A non-functional requirement describes a quality target or a constraint for a system. In other words, it defines how well the system must perform or which limits it must respect. For example, it can address reliability, security, scalability, usability, or performance. Therefore, it often sets measurable expectations such as response time, throughput, and resource use.
Functional requirements describe what the system must do. By contrast, non-functional requirements describe how the system must behave or which boundaries apply. For example, a functional requirement may ask for a login feature. However, a non-functional requirement may state that the login completes in less than three seconds. Thus, the distinction becomes clear.
Common categories help structure the topic. First, quality requirements cover measurable concerns such as performance, availability, and security. Second, constraints limit the solution space. For example, a constraint can mandate a technology, a legal rule, or a deployment schedule. Third, interface and environment requirements define where and how the system must operate, such as compatibility with a specific API or hardware platform.
Solution dependence matters. First, goals stay solution-independent, such as “Provide secure access for all users.” Second, process expectations may hint at an approach, such as requiring an audit trail for all transactions. Third, constraints can steer architecture choices, such as demanding a specific database engine. Therefore, the text should make clear which statements influence design decisions.
Measurability turns non-functional requirements into verifiable commitments. For example, a statement like “Handle 5,000 concurrent users with 99.9% uptime” enables objective checks. Furthermore, test criteria such as load tests and acceptance criteria support validation. Thus, stakeholders can verify compliance. Moreover, vague words like “fast” or “secure” need metrics to avoid misunderstandings.
Standards and organizations classify requirement types in different ways. Some models separate business, user, and product requirements, while others use different families. However, the core idea stays the same: teams must define a consistent requirements landscape. Therefore, it helps to choose a type classification, define solution-dependence levels, set abstraction rules for artifacts, and select clear presentation forms.
Practical examples make the concept concrete. For example, a security requirement can state, “Encrypt personal data in transit and at rest using AES-256.” A performance requirement can state, “Display search results within 2 seconds for 95% of queries.” A legal constraint can state, “Store EU user data within EU data centers.”
Non-functional requirements shape architecture and guide design decisions. Moreover, they protect quality and compliance across the system lifecycle. Therefore, they should be clear, measurable, and linked to tests.