There are different definitions of coupling types in software development, and each of these has a different perspective. There is one shared concept among the definitions though; Coupling in software is about the dependency relationship of modules. This leads us to the generic definition of coupling; “Coupling is the degree of interdependence between software modules …“[1]. Granted that, anyone who has dealt with coupling must have heard the widely known statement that it’s crucial to seek low coupling, and high cohesion between software modules to get a well structured, reliable, easy-to-change software. However, how can we know that our software design has the correct level of coupling? In order to answer this, first, we are going to revisit afferent coupling, efferent coupling concepts in this article. Secondly, we are going to explain the instability index concept, which relates these metrics to each other.
Afferent and Efferent Coupling as the Metrics of Coupling
A software quanta, -a module, a class, a component- is considered to be more stable if they have loose coupling to the other quanta in the system because it will remain undisturbed by the changes introduced to the others. The metrics afferent and efferent coupling, which were initially defined by Robert C. Martin in his books Agile Software Development, and Clean Architecture, help us understand the probability that a change in a software module will force a change in others. Similarly, they guide us to see the tendency of a module to be affected if an error in other modules of the system occurs.
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