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Architecture & System Design › Architecture Styles

Non-Functional Requirements

Requirements about how well a system works rather than what it does.

Backend Engineer · Mid-levelFrontend Engineer · Mid-levelData Engineer · Mid-level

Also known as: non-functional requirements, NFRs, quality requirements

Non-functional requirements (NFRs) specify how well a system must work rather than what it does: how fast (latency percentiles, not averages), how available (nines with a measurement window), how much load (throughput, concurrent users), how safe (recovery objectives), how secure, how maintainable. Without them, “fast” and “reliable” mean nothing testable — and architecture can’t be judged against wishes.

functional:   "users can reset passwords"
non-functional: "p99 reset-email delivery under 60s; 99.9% monthly availability"

Good NFRs are quantified, measured, and tied to consequences: a percentile plus a window plus what happens on breach. Vague NFRs (“system must be scalable”) produce arguments; quantified ones produce designs and fitness functions that enforce them.

The classic mistakes:

  • Writing unmeasurable NFRs. “Fast, scalable, secure” with no numbers is decoration. Every NFR needs a metric, a target and a measurement method.
  • Averages instead of percentiles. “Average response under 200ms” hides a miserable tail; p95/p99 describe what users actually feel.
  • No load numbers. Latency targets without concurrent-user and throughput figures can’t size anything. Pair every latency NFR with load.
  • Availability without a window. “99.9%” means nothing until you add “measured monthly” (43 minutes of budget) versus “yearly.”
  • Security and operability as afterthoughts. Recovery time, backup verification, auditability and patchability are NFRs too — discovered in the first incident if unwritten.
  • Conflicting NFRs unacknowledged. “Sub-50ms globally” and “single-region to save cost” can’t both hold; surfacing the conflict early forces a real decision.

How to use them: quantify early, measure continuously, and review like features — NFRs are the contract architecture is held to. See quality attributes for the vocabulary and SLOs for the operational form.