Functional vs Non-Functional
Behaviour versus quality of behaviour, and why only the second constrains structure.
4 to work through
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beginner Multiple choice
A marketplace ships saved-search alerts. The functional spec is one sentence - email a seller when a new listing matches their saved query - and it works. Six weeks after launch, alerts arrive anywhere between 4 minutes and 9 hours after the listing appears, and nobody can say whether that is a defect, because no one wrote down how fresh an alert had to be. Which of these is the missing non-functional requirement?
2 min answer -
beginner Multiple choice
A news organisation's site is functionally simple - editors publish articles and readers read them - yet its architecture is unusually elaborate. Which non-functional characteristic explains that, and what structure does it force?
2 min answer -
intermediate
"Interview. Etsy has millions of sellers listing handmade items and buyers browsing and searching them. Give me the functional requirements in thirty seconds, then tell me which non-functional requirement actually determines the architecture, and why." How should a strong candidate answer?
1 min answer -
intermediate Multiple choice
"Place an order" and "an order placed during the market open surge is acknowledged within 200ms at p99" — Zerodha needs both. Which one actually constrains the architecture, and why is that not obvious?
1 min answer
3 terms in this topic
Cross-Functional Requirement
A quality the system must exhibit across its features rather than a behaviour it must perform, so named because it cuts across all functionality.
conceptFunctional vs Non-Functional
Functional requirements say what the system does; non-functional requirements say how well, and only the second constrains structure.
practiceNon-Functional Budget
Splitting a system-level quality target into per-component allowances with named owners, so a change can be shown to have spent someone's allowance r…
Neighbouring topics
Architecture Fundamentals
General material on what solution architecture is and what an architect is accountable for.
Requirements to Constraints
Turning stated requirements into the constraints that actually bound a design.
Architectural Drivers
The small subset of requirements whose change would force the structure to change.
Quality Attributes
Availability, latency, throughput, security, cost — expressed as testable scenarios.
Architecture Principles
Durable agreed rules that rule options out, stated with rationale and implications.
Coupling
How much one component must know about, or change alongside, another.
Cohesion
Whether the things inside a boundary belong together and change for the same reason.
Modularity
Composing a system from parts that can be understood and replaced independently.
Separation of Concerns
Organising so that a change to one concern touches one place.
Abstraction & Encapsulation
Hiding mechanism behind contract, and protecting invariants by owning state.
Architecture Styles
System-level organising shapes, and how they differ from problem-level patterns.
Evolutionary Architecture
Designing for guided incremental change rather than for correctness on day one.
Fitness Functions
Automated checks that an architectural characteristic still holds.
Architecture Documentation
What to write down, at what altitude, and what nobody will ever read.
Conway's Law
Systems mirroring the communication structure of the organisation that builds them.
Trade-off Fundamentals
Why every architecture is a set of purchases, and how to state what you gave up.
Technical Constraints
Existing estate, skills, licences and platforms as inputs rather than obstacles.
Architecture Roles
Solution, enterprise, domain and platform architecture, and where each is accountable.
Reference Models
Shared conceptual frames — layering, tiers, viewpoints — and their limits.