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?

These are the requirements that describe the specific features and operations that a system must provide to meet business and user needs. They focus on what the system should do and outline the interactions between the system and its users.

• Typical examples include user authentication, data entry and processing, search functionality, payment handling, and report generation.
• Unlike non-functional requirements, these are the features explicitly requested by users and can be directly observed in the final product.

Examples of questions for Functional Requirements:

• What are the features that we need to design for this system?
• What are the possible edge cases we need to consider in our design?

What are Non-Functional Requirements?

These requirements define how a system should operate, rather than what it should do. They define how well the system should perform its functions and ensure that the product is reliable, efficient, and user-friendly.
While they may not be directly visible as features, they play a vital role in shaping the user experience and ensuring the system’s long-term success.

Key Aspects in NFRs include:

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

Examples for needs in NFRs:

• How fast should the system respond to user actions?
• System should be highly available.
• How secure should the system be against unauthorized access?

Extended Requirements

Extended requirements go beyond the standard functional and non-functional requirements. They often capture business goals, domain-specific needs, or future considerations that may not directly map to system functions but are important for long-term success.

These are basically ensuring that the system fits real-world constraints and business environments, not just technical or user needs.

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

Functional and non-functional requirements cover different but equally important aspects of a system. To ensure a successful design, both must be carefully balanced. Here’s why:

• Improves User Experience: Focusing only on functional requirements may lead to a system that works but feels slow, unresponsive, or difficult to use. Non-functional aspects such as performance, usability, and availability directly influence how users perceive and interact with the system.
• Enhances System Performance: Scalability, reliability, and security ensure that the system performs effectively under real-world conditions. A product that meets functional goals but fails to scale or handle heavy loads risks failure. Balancing both guarantees smooth and dependable operation.
• Prevents Bottlenecks and Failures: A system without proper attention to non-functional needs is more prone to outages, breaches, or downtime. Incorporating reliability and security from the start builds resilience and reduces risks of major disruptions.
• Reduces Long-Term Costs: Ignoring non-functional requirements early often leads to scalability and maintenance issues later. By addressing them during design, organizations avoid expensive rework, redesign, or performance fixes in the future.
• Supports System Evolution: Requirements like maintainability and extensibility enable future upgrades, new features, and scaling with minimal friction. Balancing both ensures the system can evolve smoothly as business and user needs grow.

Common Challenges in Defining these Requirements

Defining functional and non-functional requirements is often complex and comes with several challenges:

• Ambiguity in Requirements: Requirements are sometimes vague or incomplete, making it difficult to clearly define what the system must do (functional) and how it should perform (non-functional).
• Changing Requirements: As projects evolve, requirements often shift due to changing business goals, market trends, or user expectations, making it harder to maintain stability in design.
• Difficulty in Prioritization: Determining which requirements matter most can be tricky. Functional needs frequently take priority, while critical non-functional aspects like security or scalability may be overlooked.
• Measuring Non-Functional Requirements: Functional requirements are easier to test, whereas non-functional attributes such as usability, scalability, or reliability are harder to define in measurable terms and validate effectively.
• Overlapping or Conflicting Requirements: Requirements can sometimes conflict or influence each other. For example, strengthening security may reduce 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.