Functional vs non-functional requirements in Software Engineering - Key Differences Explained

Introduction

When building software, it's important to know what the software should do and how well it should do it. This helps teams create products that meet users' needs and expectations clearly.

Explanation

Functional Requirements

These describe what the software must do. They specify tasks, features, or behaviors the system should have, like processing data, performing calculations, or responding to user actions. They focus on the core functions that solve the user's problem.

Functional requirements define the specific actions and features the software must perform.

Non-functional Requirements

These describe how the software performs its functions rather than what it does. They include qualities like speed, security, usability, and reliability. These requirements ensure the software works well under different conditions and meets user expectations beyond basic functionality.

Non-functional requirements define the quality and performance standards the software must meet.

Relationship Between Functional and Non-functional Requirements

Both types of requirements work together to create a complete picture of the software. Functional requirements tell what to build, while non-functional requirements guide how well it should work. Ignoring either can lead to software that is incomplete or unsatisfactory.

Functional and non-functional requirements together ensure the software is both useful and effective.

Real World Analogy

Imagine ordering a custom car. The functional requirements are the car's features like having four wheels, an engine, and seats. The non-functional requirements are how fast it should go, how safe it is, and how comfortable the ride feels.

Functional Requirements → The car's features like wheels, engine, and seats that make it usable.

Non-functional Requirements → The car's qualities like speed, safety, and comfort that affect the experience.

Relationship Between Functional and Non-functional Requirements → How the car's features and qualities together make it a good vehicle.

Diagram

Diagram

┌───────────────────────────────┐
│       Software Requirements    │
├───────────────┬───────────────┤
│ Functional    │ Non-functional│
│ Requirements  │ Requirements  │
│               │               │
│ - What it     │ - How well it │
│   does        │   performs    │
│ - Features    │ - Quality     │
│ - Tasks       │ - Performance │
└───────────────┴───────────────┘

This diagram shows the two main types of software requirements and their focus areas.

Key Facts

Functional Requirement → A specification of a function or feature the software must provide.

Non-functional Requirement → A specification of a quality or performance standard the software must meet.

Usability → A non-functional requirement that measures how easy the software is to use.

Performance → A non-functional requirement that defines how fast or responsive the software should be.

Security → A non-functional requirement that ensures the software protects data and resists attacks.

Common Confusions

Believing functional requirements include quality aspects like speed or security.

Believing functional requirements include quality aspects like speed or security. Functional requirements focus only on what the software does, while qualities like speed and security are part of non-functional requirements.

Thinking non-functional requirements are optional or less important.

Thinking non-functional requirements are optional or less important. Non-functional requirements are essential because they affect user satisfaction and software success, not just optional extras.

Summary

Functional requirements specify what the software must do, focusing on features and tasks.

Non-functional requirements specify how well the software performs, focusing on qualities like speed and security.

Both types are necessary to build software that works correctly and meets user expectations.

Practice

1. Which of the following best describes a functional requirement in software engineering?

easy

A. The look and feel of the user interface

B. A measure of how fast the system responds

C. A feature that specifies what the system should do

D. The hardware needed to run the software

Solution

1. Step 1: Understand the definition of functional requirements

   Functional requirements describe specific behaviors or functions the system must perform.

2. Step 2: Match the option to the definition

   A feature that specifies what the system should do states a feature specifying what the system should do, which matches the definition.

3. Final Answer:

   A feature that specifies what the system should do -> Option C

4. Quick Check:

   Functional requirement = What system does [OK]

Hint: Functional = what system does, Non-functional = how it does [OK]

Common Mistakes:

• Confusing functional with non-functional requirements
• Thinking performance is a functional requirement
• Mixing hardware needs with software functions

2. Which of the following is an example of a non-functional requirement?

easy

A. The system must allow users to reset their password

B. The system should respond to user input within 2 seconds

C. The system must save user data to a database

D. The system must support login with username and password

Solution

1. Step 1: Identify non-functional requirements

   Non-functional requirements describe how the system performs, such as speed or reliability.

2. Step 2: Evaluate each option

   The system should respond to user input within 2 seconds specifies response time, which is a performance measure and thus non-functional.

3. Final Answer:

   The system should respond to user input within 2 seconds -> Option B

4. Quick Check:

   Non-functional = performance or behavior [OK]

Hint: Look for speed, usability, or reliability for non-functional [OK]

Common Mistakes:

• Choosing features as non-functional requirements
• Confusing functional actions with performance measures
• Ignoring timing or quality aspects

3. Consider this statement: "The system shall encrypt all user data before saving." What type of requirement is this?

medium

A. Functional requirement

B. Non-functional requirement

C. Neither functional nor non-functional

D. Both functional and non-functional

Solution

1. Step 1: Analyze the statement's action

   Encrypting data is a specific action the system must perform, describing what it does.

2. Step 2: Classify the requirement type

   Since it describes a function (encryption), it is a functional requirement.

3. Final Answer:

   Functional requirement -> Option A

4. Quick Check:

   Encryption = system function = functional [OK]

Hint: If it describes a system action, it's functional [OK]

Common Mistakes:

• Thinking encryption is a performance or quality aspect
• Confusing security features with non-functional requirements
• Selecting 'both' without clear reasoning

4. A software team wrote this requirement: "The system must be user-friendly." What is the main issue with this requirement?

medium

A. It describes a functional requirement incorrectly

B. It is a security requirement

C. It is a hardware requirement, not software

D. It is too vague and not measurable

Solution

1. Step 1: Understand the requirement's nature

   "User-friendly" describes a quality but is subjective and lacks clear criteria.

2. Step 2: Identify the problem

   Requirements must be clear and measurable; this one is vague and cannot be tested easily.

3. Final Answer:

   It is too vague and not measurable -> Option D

4. Quick Check:

   Good requirements = clear + measurable [OK]

Hint: Vague terms mean poor requirements [OK]

Common Mistakes:

• Assuming all non-functional requirements are clear
• Confusing vague qualities with functional features
• Ignoring measurability in requirements

5. A project requires the system to handle 1000 users simultaneously and respond within 1 second. Which combination correctly classifies these requirements?

hard

A. "Handle 1000 users simultaneously" is functional; "respond within 1 second" is non-functional

B. "Handle 1000 users simultaneously" is non-functional; "respond within 1 second" is functional

C. Both are functional requirements

D. Both are non-functional requirements

Solution

1. Step 1: Analyze "handle 1000 users simultaneously"

   This describes a specific system function to support multiple users at once, which is a functional requirement.

2. Step 2: Analyze "respond within 1 second"

   This describes a performance measure, how fast the system reacts, so it is non-functional.

3. Final Answer:

   "Handle 1000 users simultaneously" is functional; "respond within 1 second" is non-functional -> Option A

4. Quick Check:

   Function = what system does; Performance = how system performs [OK]

Hint: Capacity = functional; speed = non-functional [OK]

Common Mistakes:

• Classifying capacity as non-functional
• Mixing performance with system capabilities
• Ignoring the difference between function and quality