27 September 2021

Efferent Coupling

Efferent Coupling definition

  • It is a code quality metric.
  • Efferent coupling is a coupling metric in software development.
    • It measures the number of data types a class knows about.
    • Efferent coupling is a coupling metric in software development, it measures the number of data types a class knows about.
    • This includes inheritance, interface implementation, parameter types, variable types, and exceptions.

Efferent Coupling purpose

  • This metric identifies the number of classes inside this component that depend on classes outside the component.

Outgoing dependencies (Fan-out)

  • Efferent (outgoing) couplings = Who do you depend on.
    • Measure of how many different packages are used by a specific package.
    • Efferent couplings signal outward.
    • This has also been referred to by Robert C. Martin as the Fan-out stability metric which in his book Clean Architecture he describes as Outgoing dependencies.

Efferent coupling calculation

  • This metric is often used to calculate instability of a component in software architecture
    • I= Fan-out / (Fan-in + Fan-out).
    • This metric has a range [0,1].
    • I = 0 is maximally stable while I = 1 is maximally unstable.
    • Instability of a class or a module is the ratio between its efferent coupling and total coupling.
    • Obviously, instability is a value between 0 and 1, inclusive.
    • Instability of zero means that the class or module is absolutely stable. It depends on nobody, everybody depend on it.
    • A large efferent coupling can indicate that a class is unfocused and may also indicate that it is unstable since it depends on the stability of all the types to which it is coupled.

Efferent Coupling practice

  • Efferent coupling measures the number of classes on which a given class depends and has the following characteristics :
    • Efferent Coupling determines the number of classes in other packages that the classes in a package depend upon.
    • Efferent Coupling is an indicator of the package’s dependence on externalities.
    • Classes with high efferent coupling will receive the effects of changes or defects in other classes.
  • Efferent Coupling includes inheritance, interface implementation, parameter types, variable types, and exceptions.
    • This has also been referred to by Robert C. Martin as the Fan-out stability metric which in his book Clean Architecture he describes as Outgoing dependencies.
    • The term “stable” itself indicates the desired quality of such class or module.
    • We want such class never to change, we want it to be absolutely stable, or otherwise we will have to maintain other classes that depend on it whenever we change it.
    • Instability equal to one means that the class or module is absolutely instable.
    • It depends on other classes and must be changed when other classes are changed.
    • On the other hand, changing this class means nothing to others.
    • That is why we call it “absolutely instable” – we can change it liberally and no harm will be done to other parts of the system.

Scrum and Metrics

The Scrum framework

More informations for the Scrum PSD certification here.

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