needs.
• Surveys: Distribute questionnaires to gather input from a larger audience.
• Workshops: Host sessions to brainstorm features and gather feedback.

2. Non-functional Requirements:

• Performance Benchmarks: Consult with IT teams to set expectations for performance and load.
• Security Standards: Consult with security experts to define the best practices for data protection.
• Usability Testing: Test the system to find areas where users might struggle and refine the interface.