Non-Functional Requirements: Definition and Examples

Non-functional requirements describe how a system should perform and behave, focusing on qualities like performance, security, and usability. They complement functional requirements by defining system constraints and standards rather than specific features.

How It Works

Think of building a house: functional requirements are like the rooms you want, such as a kitchen or bedroom. Non-functional requirements are the qualities of the house, like how strong the walls are, how well it insulates heat, or how safe it is from intruders.

In software, non-functional requirements set the rules for the system’s behavior and quality. They ensure the system is fast enough, secure, reliable, and easy to use. These requirements guide design decisions and testing to meet user expectations beyond just features.

Example

This example shows a simple check for a non-functional requirement: ensuring a function runs within a time limit (performance).

python

import time

def process_data():
    start = time.time()
    # Simulate data processing
    time.sleep(0.5)  # 0.5 seconds delay
    end = time.time()
    duration = end - start
    if duration > 1.0:
        return "Performance requirement not met"
    return "Performance requirement met"

result = process_data()
print(result)

Output

Performance requirement met

When to Use

Non-functional requirements are essential when you want to ensure your system is reliable, secure, and user-friendly. For example, in banking apps, security and availability are critical non-functional requirements. In video streaming services, performance and scalability matter most.

Use them during planning and design to set clear expectations for system quality. They help teams avoid surprises and build systems that meet user needs beyond just functionality.

Key Points

• Non-functional requirements define system qualities like speed, security, and usability.
• They complement functional requirements by focusing on how the system works, not what it does.
• They guide design, development, and testing to meet user expectations.
• Examples include performance, reliability, scalability, and maintainability.

Key Takeaways

Non-functional requirements specify system qualities and constraints beyond features.

They ensure the system meets standards for performance, security, and usability.

Include non-functional requirements early to guide design and testing.

They help build systems that satisfy user expectations and operate reliably.