The key to system quality: Non-functional requirements 

Non-functional requirements (NFRs) are often overshadowed by their functional counterparts in software development, but they are no less critical to project success. A non-functional requirement defines and clarifies the specifications of a software project, highlighting system qualities and constraints.

Let’s look at the role and importance of non-functional requirements in software architecture. By understanding and prioritising NFRs, you can build robust, reliable and high-performing systems that meet both your current and future needs.

The importance of non-functional requirements in software architecture

NFRs in software architecture directly influence the overall quality and performance of a system. They define how the system should behave under various conditions to ensure that it meets user expectations and operational standards. The quality attributes are essential in determining system metrics such as scalability, security, reliability and maintainability, all of which contribute to the long-term success and sustainability of the software.

For example, a software may meet all its functional requirements but become unusable if it crashes under heavy user load or is vulnerable to security breaches. By incorporating NFRs into the architectural design from the outset, architects and developers can anticipate potential challenges and build systems that are functional, robust, secure and adaptable to future needs.

Prioritising NFRs offers better understanding of business goals and technical constraints, allowing for a more effective breakdown of requirements into manageable scenarios. As such, NFRs are essential when designing software architectures that are resilient, efficient and capable of delivering quality user experience.

Functional requirements vs. Non-functional requirements
Functional requirements specify what the system should do in terms of specific features, functionalities, and behaviours. They define the actions the system must take when certain conditions are met, including how it handles inputs, processes data, and generates outputs. In contrast, non-functional requirements define all the underlying mechanisms required to run the application. While FRs ensure that the system delivers the required functionality, NFRs guarantee that it does so in a way that meets business standards, covering all demanded attributes.

Types of NFRs

Availability – Defines the period during which a system is fully operational and accessible, critical to ensuring business continuity as outlined in SLAs.

Example: The system must maintain 99.9% uptime, with an automatic failover to a backup server in case of hardware failure.

Disaster recovery – Refers to the process of restoring IT services and infrastructure to normal following a disruptive event such as a natural disaster, hardware failure or cyber-attack.

Example: The system must be able to recover and resume operations within a maximum downtime of 4 hours following a disaster or significant service interruption. The system should not lose more than 12 hours’ worth of data.

Interoperability – The ability of a system to integrate seamlessly with other software and hardware, requiring detailed discussions about data exchange, system interfaces and adherence to standards.

Example: The system needs to integrate with the existing CRM software via RESTful APIs, using JSON for data exchange and supporting OAuth 2.0 for authentication.

Performance – Refers to the ability of the system to meet response time, throughput and capacity requirements, and significantly influences hardware and software design decisions.

Example: The system must process up to 10,000 transactions per second with a maximum response time of 2 seconds under peak load conditions.

Reliability – Measures the ability of a system to function without failure over time, often quantified by metrics such as mean time between failures and failure rates.

Example: The system should handle errors without data loss.

Security – Focuses on protecting the system from unauthorised access and threats, including considerations such as authentication, encryption and audit trails.

Example: The system must use AES-256 encryption for all stored data, must require two-factor authentication for all user logins, and maintain detailed audit logs of all access attempts.

Usability – Concerns how easy it is for users to learn and use the system efficiently, including aspects such as error prevention, task completion time and accessibility.

Example: New users should be able to complete the onboarding process within 5 minutes, with a data entry error rate of less than 2%.

Maintainability – Describes the ease with which a system can be repaired, improved or adapted, emphasising the importance of understandable and modifiable code.

Example: The system code base must adhere to coding standards and be thoroughly documented so that any developer can implement minor updates within 2 hours.

Scalability – The ability of a system to handle increased workloads, whether through hardware upgrades or software optimisation, essential for future growth.

Example: The system must support scaling to accommodate an increase in user traffic within one year, with minimal impact on performance.

How to manage non-functional requirements

Effectively managing non-functional requirements requires a multi-faceted approach that combines technical acumen with strategic foresight. Unlike functional requirements, which typically articulate discrete capabilities, NFRs encapsulate the broader operational characteristics of a system.

To manage these requirements, it is crucial to integrate them early in the software development lifecycle to ensure that they are not relegated to an afterthought. This involves clearly defining NFRs with quantifiable metrics and embedding them in the architecture design, test frameworks and deployment strategies. A holistic approach requires ongoing validation through performance benchmarks, security audits and user feedback loops, so that the system is continually refined to meet evolving needs.

In addition, effective prioritisation is key – balancing trade-offs between competing NFRs, such as performance versus security, to align with the system’s overarching business objectives. Balancing technical rigour with business imperatives is fundamental to ensuring that the system functions and thrives in its intended operational context.

What are the potential threats of non-functional requirements?

Non-functional requirements are crucial in identifying potential threats to system performance and security. Ignoring these requirements can lead to vulnerabilities that compromise the integrity and efficiency of the system. It is essential to address NFRs early in the development process to mitigate risks and ensure robust system architecture.

Negative performance and usability

It arises when NFRs related to system speed, responsiveness and user interface are poorly defined or ignored. It can result in a system that is slow, unresponsive or difficult to navigate, leading to user frustration, reduced productivity and potential abandonment of the system. Over time, these problems can damage the organisation’s reputation and undermine customer confidence.

Deprioritisation

Very often, deprioritisation happens when functional requirements take precedence, especially under tight deadlines or budget constraints. When non-functional aspects are overlooked or inadequately addressed, the result is a system that may meet functional requirements but fails to perform reliably in real-world conditions. It may lead to frequent system failures, performance bottlenecks, security vulnerabilities, technical debt, where the cost and effort to address these requirements grows exponentially after the system is in use.

An ever-growing scope

It occurs when projects expand beyond their original intent, adding new performance, security, or compliance demands without proper evaluation or prioritisation. The expansion can lead to scope creep, making the project more complex, time-consuming, and costly than initially planned. The resulting delays can extend development timelines, causing project delivery to be pushed back and increasing costs. In addition, as the scope grows without a corresponding increase in resources or budget, the development team may become overstretched, leading to lower quality outcomes as they struggle to meet the increasing demands.

“The potential dangers of neglecting or inadequately addressing non-functional requirements cannot be overstated. When these essential facets are ignored, the system’s resilience, security and user experience is precariously compromised, exposing the organisation to the risk of failure. Non-functional requirements are the foundation of system reliability. So, the real challenge is to define the right set of NFRs for your case, ensuring that the architecture is designed to meet current needs, and hardened against unforeseen threats.”

Filip Rozanski, Head of Managed Services

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