SomeService.java

@Service
public class SomeService {

  public Mono<ServerResponse> piu(Mono<String> body) {
    final SomeProcess someProcess = new SomeProcess();
    return ServerResponse
        .ok()
        .body(
            BodyInserters.fromPublisher(
                body
                    .doOnSuccess(s -> process(someProcess))
                    .switchIfEmpty(Mono.just("empty"))
                    .flatMap(s -> Mono.just("piu hello REQUEST_DATA=" + s)), String.class));
  }

  private void process(SomeProcess someProcess) {
    int i = 900_000;
    while (true) {
      System.out.println(someProcess.calc(i));
      if (--i == 0) {
        break;
      }
    }
  }
}