Boundaries

Boundaries in monolithic applications have been extensively discussed in many texts. In general, we can observe a vertical alignment in these systems:

Usually, one can observe a modular approach in order to achieve the implementation detail isolation, where every module will only export interfaces, meant to be consumed by other modules of the same system. This is what the contract would be in this case.

A monolithic application can be broken down in different modules, that can reside inside of the same binary, or in shared libraries. A monolithic application can also consume third party shared libraries by being linked against them. This conforms a pattern on boundary separation and code reusability.

API and ABI stability in this context is what matters the most for other modules that are consuming a certain module from third party libraries. There are many examples of this, but some would be the Qt Framework, Glib or Gtk.