Testing component interactions with contract tests

We have chosen to create our application using the modular monolith pattern. Here, we have each module communicate with the others using either gRPC or by publishing messages into a message broker. These would be very common to see on any distributed application but would be rare or not used at all on a monolith or small application. What’s more common to see across all applications is the REST API we use. This demonstration application does not have any true UI, but we have the API to support one. This API represents a third form of communication in our application, which is between an API provider and the API consumer.

We could test these interactions using integration tests since the definition of what an integration test covers is testing the interactions or integration between two components. However, the integration tests we wrote before tested smaller components, and the scope for the system under test was not very...