Designing for Non-Functional Requirements
Introduction
When designing a system, it's important to consider both the functional and non-functional requirements. Functional requirements are the features that the system must have to meet the needs of the users. Non-functional requirements are constraints on the system that it must operate within. Things like performance, scalability, security, and maintainability are all non-functional requirements.
While functional requirements are often straightforward to isolate and implement incrementally, non-functional requirements are broader in scope and can have a profound impact on the system’s overall architecture.
"Inside every non-functional requirement, is a feature that matters to users."
-- Dave Farley
Non-functional requirements frequently affect every layer of a system. Because of their systemic nature, they must be considered early in the design process. Ignoring them can lead to architecture that is fragile, inflexible, or costly to scale or secure, ultimately requiring significant and expensive rework.
- Scalability may influence your decision to use stateless services, distributed storage, or asynchronous processing.
- Security may require specific access control boundaries, encryption strategies, or network segmentation.
- Maintainability might demand better modularity, documentation practices, or automated testing frameworks.
Failing to meet a critical non-functional requirement can undermine the entire system, regardless of how well it delivers on functional expectations. For example, if a system performs all the right tasks but cannot scale to handle user load, it will fail in practice.
Considering non-functional requirements at the beginning of a project allows us to establish a solid foundation for the system that can adapt to future needs. They are the main drivers of our architecture decisions and must be understood and prioritized from the outset.
The term "non-functional requirements" is a bit of a misnomer. These are better thought of as the constraints that the system must operate within. They are not "non-functional" in the sense that they don't do anything, but rather that they are not directly related to the system's primary function.
Functional vs Non-Functional Requirements
Functional requirements are the features that the system must have to meet the needs of the users. These are the things that the system must do in order for it to provide value. Non-functional requirements are constraints on the system that it must operate within. Things like performance, scalability, security, and maintainability are all non-functional requirements.
Typical functional requirements are atomic and can be implemented independently. Non-functional requirements are often interdependent and can require changes to the entire system to meet. For example, if you need to scale your system to meet demand, you may need to change the architecture of the system to support that scale. This can have a cascading effect on other non-functional requirements like performance and security.
Identifying Non-Functional Requirements
While functional requirements are usually straightforward to gather through conversations with users and stakeholders, non-functional requirements are often more elusive. These requirements aren’t about what the system does, but how it does it. Because of this, they are frequently implied rather than stated outright, and identifying them requires careful analysis and informed questioning.
One of the challenges in uncovering non-functional requirements is that stakeholders often aren’t equipped to express them in concrete terms. They may say things like “the system should be fast” or “we want it to be reliable,” but these statements lack the specificity needed for design and implementation. What does “fast” mean in context? Is one second acceptable? Half a second? And when they say “reliable,” do they mean 99% uptime, 99.99%, or something else entirely? Each level of reliability carries dramatically different cost and complexity implications.
It’s also common for stakeholders to default to idealistic answers such as “as fast as possible,” or “never go down,” without fully understanding the trade-offs involved. As a developer or architect, part of your job is to guide these conversations toward more realistic, measurable expectations. This may involve translating vague desires into concrete metrics (e.g., response time under 300ms for 95% of requests), and helping stakeholders understand how choices around performance, cost, and complexity are inherently linked.
When identifying non-functional requirements:
- Ask open-ended questions, but push for specifics.
- Use examples or historical performance baselines to anchor expectations.
- Document assumptions and constraints clearly.
- Clarify which priorities matter more: speed, reliability, scalability, or cost?
Being proactive in identifying and articulating non-functional requirements is essential for building a system that not only works but works well in the context in which it will operate.
Architectural Implications
Meeting non-functional demands, such as performance, reliability, security, scalability, and maintainability, often requires architectural decisions that fundamentally shape how a system is structured and how its components interact.
For example:
- To handle high traffic volumes, you might need load balancing, asynchronous workflows, or a distributed architecture.
- For high reliability, you’ll likely introduce redundancy, failover mechanisms, and active monitoring.
- Security and compliance needs can dictate approaches to authentication, encryption, and how data is stored or accessed.
These considerations influence everything from module boundaries and communication patterns to the choice of frameworks and deployment strategies. Overlooking them early in the design process can lead to expensive rework or architectural overhauls later on.
In legacy systems, it's just as important to understand the non-functional expectations that shaped the original design. Many older systems were built under very different assumptions, such as limited scale, minimal uptime requirements, or less stringent security needs. Imposing modern constraints on such systems without accounting for their design history often results in brittle code or unreliable behavior.
Non-functional requirements tend to remain invisible until they’re violated. Whether you’re designing something new or modifying what already exists, surfacing and respecting these needs is key to creating systems that are not only functional, but robust, adaptable, and sustainable.
Trade-offs and Prioritization
In an ideal world, every system we build would be perfectly secure, highly performant, and infinitely scalable. However, in the real world, we must make trade-offs between these non-functional requirements based on the system's context, budget, and timeline. Optimizing a system for security may introduce latency or hamper availability. Prioritizing performance may lead to compromises in maintainability or security.
In each system, we must evaluate the relative importance of each non-functional requirement and make informed decisions about where to invest our resources. This often involves trade-offs that require careful consideration of the system's goals and the needs of its users.
Not all non-functional requirements are equally important in every system. For example:
- A real-time game prioritizes performance and latency.
- A healthcare system prioritizes security and data integrity.
- A startup prototype may prioritize speed of delivery and changeability over scalability.
Common Trade-offs
Non-functional requirements frequently conflict, where improving one may come at the cost of another. Some of the most common trade-offs include:
- Performance vs. Security: Adding encryption and security checks can slow down performance.
- Scalability vs. Simplicity: A distributed architecture may scale better but introduces more complexity and failure modes.
- Maintainability vs. Time-to-Market: Investing in clean abstractions and testability may slow down initial delivery but pays off long-term.
- Availability vs. Consistency: In distributed systems, the CAP theorem forces you to choose between consistency and availability under network partition.
These trade-offs cannot be eliminated, only managed. Good architects recognize that every decision has a cost and optimize for what matters most to the system's goals and constraints.
Identifying the Most Important Non-Functional Requirements
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Understand the Domain: Different domains have different priorities. For example, financial systems prioritize security and data integrity, while gaming applications prioritize performance and responsiveness. Understanding the domain helps identify which non-functional requirements are most critical.
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Consider the Organization's Goals: Align non-functional requirements with the organization's strategic objectives. If the goal is to enter a new market quickly, speed and flexibility may take precedence over scalability. If the company is small and resource-constrained, affordability may be a top priority.
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Focus on What Matters to Stakeholders: Work with stakeholders to uncover the system qualities that are truly critical. Ask questions like, “What would make this system fail in production?” to surface real-world priorities.
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Turn Ambiguity into Metrics: Avoid vague goals like “fast” or “secure.” Define concrete, measurable criteria—such as response time thresholds, uptime percentages, or data protection standards—so expectations are clear and progress is trackable.
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Design for Uncertainty: When requirements are unclear or expected to evolve, aim for flexibility. Modular, loosely coupled designs make it easier to respond to new constraints without major rework.
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Reassess as the System Grows: What matters today may not be what matters tomorrow. As the system evolves, periodically revisit which qualities are most important to users and adjust priorities accordingly.
Every system makes trade-offs. but strong architecture makes those trade-offs consciously and to explicitly define them during the planning process. Prioritizing non-functional requirements based on real-world needs ensures that the system not only works, but works reliably, sustainably, and in line with what matters most to the people who depend on it.
Conclusion
Non-functional requirements play a critical role in shaping the architecture and behavior of a software system. While functional requirements define what a system does, non-functional requirements determine how well it performs under real-world conditions and affect everything from performance and scalability to security, reliability, and maintainability.
These requirements often drive key architectural decisions, such as technology choices, deployment strategies, data partitioning, and fault tolerance mechanisms. Ignoring them can lead to systems that technically work, but fail to meet user expectations or business goals.
By identifying and addressing non-functional requirements early in the design process, teams can create systems that are not only functionally correct but also robust, responsive, and sustainable. In short, designing with these constraints in mind is essential to building software that delivers long-term value.