Stable Dependencies Principle (SDP)

Clean Architecture 達人に学ぶソフトウェアの構造と設計

Depend in the direction of higher stability.

A component expected to change should not be depended on by other components that are hard to change.
Even a module designed to be easy to modify becomes hard to change if dependent on by other components.
If SDP is satisfied, modules intended to be easy to change will not be depended on by inflexible modules.
The instability I of a component should be greater than that of the components it depends on.
As you follow dependency direction in component structure, I should decrease.
In design, keep both highly stable and low-stable components, not just one side.

Stability metric

One method for measuring component stability is to examine how many components this component depends on, and how many depend on it.

Fan-In
- Incoming dependencies: the number of external components depending on classes in this component.
Fan-Out
- Outgoing dependencies: the number of external components that classes in this component depend on.
I (Instability)
- Instability.
- I = Fan-Out ÷ (Fan-In + Fan-Out)
- I = 0 is the most stable component
- I = 1 is the least stable component

To compute fan-in and fan-out, count the classes in the component and the classes in dependent external components.