code: public void subscriber(String path,
                          @Pin ServiceRefAmp serviceRef,
                          Result<? super Cancel> result)
    {
      if (path.isEmpty()) {
        result.fail(new ServiceException(L.l("Invalid event location '{0}'", path)));
        return;
      }
  
      String address = address(path);
  
      EventNodeAsset node = lookupPubSubNode(address);
  
      Cancel cancel = node.subscribeImpl(serviceRef);
  
      result.ok(cancel);
    }
docs: Subscribe a callback to a location.