In software development, users have specific expectations regarding the performance and quality of applications. These include performance levels, usability, availability, speed of execution, reliability, security against unauthorized access, and the ability to manage unexpected situations effectively.
These characteristics are known as non-functional requirements (NFRs) or quality attributes. Let’s delve into what NFRs are and the primary types that exist.
What Are Non-Functional Requirements?
Requirements analysis is vital for the success of any software development project. Requirements can be categorized into two main types: functional and non-functional. Both are essential for meeting the needs of stakeholders and the business.
Performance and Scalability
Performance and scalability are critical non-functional requirements that no system can overlook, as they are closely interconnected.
Performance
Performance is one of the most prevalent NFRs, ensuring that systems are designed to meet an acceptable level of efficiency. Factors such as latency, load handling, and resource usage can degrade performance. Common issues include excessive API calls, poor caching strategies, and overloaded third-party services. To maintain high performance, it’s advisable to monitor these areas and utilize separate task queues to enhance user experience.
Example: A webpage displaying customer data should load within five seconds on a 30 Mbps or faster internet connection.
Scalability
Scalability measures how well a solution can adapt to increasing workloads, such as user requests, events, and data traffic. A scalable system might require upgrades in hardware or IT infrastructure—like faster processors, additional servers, or improved network capacity—as well as software optimizations such as performance tuning and data compression.
Example: The system should support an additional 5,000 users per month for the next six months without noticeable performance degradation.
Portability and Compatibility
Portability and compatibility are essential considerations in non-functional requirements.
Portability
Portability assesses how well a system performs across different environments, including hardware and software platforms. It reflects how seamlessly operations can transition from one platform to another.
Example: If a user downloads a mobile application from an old device to a new one and it functions as expected, the app is considered highly portable. Developers can enhance portability by ensuring applications work across various devices.
Compatibility
Compatibility refers to a system’s ability to interact with other systems and software effectively. It ensures users can customize settings based on their preferences without issues.
Example: If a user sets their phone to vibrate for incoming calls, the device should retain that setting after a restart. Systems with larger storage capacities allow for more settings and data storage.
Reliability, Maintainability, and Availability
While these three requirements are often documented separately, they tackle similar challenges from different angles and can be complex to quantify.
Reliability
Reliability indicates a system’s ability to function consistently over time. Key metrics include:
- Probability of Failure: A higher failure rate suggests lower reliability.
- Critical Failures Count: Fewer critical failures during testing indicates better reliability.
- Time Between Critical Failures: Longer intervals suggest a more dependable system.
Example: The database upgrade process should ensure that if one upgrade fails, all related upgrades are rolled back.
Maintainability
Maintainability focuses on the ease and cost-effectiveness of maintaining a system throughout its lifecycle. It encompasses factors like convertibility, extensibility, and interoperability, with an emphasis on monitoring code and architecture to identify improvement areas.
Example: The system should allow for the addition of new product categories, extending existing functionality to accommodate them.
Availability
Availability encompasses multiple aspects, including reliability and resilience. It ensures that systems are operational as much as possible, minimizing downtime. Considerations include the impact of downtime on business objectives and scheduling for maintenance.
Example: The system must maintain at least 99% availability on weekdays from 9:00 AM to 6:30 PM Pacific Time.
Security
Security is crucial when handling sensitive data, such as personal or financial information. It includes confidentiality and authentication measures to safeguard this data. Software engineering practices like encryption, secure integration points, and regular vulnerability assessments help maintain security.
Example: Only users with administrator privileges should be able to create new customer accounts and manage data access rights.
Localization
Localization defines how well a system fits within the context of its local market. It includes features that align with the geographic and cultural specifics of its users, such as:
- Language
- Currency
- Measurement systems (e.g., pounds vs. kilograms)
- Time zones
Example: The date format must conform to month.day.year standards.
Usability
Usability ensures that systems are user-friendly for diverse audiences, including end-users and administrators. Accessibility is vital, especially for users with specific needs or limited digital skills. Investing in user experience (UX) is essential for creating an inclusive system.
Example: At least 99% of trained users should successfully complete the product form in a single attempt using the metadata management system.
Non-Functional Requirements Documentation
While there isn’t a standardized document solely for non-functional requirements, they can be effectively integrated into broader project documentation. Common documents include:
- Business Requirements Documents (BRD)
- Technical Requirements Documents (TRD)
- Product Requirements Documents (PRD)
- Software Requirements Specifications (SRS)
Key considerations for documenting non-functional requirements:
- Ensure they are measurable and testable.
- Avoid combining multiple requirements into one.
- Link NFRs to business objectives.
- Consider third-party and architectural limitations.
- Reference existing standards and guidelines.
By addressing these elements, you can ensure that non-functional requirements contribute effectively to the overall quality and success of your software project.
