What Are Non-Functional Requirements?
Non-functional requirements (NFRs) describe essential system characteristics and environmental conditions that a solution must meet to be effective, rather than focusing on specific functionalities. They define the qualities and operational expectations, influencing the overall user experience and ensuring the solution remains robust and secure.
Unlike functional requirements, which specify actions or tasks a system must perform, NFRs encapsulate broader qualities like performance, security, and usability that support compliance, scalability, and business continuity. Two systems may have identical functionalities, but their NFRs can set them apart by determining how effectively they operate under various conditions.
For example, while both a BMW and a Mini may function as cars, the quality attributes of each—such as durability, comfort, and handling—define the driving experience and differentiate them significantly.
Importance of Non-Functional Requirements in Compliance
Non-functional requirements are crucial in maintaining regulatory and organisational compliance, particularly in sectors like finance, healthcare, and public services where strict standards govern data security, reliability, and accessibility. Non-compliance with these requirements can lead to legal repercussions, financial penalties, and loss of user trust. By defining NFRs clearly, organisations can ensure that their solutions align with statutory regulations and industry standards, safeguard sensitive data, and mitigate potential security risks.
For instance, compliance-related NFRs might specify how user data should be encrypted, the permissible downtime for systems, or the recovery protocols for data breaches. Failure to meet these requirements could result in security vulnerabilities, breaches of privacy laws, or penalties for failing to meet regulatory standards. Non-functional requirements, therefore, play a foundational role in the operational integrity of systems, especially those subject to compliance requirements.
Main Categories of Non-Functional Requirements
Non-functional requirements span several core categories, each addressing distinct aspects of system performance and quality:
1. Performance
Performance requirements define the speed, efficiency, and responsiveness of the system. They ensure that the system meets user expectations even during high-usage periods or with increasing workloads.
- Transaction speed: Expected response time for key actions, such as login, data retrieval, and page loading.
- Scalability: Handling of increased transactions over time, expected growth in users, and geographical distribution of users.
- Business hours: Operating and peak hours, anticipated daily or seasonal changes in user load.
Performance metrics are critical for compliance with service-level agreements (SLAs), especially when downtime or slow response times could disrupt business operations.
2. Reliability and Recoverability
These requirements focus on system stability and resilience. Reliability involves maintaining consistent availability and data accuracy, while recoverability defines the system’s ability to restore functionality following a failure.
- System availability: Expected uptime, the impact of system downtime on business operations, and recovery protocols.
- Backlog handling: Impact on processing if systems are down and mechanisms to resume normal operations.
- End-of-cycle processing: Batch jobs and critical processing cycles at key intervals (end of day, week, month, etc.).
For compliance, systems must have robust disaster recovery and data integrity measures to prevent data loss, ensure business continuity, and support regulatory mandates for data availability.
3. Security
Security NFRs safeguard data and applications from unauthorised access, cyber threats, and data breaches. These are critical in ensuring compliance with data protection laws and internal security standards.
- Access control: Limit access to sensitive data and functions based on user roles.
- Data integrity: Protect data from unauthorised changes, accidental deletion, or tampering.
- Compliance with regulations: Adhere to data privacy and protection laws, like GDPR, which mandate specific security measures.
NFRs for security help prevent data breaches, which are often subject to regulatory scrutiny and can incur heavy fines and reputational damage.
4. Usability
Usability requirements define how intuitive and accessible the system is for end-users. They enhance user experience and ensure that system design aligns with user expectations.
- Ease of learning and use: System design, user manuals, and help facilities.
- Consistency: Align the look and feel with other corporate applications.
- Accessibility: Adherence to accessibility standards to support users with disabilities.
Well-defined usability requirements support compliance with accessibility laws, such as the Equality Act 2010 in the UK, which mandates accessible design for digital services.
5. Interoperability
Interoperability requirements specify the system’s ability to operate across various platforms and integrate with other systems.
- Device compatibility: Support across different devices, operating systems, and browsers.
- Infrastructure compatibility: Compatibility with specific hardware, virtual environments, and cloud services.
- Data integration: Ability to connect and interact with legacy systems or third-party services.
Interoperability ensures seamless system integration, particularly in environments where multiple platforms must work in tandem without compromising security or functionality.
6. Data Migration
Data migration NFRs guide the transfer of data from existing systems to the new system, ensuring data integrity and continuity.
- Data accuracy: Ensure data accuracy and completeness after migration.
- Migration timeline: Set specific timelines for data cutover and recovery.
- Impact of migration failures: Define acceptable downtime and contingency measures in case of failed migration.
Data migration requirements are particularly important for organisations handling large volumes of sensitive data, where data loss or inaccuracies could breach compliance requirements.
Eliciting Non-Functional Requirements
Developing non-functional requirements involves gathering input from diverse stakeholders to address technical and operational concerns:
- Stakeholder Goals and Concerns: Understand what qualities stakeholders prioritise, such as security and ease of use.
- Legacy System Constraints: Review the limitations of existing systems and how the new system should integrate or overcome these.
- Industry Trends: Analyse market trends and competitive requirements for benchmarks.
- Standard NFR Templates: Use templates to ensure all categories of non-functional requirements are covered.
Key Stakeholders in the NFR Process
Consulting a wide range of stakeholders can improve the accuracy and comprehensiveness of non-functional requirements:
- Business SMEs for alignment with business requirements
- IT and Cybersecurity Specialists for system security and compliance
- Solutions Architects for integration and infrastructure alignment
Non-Functional Requirements and Architectural Principles
When defining NFRs, organisations may also adopt architectural principles, such as:
- Avoid Customisation: Minimise customisation to reduce complexity and improve compatibility with updates.
- Adopt Best Practices: Utilise industry best practices and standards to align with regulatory and operational expectations.
- Ensure Security and Compliance: Integrate compliance with regional data protection laws into the core system design.
Conclusion
Non-functional requirements play a vital role in building robust, compliant systems that meet both operational and regulatory standards. By carefully defining and meeting these requirements, organisations can ensure system effectiveness, enhance user experience, and prevent compliance risks, thereby securing long-term value and trust in their digital solutions.