code: public void subscriptionRequestReceived(SubscriptionRequest sr) throws SubscriptionException {
  
          LOG.info("Subscriber -> (Hub), new subscription request received.", sr.getCallback());
  
          try {
  
              verifySubscriberRequestedSubscription(sr);
  
              if ("subscribe".equals(sr.getMode())) {
                  LOG.info("Adding callback {} to the topic {}", sr.getCallback(), sr.getTopic());
                  addCallbackToTopic(sr.getCallback(), sr.getTopic());
              } else if ("unsubscribe".equals(sr.getMode())) {
                  LOG.info("Removing callback {} from the topic {}", sr.getCallback(), sr.getTopic());
                  removeCallbackToTopic(sr.getCallback(), sr.getTopic());
              }
  
          } catch (Exception e) {
              throw new SubscriptionException(e);
          }
  
      }
docs: Input method, called by a Subscriber indicating its intent into receive notification about a given topic.
  
  @param sr DTO containing the info given by the protocol