Architecture and design

Vocabulary

English	Meaning
layers/ˈleɪəz/	Software levels separating concerns such as presentation, domain logic and storage.
dependency rule	In clean architecture, the rule that source-code dependencies point toward inner policies rather than outer mechanisms.
coupling	The degree of dependency between software components.
cohesion/kəʊˈhiːʒn/	How closely related the responsibilities within a software component are.

Layers pointing inward

BookShare in three layers: presentation (web app), domain (loans, members, catalog), storage (database). The dependency rule: dependencies point inward — the domain knows no framework, and that is what makes it testable.

{exercise:uni-cs1202-layers}

Practice

The dependency rule points —

Difficulty 1/5 · 100 coins + a small bonus on first completion today

Cohesion keeps related change in one module; coupling spreads it. The loan-limit rule change: 1 module, 2 files (high cohesion, low coupling). The same rule smeared across presentation: 3 layers, 5 files. The count IS the design metric.
The borrow sequence: presentation calls domain.borrow(member, copy); the domain checks limit and status; storage records with dates; presentation confirms — four participants, five labelled messages.

{exercise:uni-cs1202-counting}

Practice

The loan-rule change touches 1 module, 2 files. The smeared version touches —

Difficulty 1/5 · 100 coins + a small bonus on first completion today

Reminder delivery is a strategy behind a Notifier interface: email today, push tomorrow — swapping costs one configuration line, zero domain edits.
The domain importing the web framework inverts the rule and becomes untestable; diagrams as decoration (unlabelled arrows) review nothing.

{exercise:uni-cs1202-boundary}

Practice

Swapping the Notifier strategy costs —

Difficulty 1/5 · 100 coins + a small bonus on first completion today