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.