I think it's akin to the original conception of OOP in Smalltalk, where objects are passed messages. (I've never written ObjectiveC, but I believe that's its paradigm as well.) In that, it's similar to the actor model used in Akka and in Erlang.
If you think of a service as being like a class -- as originally conceived by Smalltalk -- then sending it messages and acting on responses is the very definition of OOP, at least according to Alan Kay, the inventor of Smalltalk and of the term "object oriented programming."
If you think of a service as being like a class -- as originally conceived by Smalltalk -- then sending it messages and acting on responses is the very definition of OOP, at least according to Alan Kay, the inventor of Smalltalk and of the term "object oriented programming."
There's an interesting discussion of the definition of OOP here. https://wiki.c2.com/?AlanKaysDefinitionOfObjectOriented