Question 2 of 28: Functional and Non-Functional Requirements
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, May 2016. 3 hours, closed book exam with one aid sheet allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any five (5) questions in Part I, any three (3) in Part II, any four (4) in Part III, any two (2) in Part IV, and any five (5) in Part V — only the first questions answered, in each part, as they appear in the answer book are marked. All questions carry equal weight, so the 19 questions actually marked (5+3+4+2+5 of 28) each count for 100/19 ≈ 5.26% of the paper. All 28 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, agile methods, design principles, dependability; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process and quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — creational/structural/behavioural pattern catalogue (Singleton, Proxy, Template Method, Observer, etc.); Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/invariants, the open–closed principle; Barbara Liskov's 1987 substitutability paper for Question 11; Rogers, Sharp & Preece, Interaction Design, and Nielsen, Usability Engineering, for Question 21's HMI-specific non-functional requirements; Myers, The Art of Software Testing, for Question 28's boundary value analysis.
PART I — General Principles (answer any 5 of 7)
Question 2: Functional and Non-Functional Requirements (Part I)
A functional requirement (FR) is a statement of a service the system must provide, or of how the system must behave in response to particular inputs — it describes something the system does. Example: "The system shall allow a registered user to search for a book by ISBN and display the title, author, and current availability."
(b) Non-Functional Requirement
A non-functional requirement (NFR) is a constraint on the services the system offers, or on the system as a whole, rather than a service in its own right — it constrains how well, how fast, how securely, or under what conditions functions must operate. Example: "The search function shall return results within 2 seconds for 95% of queries when up to 200 users are searching concurrently," or "The system shall be available at least 99.9% of the time, measured over a rolling 30-day window."
An important asymmetry between the two: failing a single functional requirement typically makes one feature unavailable, but failing a pervasive non-functional requirement (unacceptable response time, unacceptable security) can make an otherwise functionally complete system unusable or unacceptable outright — which is why NFRs, though fewer in number, often dominate acceptance testing and architectural decision-making (elaborated further in Question 3).
