This online resource is a unique way to frame a conceptual model for evented APIs.  Sam Curren and Phillip J. Windley discuss the fundamentals of evented APIs, how evented systems work, and a proposed protocol.

Events indicate something has happened. In this they differ from the request-response interaction style popular on the Web. Event-based systems are declarative whereas request-response systems are interrogatory. The difference between events (“this happened”) and requests (“will you do this?”) offers benefits in looser coupling of components as well as semantic encapsulation (see On Hierarchies and Networks for more detail).

APIs have become an economic imperative for many companies. But APIs based solely on request-response style interactions limit integrations to those where one system always knows what it wants from the other. The calling service must script the interaction and the APIs simply follow along.

We envision a world where applications integrate multiple products and services as equals based on event-driven interactions. Evented APIs, following the form described in this document, enable building such applications.