Non-functional requirements (NFRs) are specifications that define the quality attributes of a system. Unlike functional requirements that describe what a system should do (like features and functionalities), non-functional requirements describe how a system should behave in terms of its performance, usability, reliability, and other operational characteristics. Here’s a simplified explanation suitable for someone new to the concept:

Understanding Non-Functional Requirements

Imagine you're buying a car. The functional requirements would be what the car needs to do: accelerate, brake, play music, turn on the headlights, etc. The non-functional requirements, however, are about how well the car performs these functions: how fast it accelerates (performance), how smoothly it rides (usability), how rarely it breaks down (reliability), or how secure it is against theft (security).

Here are some common types of non-functional requirements:

1. Performance

• Example: A website must load within 3 seconds.
• This affects how quickly a system responds to inputs or how fast it processes data.

2. Scalability

• Example: An app should be able to handle 10,000 concurrent users without slowing down.
• This is about the system’s ability to increase capacity to handle growth, like more users or more transactions, without affecting performance.

3. Reliability

• Example: A payment system should be operational 99.99% of the time.
• This refers to how dependable and consistent the system is in terms of uptime and its ability to perform under expected conditions.

4. Usability

• Example: Software should be usable with no prior training, and all main features should be accessible within three clicks.
• This encompasses how easy and pleasant the system is to use.

5. Security

• Example: User data transmitted over the network must be encrypted.
• This involves protecting the system from malicious attacks and ensuring data is safe from unauthorized access.

6. Maintainability

• Example: The system should allow developers to update code with new features within a maximum of four hours.
• This is about how easy it is to maintain, update, or enhance the system.

7. Portability

• Example: The software should run on both Windows and macOS.
• This describes the system's ability to operate across different environments and platforms.

How to Identify Non-Functional Requirements

To identify non-functional requirements, think about:

• User expectations: What quality of service do they expect?
• Operational environments: Where and how will the system be used?
• Business goals: What are the critical success factors for the system from a business perspective?
• Regulatory standards: Are there any legal or compliance aspects the system needs to meet?

Why Non-Functional Requirements Matter

Non-functional requirements are crucial because they directly impact user satisfaction and system performance. Poor non-functional characteristics can lead to systems that are slow, unreliable, difficult to use, or insecure, which can affect the overall success of the software.

In Summary

For "dummies" or beginners, think of non-functional requirements as the qualities or characteristics that determine how well a system performs and meets user and operational needs beyond its basic functionalities. They are what make a system robust, user-friendly, and effective in real-world scenarios.