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Summary – In a custom project, limiting yourself to functional requirements exposes you to technical drift, cost overruns, delays and user dissatisfaction. Nonfunctional Requirements cover performance, security, scalability, maintainability and user experience and must be defined from scoping as SMART metrics to shape the architecture, guide testing and prevent risks throughout the lifecycle.
Solution: formalize NFRs in the SRS/PRD, integrate tests
In a custom software development project, meeting only the functional requirements is not enough to guarantee the solution’s robustness, security, and longevity. Nonfunctional Requirements (NFRs) cover performance, security, scalability, maintainability, and user experience criteria that directly impact overall quality. If these criteria are not defined from the outset, the risk of technical drift, cost overruns, and user dissatisfaction increases significantly. This approach aligns deliverables with business objectives and manages technical risks throughout the software lifecycle.
The Fundamentals of Nonfunctional Requirements
Nonfunctional Requirements (NFRs) define the quality criteria and constraints of a software solution beyond its immediate application features. They ensure that performance, security, maintainability, and user experience meet business expectations and technical challenges.
What Is an NFR?
A Nonfunctional Requirement (NFR) specifies a requirement related to the quality, constraint, or operating environment of software rather than its functional behavior. It covers aspects such as response time, availability rate, data security, or compatibility with other systems.
Unlike user stories or functional specifications, an NFR does not directly define a feature visible to the user but determines how that feature must be delivered or executed. It focuses on guaranteeing service levels and reliability essential for operation and maintenance.
NFRs come into play at every stage of the software lifecycle: from requirements gathering to architecture, from development to acceptance testing, and then to operation. They serve as a reference when creating non-regression, performance, and security tests to validate that quality objectives are met.
Distinction from Functional Requirements
Functional requirements describe what the system must do (use cases, workflows, manipulated data) while nonfunctional requirements describe how the system must do it (service level, security constraints, performance). This distinction is essential for structuring a complete requirements specification.
Functional requirements are translated into user stories or use-case diagrams, whereas NFRs are formalized as metrics, thresholds, or acceptance criteria (for example, a response time under 200 ms). Precise wording of these criteria avoids ambiguity and simplifies validation.
A set of functional requirements without NFRs exposes the project to quality drift and misunderstandings among stakeholders. NFRs ensure that delivered results are not only functional but also operable, secure, and evolvable over time.
Importance in the Project Lifecycle
Integrating NFRs from the scoping phase allows you to anticipate architectural challenges, plan testing efforts, and allocate the necessary resources to meet quality targets throughout the project. This foresight limits the risk of backtracking and late-stage fixes.
During design, architects and engineers rely on NFRs to select technologies, develop infrastructure diagrams, and define appropriate development and security patterns. Without these guidelines, technical choices can be unsuitable and lead to high maintenance costs.
For example, a mid-sized Swiss fintech firm defined strict availability and data encryption requirements in its initial specifications. This approach revealed the need for a multi-availability-zone redundant architecture and encryption modules compliant with banking standards, reducing service incidents and strengthening user trust.
Key Dimensions of Nonfunctional Requirements
NFRs cover several essential dimensions that influence the stability, scalability, security, and compatibility of a solution. Each dimension must be defined precisely and measurably to manage quality and limit technical risks.
Performance and Scalability
The performance dimension sets thresholds such as maximum response time, transactions per second, or acceptable latency under load. It determines the application’s efficiency and responsiveness under real-world usage.
Scalability describes the system’s ability to handle increased load without service degradation. It can be vertical (adding resources to a server) or horizontal (adding more nodes).
Clearly defining these criteria allows you to plan load tests and simulate traffic spikes before production deployment, preventing unexpected outages during high demand.
For example, a Swiss online retailer specified an NFR for handling 5,000 concurrent orders with a response time under 300 ms. This requirement demonstrated the need for a microservices architecture and a distributed cache to meet performance goals and avoid downtime during sales promotions.
Security and Availability
Security covers data protection, access management, resistance to attacks, and compliance with standards (ISO 27001, GDPR, Swiss Federal Data Protection Act (FADP), etc.). It relies on criteria such as encryption in transit and at rest, strong authentication, and regular code reviews.
Availability defines the percentage of time the service must remain operational (for example, 99.9%). Achieving this level involves redundant architectures, disaster recovery plans, and monitoring procedures.
Implementing vulnerability tests, security scans, and incident simulations verifies that security and availability objectives are met. Without these checks, any incident can become critical.
Compatibility and Portability
Compatibility ensures that the application runs on various environments (browsers, operating systems, databases) and interoperates with other systems via APIs or standard data formats. An NFR might specify support for multiple browser versions or OS releases.
Portability refers to the ability to deploy the solution on diverse infrastructures (cloud, on-premises, containerization). It prevents vendor lock-in and provides flexibility to move to other platforms.
Compatibility and portability NFRs often boost agility and solution lifespan. They allow progressive migrations and encourage the adoption of open-source components to limit long-term costs.
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Formulating and Documenting Nonfunctional Requirements
Well-formulated NFRs rely on SMART criteria and their integration into technical and functional documentation. This facilitates validation, testing, and alignment with business objectives.
SMART Criteria for NFRs
Each NFR should be Specific, Measurable, Achievable, Realistic, and Time-bound. SMART criteria ensure that the requirement is clear, verifiable, and aligned with the delivery schedule.
For example, replace a vague statement like “the system must be fast” with “critical API response times must be under 200 ms for 95% of requests” to eliminate ambiguity and allow quantitative tracking.
Thresholds, metrics, and failure conditions must be validated by business and technical stakeholders to ensure objectives are coherent and achievable within the project context.
Scenarios and KPIs
Describing concrete scenarios (e.g., traffic spikes, load-increase cases, penetration tests) illustrates usage and validates expected performance. These scenarios form the basis for automated and manual test campaigns.
Defined KPIs may include Mean Time to Recovery (MTTR), average latency, allowable error rate, and security test coverage rate. Each KPI must have a critical threshold and an alert threshold.
Regular measurement of these indicators during development and in production ensures continuous compliance with NFRs and enables rapid detection of drift.
For example, a Swiss manufacturing SME documented an MTTR KPI of under 30 minutes for its supervision module. This definition highlighted the need for automated failovers and proactive alerts to reduce downtime and secure the production chain.
Integration in SRS and PRD
The Software Requirements Specification (SRS) consolidates all functional and nonfunctional requirements into a reference document for development and testing teams. NFRs appear in a dedicated section with their wording, acceptance criteria, and priority.
The Product Requirements Document (PRD) is aimed at product managers and defines the overall vision, objectives, and technical constraints. NFRs are often broken down into cross-cutting themes to inform the roadmap and risk management.
Traceability must link each NFR to one or more automated or manual tests. This traceability ensures full coverage and simplifies auditing during quality reviews and certifications.
Business Impact and Best Practices for Validation
Poorly defined NFRs can cause financial risks, service incidents, and dissatisfaction, while rigorous validation secures delivery and operation. Implementing review, testing, and alerting processes guarantees ongoing compliance with quality requirements.
Risks of Poorly Defined Requirements
When NFRs are vague or omitted, the technical team may underestimate required resources, leading to delays and substantial rework costs. Production incidents can then multiply.
Lack of measurable criteria exposes the project to divergent interpretations among stakeholders, making validation complex and often postponed. Documentation becomes incomplete and tests unreliable.
Insufficient monitoring in production can lead to undetected service degradation, impacting user satisfaction and the organization’s credibility with clients or partners.
Alignment with Business Objectives
For each NFR, specify its impact on return on investment, time-to-market, and user satisfaction. This alignment ensures that technical quality truly supports strategic business goals.
For example, an optimized response-time NFR can translate into increased conversion rates on an e-commerce platform or fewer support calls for an internal application.
Documenting the business impact of each criterion strengthens NFR prioritization in the roadmap and facilitates decision-making when balancing features and quality.
Validation and Testing Processes
Integrating NFRs into CI/CD pipelines enables non-regression, performance, and security tests with each release. This ensures every change meets defined service levels.
Code reviews and specialized audits (penetration tests, static analyses) complement these validations with in-depth technical expertise. They help anticipate security vulnerabilities and performance bottlenecks.
Setting up alerts and automated reports tracks production KPIs continuously. Teams can then trigger preventive or corrective actions before incidents affect users.
Leverage Nonfunctional Requirements as a Software Quality Driver
NFRs are essential to guarantee the performance, security, scalability, and maintainability of custom software. Their precise formulation, documentation in the SRS and PRD, and continuous validation through tests and KPIs secure delivery and operation.
By linking each quality criterion to business objectives, decision-makers align technical investments with ROI, time-to-market, and user satisfaction. This approach reduces risks, optimizes maintenance costs, and strengthens solution longevity.
Our Edana experts are available to assist you in defining, formalizing, and implementing your nonfunctional requirements from scoping to operation. Together, let’s build robust and scalable digital solutions perfectly aligned with your business challenges.
