Functional vs non-functional requirements in Software Engineering - Visual Side-by-Side Comparison

Concept Flow - Functional vs non-functional requirements

Start

↓

Gather Requirements

↓

Classify Requirements

↓

Functional Req.

↓

Define What

↓

Develop Features

↓

Test & Validate Both

↓

End

The flow shows starting from gathering requirements, then splitting them into functional (what the system should do) and non-functional (how the system should be) requirements, followed by development and testing.

Execution Sample

Software Engineering

Requirement: User login
Functional: System must allow user to login
Non-functional: Login must be secure and fast

This example shows a requirement split into functional (login feature) and non-functional (security and speed quality) parts.

Analysis Table

Step	Requirement Type	Description	Purpose	Example
1	Functional	Defines specific behavior or function	What the system should do	User can submit a form
2	Non-functional	Defines quality or constraint	How the system performs or behaves	Form submits within 2 seconds
3	Functional	Focus on features	Direct user actions or system tasks	System sends email notification
4	Non-functional	Focus on system attributes	Performance, security, usability	Email system uptime 99.9%
5	Termination	No more requirements to classify	Process ends	All requirements categorized

💡 All requirements have been classified as either functional or non-functional, completing the process.

State Tracker

Requirement	Initial	After Classification	Final
User login	Unclassified	Functional	Functional
Login speed	Unclassified	Non-functional	Non-functional
Email notification	Unclassified	Functional	Functional
System uptime	Unclassified	Non-functional	Non-functional

Key Insights - 3 Insights

Why is 'User login' a functional requirement and not non-functional?

Because it describes what the system must do (allow login), as shown in execution_table step 1 and variable_tracker where it is classified as functional.

Can a requirement be both functional and non-functional?

No, each requirement is classified as either functional (what system does) or non-functional (how system behaves), as seen in execution_table steps 1-4.

Why do non-functional requirements focus on qualities like speed or security?

Because they define how well the system performs or behaves, not specific features, as explained in execution_table step 2 and 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what type of requirement is 'System sends email notification'?

A Non-functional

B Functional

C Both

D Neither

💡 Hint

Refer to execution_table row with Step 3 under 'Requirement Type' column.

According to variable_tracker, what is the classification of 'Login speed' after classification?

A Unclassified

B Functional

C Non-functional

D Both

💡 Hint

Check variable_tracker row for 'Login speed' under 'After Classification' column.

If a requirement describes 'System must be easy to use', where would it appear in the execution_table?

A Non-functional requirements

B Termination step

C Functional requirements

D Not classified

💡 Hint

Non-functional requirements describe qualities like usability, see execution_table steps 2 and 4.

Concept Snapshot

Functional requirements describe what the system should do (features and behaviors).
Non-functional requirements describe how the system performs (quality, speed, security).
Classify requirements early to guide development and testing.
Functional = actions; Non-functional = qualities.
Both are essential for successful software.

Full Transcript

This visual execution mode explains the difference between functional and non-functional requirements. The process starts by gathering all requirements, then classifying each as functional or non-functional. Functional requirements define what the system should do, like allowing user login or sending notifications. Non-functional requirements define how the system should behave, such as performance speed or security levels. The execution table shows step-by-step classification examples, and the variable tracker follows requirement states from unclassified to final type. Key moments clarify common confusions, like why a login feature is functional and qualities like speed are non-functional. The quiz tests understanding by referencing the execution visuals. The snapshot summarizes the core idea: functional requirements are about system actions, non-functional about system qualities.

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