It is analogous to a telephone call, in which the caller must wait for the recipient to pick up before anything can be discussed.
This is a simple but powerful messaging pattern which allows two applications to have a two-way conversation with one another over a channel; it is especially common in client–server architectures.
[1] Request–response pattern can be implemented synchronously ( such as web service calls over HTTP) or asynchronously.
[1] In contrast, one-way computer communication, which is like the push-to-talk or "barge in" feature found on some phones and two-way radios, sends a message without waiting for a response.
(Most of these systems use a "listen before talk" or other contention-based protocol so multiple sensors can transmit periodic updates without any pre-coordination.)