A simple classification of discrete system interactions and some consequences for the solution of the interoperability puzzle

Abstract In this article, we introduce a classification of system interactions to guide the discourse on their interfaces and interoperability. It is based on a simple, but nevertheless complete classification of system interactions with respect to information transport and processing. Information transport can only be uni- or bidirectional and information processing is subclassified along the binary dimensions of state, determinism and synchronicity. For interactions with bidirectional information flow we are able to define a criterion for a layered structure of systems: we name a bidirectional interaction ”horizontal” if all interacting systems behave the same with respect to state, determinism and synchronicity and we name it “vertical” — providing a semantic direction — if there is a behavioral asymmetry between the interacting systems with respect to these properties. It is shown that horizontal interactions are essentially stateful, asynchronous and nondeterministic and are described by protocols. Vertical interactions are essentially top-down-usage, described by object models or operations, and bottom-up-observation, described by anonymous events. The interaction classification thereby helps to better understand the significant relationships that are created between interacting discrete systems by their interactions and guides us on how to talk about discrete systems, their interfaces and interoperability. To show its conceptual power, we apply the interaction classification to assess several other architectural models, communication technologies and so called software design or architectural styles like SOA and REST.