docs: @Override
      public Collection<EventRegistration> getRegistrations(String serviceName, String topic) {
          EventServiceSegment segment = getSegment(serviceName, false);
          if (segment == null) {
              return Collections.emptySet();
          }
  
          Collection<Registration> registrations = segment.getRegistrations(topic, false);
          if (registrations == null || registrations.isEmpty()) {
              return Collections.<EventRegistration>emptySet();
          } else {
              return Collections.<EventRegistration>unmodifiableCollection(registrations);
          }
      }