Stability

Stability is not directly related with frequency of change, instead, it is related to the amount of work required to make a change.

Many factors make a software component hard to change, for example, its size, complexity, and clarity, among other characteristics. One sure way to make a software comonent difficult to change, is to make lots of other software components depend on it.

A component wiwth lots of incoming dependencies is very stable because it requires a great deal of work to reconcile any changes with all the dependent components.

Component example

Suppose we have a component X, which is a stable component. Three components depend on X, so it has three good reasons not to change. We say that X is responsible to those three components. Conversely, X depends on nothing, so it has no external influence to make it change. We say it is independent.

Suppose on the contrary, we have a component Y, which is a very unstable component. No other components depend on Y, so we say that it is irresponsible. Y also has three components that it depends on, so changes may come from three external sources. We say that Y is dependent.