Quality Concerns Driving Different Design Decisions: 25-Comp-B11 Advanced Software Design, December 2019, Q5

Question 5 of 28: Quality Concerns Driving Different Design Decisions

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, December 2019. 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, design principles, testing, 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 and the "program to an interface, not an implementation" / "favor object composition over class inheritance" principles; Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, visibility, and multiple inheritance semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Karl Wiegers, Software Requirements (3rd ed.); Myers, The Art of Software Testing, for Question 6.

PART I — General Principles (answer any 5 of 7)

Question 5: Quality Concerns Driving Different Design Decisions (Part I)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Quality concern 1 — Performance. If minimizing response time is the dominant concern for a data-access layer, the design decision favours a tightly-coupled, direct approach: embed SQL queries close to the call site, cache frequently-read data in memory, and accept some duplication if it avoids an extra indirection layer at runtime.

Quality concern 2 — Maintainability. If ease of future change is the dominant concern for the same data-access layer, the design decision goes the other way: introduce a Repository or Data Access Object abstraction between business logic and the database, accept the small overhead of an extra method call, and keep SQL centralized behind one interface so the database technology can change, or the query can be unit-tested with a mock, without touching business logic.

The two decisions are not simply "good vs. bad" — they are the objectively correct call for their respective concern, and the same design problem yields opposite structures depending on which quality attribute is weighted most heavily. In practice a design typically trades several quality concerns off simultaneously and documents which one was prioritized and why (Question 3's "quantifiable" metrics are exactly what makes this trade-off an explicit, comparable decision rather than a guess).