When designing scalable software systems, it’s easy to focus only on the core features, like data flow, architecture, databases, and APIs. But in real-world production systems, it’s often the non-functional requirements in system design that make or break your design. These are the behind-the-scenes characteristics that define how your system behaves, not just what it does.
In this guide, we’ll break down what are the non-functional requirements system design interviews expect you to address, how to talk about them, and how to bake them into your diagrams and discussion.
What are non-functional requirements?
Non-functional requirements (NFRs) are the qualities and constraints that define a system’s behavior beyond its specific functions. While functional requirements describe what the system should do (e.g., store user data, process orders), non-functional requirements describe how well it should do it.
These include areas like:
- Performance
- Scalability
- Availability
- Reliability
- Security
- Maintainability
- Observability
Think of them as the invisible guardrails that shape your system’s user experience, engineering complexity, and production stability.
Why non-functional requirements matter in system design
In interviews and in production, ignoring non-functional requirements for system design considerations is a red flag. Engineers who focus only on data models and APIs but overlook scalability or fault tolerance often miss what makes systems work at scale.
Here’s why non-functional requirements matter:
- They reflect real-world constraints: Any system can look good in theory. NFRs ensure it survives reality.
- They reveal your engineering maturity: How you approach consistency, failover, and observability signals senior-level thinking.
- They directly affect user experience: Downtime, slowness, or poor error handling are often related to neglected NFRs.
- They shape trade-offs and priorities: You often can’t optimize for all NFRs at once. Choosing between them shows how you handle complexity.
The best candidates call out NFRs before they’re asked. They show proactive awareness of performance, availability zones, rate limits, and system monitoring, which are exactly the kinds of concerns production teams face every day.
7 key non-functional requirements system design interviews care about
In system design interview questions, your ability to talk about non-functional requirements is often what separates intermediate engineers from senior-level candidates. These requirements ensure your system doesn’t just function, but that it scales, performs reliably, and provides a seamless user experience even under unpredictable conditions.
Let’s break down the most essential non-functional requirements system design interviews expect you to cover. For each one, you’ll get a clear definition, discussion points, and examples of how to integrate it into your design answers.
1. Performance
What it means: Performance refers to how quickly your system responds to user actions, internal requests, or background operations. It is one of the most critical non-functional requirements in system design because it directly impacts the user experience and infrastructure efficiency.
In system design interviews, highlight:
- Target latencies (e.g., 200ms response time for 95% of requests)
- System throughput (requests per second your backend can handle)
- Techniques like indexing, denormalization, and read replicas to optimize performance
Where to apply this:
If you’re designing an API for fetching personalized recommendations, mention that response time must stay under a specific threshold (e.g., 100ms) even under load. Describe how you’d use caching or a precomputed data layer to meet that performance requirement.
2. Scalability
What it means: Scalability is your system’s ability to handle increasing amounts of traffic, data, or users without a significant drop in performance or reliability. It is central to non-functional requirements system design discussions because growth is a given in real-world applications.
In interviews, explain:
- How you will scale the system horizontally (adding more servers)
- How you might shard or partition data for scale
- How stateless services allow easier horizontal scaling
Where to apply this:
In designing an event ticketing system, describe how you’d distribute incoming traffic across multiple app servers, use a CDN for static assets, and horizontally scale your database by sharding event data by region.
3. Availability
What it means: Availability measures how often your system is up and running as expected. It’s a cornerstone of non-functional requirements system design discussions, especially for mission-critical systems.
Make sure to cover:
- Uptime targets (e.g., 99.9%, 99.99%)
- Redundancy strategies (e.g., active-active, failover)
- Health checks, heartbeats, and graceful degradation
Where to apply this:
If you’re designing an online payments system, discuss strategies for multi-region failover, real-time monitoring, and fallback logic that keeps the core transaction path live even if secondary features fail.
4. Reliability
What it means: Reliability refers to your system’s ability to operate correctly over time, even in the presence of hardware failures, spikes in usage, or network issues. In interviews, it’s not enough to describe the happy path. Reliable system design means handling the edge cases, too.
Make sure to address:
- Retry logic, backoff strategies, and circuit breakers
- Idempotency in operations (especially POST and PUT)
- Data integrity guarantees and consistency mechanisms
Where to apply this:
When designing a messaging platform, you could explain how messages are persisted in a queue, retried on failure, and deduplicated on the client side to prevent duplicate deliveries.
5. Security
What it means: Security ensures your system resists unauthorized access, data breaches, and attacks like XSS or SQL injection. It’s often overlooked in system design interviews, but mentioning it demonstrates maturity and awareness.
What to bring up:
- Authentication (OAuth, JWT, session tokens)
- Authorization (RBAC, ACLs)
- Encryption at rest and in transit (HTTPS, TLS, KMS)
- Secure storage of credentials and tokens
Where to apply this:
In a design for a healthcare platform or banking app, emphasize encryption policies, multi-factor authentication, and principles like least privilege access.
6. Maintainability
What it means: Maintainability is about how easily engineers can understand, update, debug, or extend a system over time. This is an essential yet under-discussed part of non-functional requirements for system design that shows you think about long-term sustainability, not just launch-day delivery.
Topics to highlight:
- Clear service boundaries and APIs
- Modular design patterns (e.g., microservices, layered architecture)
- CI/CD pipelines and rollback mechanisms
Where to apply this:
In designing a content management system, show how each module (auth, editor, asset management) is loosely coupled and independently deployable.
7. Observability
What it means: Observability is your system’s ability to provide insight into its current state. When systems fail or slow down, observability tools help teams diagnose and fix the issue quickly.
What to include:
- Logging (structured, searchable)
- Metrics (request counts, latency, error rates)
- Distributed tracing (e.g., OpenTelemetry, Jaeger)
- Real-time dashboards and alerts
Where to apply this:
For a streaming platform, you might mention logging request latencies across edge nodes, alerting on error rate spikes, and tracing end-to-end video delivery paths.
How to include non-functional requirements in your system design interview answers
Here are a few strategies to naturally include non-functional requirements for system design elements in your interviews:
- Address them up front: After outlining your high-level design, proactively mention which NFRs you’ll prioritize (e.g., “Since this is a real-time system, latency and availability will be key”).
- Mark them on your diagram: Label your system design diagrams directly with labels like “multi-AZ,” “replica set,” or “cache hit ratio.”
- Tie NFRs to product goals: Frame them in terms of user experience or business impact. For example, “If this fails, users can’t log in. So let’s make auth highly available.”
- Discuss trade-offs: Recognize when optimizing for one NFR may harm another (e.g., adding encryption might slightly increase latency).
The more fluently you integrate NFRs into your thinking, the more senior and production-aware you’ll come across.
Final word
Understanding how to design a system is only part of the story. The real skill lies in ensuring that the system is scalable, secure, resilient, and observable in the real world. That’s where non-functional requirements in system design thinking become your superpower.
If you want to see how strong system designs balance performance, availability, and trade-offs in real interview scenarios, Grokking the System Design Interview offers examples that walk through exactly that, step by step.
