In this communication approach, each service client request is processed by one instance of a service. There are the following kinds of one-to-one interaction:

Notification: In this case, a Client sends a request to the Service; the Client does not wait for a reply from that Service

Request/async response: Here, a Client sends a request to a Service, which replies asynchronously; the client does not block the thread while waiting and it is designed with the assumption that the response may not immediately arrive

The following diagram demonstrates one-to-one asynchronous service communication:

As you can see in the preceding diagram, each service has only one instance and the services are communicating through the message broker queue.