After implementing a Reactive REST GET endpoint with Quarkus / Mutiny using a callback structure and checking the variant with a blocking service i finally played with a CompletionStage / CompletableFuture API version;
How do i call a CompletionStage / CompletableFuture API service from my Reactive REST GET endpoint with Quarkus/Mutiny
Again it turned out to be quite simple (although the underlying idea may be more complex);
The ServiceResource just forwards the call to the Service.
MyRequestService creates a MyJsonResultCompletableFuture (CompletableFuture implements CompletionStage) and delivers this to the Mutiny Uni with method completionStage(). Another possibility would be using;
The resulting Uni is returned to the ServiceResource.
Finally MyJsonResultCompletableFuture blocks the call from MyReactiveServiceResource / MyRequestService waiting for a completionStage. Method ready() accomplish this stage and returns the MyJsonResult to Mutiny (acting like some kind of callback).