Question

In your own words, describe the difference between functional and nonfunctional requirements.

Short answer

Functional requirements describe what a system should do, while nonfunctional requirements define how the system should perform its functions.

Step-by-step solution

Understanding Functional Requirements

Functional requirements describe what a system should do. They define specific behaviors or functions of a system, often detailing inputs, outputs, process workflows, and interactions with other systems. For example, in a software application, functional requirements might specify that users need to be able to log in, upload files, or process transactions.

Exploring Nonfunctional Requirements

Nonfunctional requirements characterize how a system performs a function rather than what the system does. They encompass quality attributes such as performance, usability, reliability, and security. These requirements often deal with constraints or criteria that evaluate the system's operation, such as response time for a task or user interface accessibility standards.

Comparing Functional and Nonfunctional Requirements

The main difference is that functional requirements outline specific functionalities or operations, while nonfunctional requirements specify the quality or performance standards for those operations. While functional requirements are task-specific, nonfunctional requirements are more holistic, influencing the overall system architecture and user experience.

Key concepts

Functional Requirements

Functional requirements detail what a system should do. These are the tasks or operations a software system must perform to function correctly. Think of them as the step-by-step interactions that define the system's capabilities. For example, in a retail application:

• Users should be able to search for items.
• Customers must be able to add items to their carts.
• The system should process payments securely.

These requirements are essential to ensure that every user operation, from login to logout, happens smoothly.
Functional requirements are often documented in use cases, user stories, or functional specifications. They aim to meet user needs by providing precise and clear information about system behavior. They are crucial during the design phase as they guide developers on what functionalities to implement.

Nonfunctional Requirements

Nonfunctional requirements focus on how a system performs its operations. They define the system's quality attributes, including performance, security, reliability, usability, and scalability. Instead of detailing actions, they address the criteria and standards by which the functionality is evaluated.
For instance, a system should load a page within two seconds, handle up to 1000 users simultaneously, and ensure data encryption for security. Nonfunctional requirements can often be seen as constraints or restrictions on the design of the system.

• Performance: How fast the system provides a response.
• Security: Protection of data against unauthorized access.
• Usability: Ease of use for the end-user.

These requirements play a significant role in the system's architecture and design, ensuring not just "what" the system will do, but "how well" it will do it.

System Architecture

System architecture is the structure that defines how a software system is organized and works. It's crucial because it gives a blueprint for both how the system will function and how it will satisfy its functional and nonfunctional requirements.
Think of it as a high-level design plan that shows all the components of a system and how they interact. Proper architecture helps ensure that complex software systems remain scalable, maintainable, and flexible over time. Key aspects include:

• Components: distinct parts that encapsulate specific functionality.
• Interfaces: how components communicate and interact with each other.
• Topology: the arrangement and connectivity of components within the system.

Good system architecture must anticipate possible future changes and adapt easily to new requirements, ensuring the system's sustainability and robustness over the long term.

Quality Attributes

Quality attributes are the properties that determine how well a software system meets both user needs and business goals. They directly influence the user experience and the system's success in fulfilling its intended purpose.
Apart from nonfunctional requirements, quality attributes include specific goals for:

• Reliability: The likelihood that the system will operate without failure over a given period under stated conditions.
• Maintainability: How easily the system can accommodate changes and updates.
• Portability: The ability of the system to run in different environments or on different platforms.

By focusing on quality attributes, developers and designers can ensure not just functional success, but also a positive user experience and efficient system performance. Aligning system architecture with these attributes is crucial to achieving comprehensive system reliability and value.