code: public ServiceCall<NotUsed, NotUsed> audit() {
          //#subscribe-to-topic
          helloService.greetingsTopic()
              .subscribe() // <-- you get back a Subscriber instance
              .atLeastOnce(Flow.fromFunction((GreetingMessage message) -> {
                  return doSomethingWithTheMessage(message);
              }));
          //#subscribe-to-topic
          return name -> completedFuture(NotUsed.getInstance());
      }
docs: #inject-service