Request-Response APIs

At its core, request–response is a message exchange pattern in which a requestor sends a request message to a replier system. The replier system receives and processes the request, and if all goes well, it returns a message in response. While this exchange format works well for more structured requests, it limits integrations to those where the expectant system has a clear idea what it wants from the other. These request-response style APIs, therefore, must follow the interaction script from the calling service.

Pushpin Fanout - Request/Response vs Evented APIs