Functional vs Non-Functional

When Good Features Meet Reality

Picture this: You’ve just launched your mobile app. The feature set is perfect—users can create accounts, browse products, add items to a cart, and checkout. Your team celebrates. Then, the product goes viral. Fifty thousand users sign up in the first week.

By day three, the checkout page takes thirty seconds to load. By day five, users report that their carts are losing items randomly. By day six, you’ve got a crisis: the system works beautifully, but only for a few dozen concurrent users. Your engineering team is frantically scaling databases, adding caches, and rewriting database queries. You’re hemorrhaging users who can’t complete purchases.

What went wrong? You focused on what the system does, but not on how well it does it. You nailed the functional requirements but overlooked the non-functional ones. In this chapter, we’ll explore both types and show you why they’re equally critical to building systems that real people can actually use.

What Your System Does vs How Well It Does It

Let’s start with a clear distinction because it changes how you approach system design.

Functional requirements describe what your system does. They’re the features and behaviors users directly interact with. When you say “users should be able to upload a video,” that’s functional. When you specify “the checkout process must include a payment gateway integration,” that’s functional. Functional requirements are about the core business logic—what makes your product valuable.

Think of functional requirements as your product’s feature checklist. They answer questions like:

What actions can users perform?
What data does the system manage?
How do different features interact?
What are the business rules that govern behavior?

Non-functional requirements, by contrast, describe how well the system performs. They’re often called quality attributes. Non-functional requirements aren’t about what the system does; they’re about properties like speed, reliability, security, and how the system handles growth. “The system must handle 10,000 concurrent users without degrading performance” is non-functional. “The system must be 99.99% available” is non-functional. “All user data must be encrypted at rest and in transit” is non-functional.

Non-functional requirements fall into several categories, and understanding each is crucial:

Performance refers to response time and throughput. How fast should the system respond to user requests? How many transactions per second can it handle? A video streaming platform needs different performance characteristics than a tax filing system—one optimizes for low latency, the other can tolerate slight delays if it processes more volume.

Scalability is about growth. As your user base grows from 1,000 to 1 million, does the system maintain the same performance? Non-functional requirements specify how the system should scale: “Linear scaling up to 100 million users” or “The system should support 1000x growth without architectural changes.”

Availability addresses reliability. How often is the system up and running? Requirements like “99.99% uptime” (about 52 minutes of downtime per year) demand very different architecture than “95% uptime” (about 18 hours per year). Financial systems and healthcare platforms need higher availability than social networks, where brief outages are merely annoying.

Security protects data and users. This includes authentication (proving who you are), authorization (what you can do), encryption, and protection against attacks. Non-functional security requirements might specify “All passwords must be hashed with bcrypt,” “API calls must use OAuth 2.0,” or “The system must be HIPAA-compliant.”

Maintainability enables long-term success. Can your team understand the code? Is it easy to deploy changes? Non-functional requirements here might include “Deployment should take less than 5 minutes,” “Onboarding a new engineer should take less than one week,” or “Code must maintain >80% test coverage.”

User experience goes beyond raw performance. “Page load time must feel instant” is vague, but “95th percentile page load must be under 2 seconds” is measurable and concrete. Non-functional requirements about UX are often the most business-critical.

Pro tip: The best way to distinguish them is to ask, “Does this directly provide value to the user’s business goal?” If yes, it’s likely functional. If it’s about how reliably, quickly, or securely that value is delivered, it’s non-functional.

A House Is Not Just Four Walls

Let’s use a concrete analogy. Imagine you’re building a house.

Functional requirements are the rooms and layout: “We need a kitchen, three bedrooms, two bathrooms, and a living room. The kitchen should have space for a dining table.” These define what your house does—where you cook, sleep, and gather.

Non-functional requirements are everything else: earthquake-proofing, insulation for the climate you live in, plumbing and electrical systems rated for the load you’ll place on them, and a roof that won’t leak. These don’t add new functionality—you can’t “use” earthquake-proofing in the way you use a bathroom—but they determine whether the house is actually livable and lasts decades instead of years.

Just as a house needs both the rooms and the structural integrity, a software system needs both features and quality attributes. Skip the insulation and your heating bill becomes astronomical. Skip non-functional requirements and your system collapses under real-world conditions.

Gathering Requirements and Turning Them Into Architecture

Now let’s talk about how to actually work with these requirements in practice.

Documenting Requirements

When you gather functional requirements, you typically create a feature list or use case document. A use case might read: “A user can view their purchase history. When they visit the ‘My Orders’ page, they see a list of all past orders sorted by date, with order status, total amount, and a link to view details.” This is concrete and testable.

For non-functional requirements, you need to be similarly specific. Vague requirements like “the system should be fast” or “it should be reliable” are useless. Instead, you write measurable specifications:

“95th percentile latency for the checkout page must be less than 500ms”
“System must maintain 99.95% availability over a rolling 30-day window”
“All user passwords must be hashed using bcrypt with a minimum cost factor of 10”
“The system must support 100,000 requests per second at peak load”

Here’s a simple template for documenting non-functional requirements:

Requirement	Specification	Rationale
Response Time	95th percentile under 500ms	Users abandon carts if checkout takes over 1 sec
Availability	99.95% uptime	Peak revenue hours are 7-9pm EST; 5 min/week downtime is acceptable
Data Durability	99.999999%	Financial records must never be lost
Scalability	Linear to 10M users	Projected growth over 5 years
Security	OAuth 2.0 + TLS 1.2 minimum	Industry standard; required for payment card compliance

How Non-Functional Requirements Shape Architecture

Here’s the critical insight: non-functional requirements force architectural decisions. They’re not nice-to-haves—they fundamentally shape what you build.

Let’s trace an example. Suppose your requirement is “99.95% availability.” What does that mean architecturally?

Single server: If you run on one server, that server has maybe 99.5% uptime if we’re generous. You fail the requirement.
Two servers with failover: When one fails, the other takes over. This gets you to ~99.95%. But you need health checks, load balancers, and orchestration. Your architecture just got 3x more complex.
Geographic redundancy: For 99.99%, you need multiple data centers in different regions. Network calls between them introduce latency. Your database now needs replication. Your infrastructure costs spike.

See how “99.95% availability” immediately dictates that you cannot use a single server? The non-functional requirement has forced an entire architectural pattern.

Here’s another example: “Handle 10,000 concurrent users.” What does that imply?

You can’t use an in-memory session store on one server (it’ll run out of RAM).
You need a distributed session store like Redis.
Your database queries must be optimized; you can’t afford slow queries multiplied by 10,000 users.
You probably need caching layers.

Again, a single non-functional requirement cascades into multiple architectural decisions.

graph LR
    NFR["Non-Functional Requirement<br/>10,000 concurrent users"] --> A["Need Load Balancer"]
    NFR --> B["Need Distributed Cache<br/>Redis/Memcached"]
    NFR --> C["Need Database Optimization<br/>Indexing, Query Tuning"]
    NFR --> D["Need Monitoring & Observability<br/>Detect Bottlenecks"]
    A --> E["Distributed Architecture"]
    B --> E
    C --> E
    D --> E
    E --> F["Scalable System"]

Documentation vs. Discovery

In practice, you don’t always have all requirements upfront. You discover them through conversations with stakeholders, analyzing competitor products, and prototyping. A good process looks like:

Identify functional requirements through user interviews and product specs.
Ask “how well?” questions to uncover non-functional requirements:
- How many concurrent users do we expect?
- What’s acceptable latency for this feature?
- How often is downtime tolerable?
- What data is sensitive and needs encryption?
Challenge assumptions: “Is 99.99% availability necessary, or is 99% acceptable and cheaper?”
Write them down in a testable, measurable format.

Real Systems: E-Commerce Checkout

Let’s walk through a real example: an e-commerce checkout system. On the surface, the requirements seem simple. But functional and non-functional requirements diverge significantly, and each shapes the design.

Functional Requirements:

User selects items to purchase
User enters shipping address
User enters payment information
System validates address and payment
System creates an order and sends confirmation
User receives order confirmation email

Non-Functional Requirements:

Attribute	Requirement	Reasoning
Response Time	Payment validation under 500ms	Users abandon if they see a spinning wheel
Availability	99.99% during peak hours (6pm-11pm)	Peak revenue times; downtime = lost sales
Data Durability	99.9999999% on orders	A lost order = fraudulent charge or refund—worst case scenario
Security	PCI-DSS Level 1 compliance	Handles credit cards; federal requirement
Scalability	50,000 checkout requests/second	Black Friday peak load
Throughput	Process orders in under 1 second end-to-end	Prevent transaction timeouts

Notice how the functional requirements could be built on a single server. A developer might prototype the entire checkout flow in a day on their laptop. But the non-functional requirements? They demand:

Payment gateway integration with redundancy (what if the primary gateway is slow?)
Caching to speed up address validation
Database replication to handle write throughput
Load balancing across multiple checkout servers
SSL/TLS encryption for all communication
PCI-DSS audit of infrastructure and code
Fraud detection algorithms running in real-time
Circuit breakers that fail gracefully if payment services are down
Monitoring and alerting to detect and respond to problems

A working prototype and a production-grade system serving Black Friday traffic are fundamentally different beasts, and it’s the non-functional requirements that force the difference.

When Requirements Collide

In real projects, you’ll often face conflicts. A customer wants both “complete feature set” and “deploy to five regions for redundancy” and “launch in three months.” You can’t have all three.

When functional and non-functional requirements conflict, the hierarchy usually looks like:

Business requirements (we need profitability or we die)
Regulatory requirements (security, privacy, compliance—these are non-negotiable)
User experience requirements (the core reason people use the product)
Performance and scalability (important but sometimes can be added later)
Developer experience and maintainability (nice to have but often sacrificed under pressure)

A common mistake is implementing every feature before realizing the system can’t scale. You spend months building features that impress in demos but crumble under real user load. A better approach: define your non-functional requirements first, then build features within those constraints.

Did you know? Many startups ship features without nailing non-functional requirements, then hit a “scaling wall” around Series A. They have great products that work for thousands of users but fail for millions. Rebuilding the architecture takes months and delays everything else. Get non-functional requirements right early and you’ll save yourself months of pain.

Key Takeaways

Functional requirements describe what the system does (features, business logic, user interactions). Non-functional requirements describe how well it does it (performance, reliability, security, scalability).
Non-functional requirements force architecture decisions. A requirement like “99.99% uptime” or “10 million concurrent users” immediately rules out simple designs and demands complex infrastructure.
Non-functional requirements must be measurable. “Fast” is useless. “95th percentile latency under 500ms” is actionable and testable.
Get non-functional requirements early. Discovering them late means rearchitecting your system—an expensive, demoralizing process.
Prioritize ruthlessly. You can’t optimize for every dimension. Choose which non-functional attributes matter most for your business.
Non-functional requirements are not add-ons. They’re foundational design constraints, as important as the features themselves.

Practice Scenarios

Social Media Feed: Write both functional and non-functional requirements for a Twitter-like feed system. Assume 100 million monthly active users. What’s different if you only expect 1 million users?
Internal Admin Tool: Your company builds an internal admin tool used by 50 employees to manage customer data. What non-functional requirements can you relax compared to a public-facing social media platform? Why?
Conflict Resolution: Your product manager wants to launch with 20 features by month three. Your infrastructure team says supporting 99.99% availability across all features requires a six-month build. How do you resolve this? What would you cut, and why?

From Requirements to Scale

You now understand the two types of requirements and why they both matter. But here’s the reality: as your system grows, non-functional requirements become harder to meet. A feature that works for 1,000 users may not work for 100,000 without rethinking.

In the next chapter, “Understanding Scale,” we’ll explore what happens when your user base grows and how to design systems that scale gracefully. You’ll learn why some systems crumble under load while others handle 100x growth with minimal changes. The answer lies in understanding the constraints of hardware, networks, and databases—and how to overcome them.

The foundation you’re building here—understanding functional and non-functional requirements—will guide every architectural decision ahead.