Coupling & Instability

Coupling measures how dependent classes are on each other. High coupling makes code rigid and fragile.

Afferent Coupling (Ca)

"Who uses me?" - The number of classes that depend on this class. - High Ca: This class is Critical or Responsible. - Examples: Core domain entities, Utility classes, Shared libraries. - Risk: If you change this class, you might break many things (high impact).

Efferent Coupling (Ce)

"Who do I use?" - The number of classes this class depends on. - High Ce: This class is Dependent. - Examples: Orchestrators, Facades. - Risk: This class is fragile because it breaks if any of its dependencies change.

Instability (I)

Instability is a ratio between 0 and 1 derived from coupling.

$$ I = \frac{Ce}{Ca + Ce} $$

0: Stable

I am used by many, but I use no one. - Hard to change because many depend on it. - Should be very robust and abstract. - Example: String, Integer, Core Interfaces.

1: Unstable

I use many, but nobody uses me. - Easy to change because nobody depends on it. - Can be concrete and volatile. - Example: Controllers, CLI Commands, Scripts.

The Stable Dependencies Principle (SDP)

Dependencies should point in the direction of stability. A component should only depend on components that are more stable than itself.

Unstable (Variable) $\rightarrow$ Stable (Abstract)