code: def start(self, on_exit_callback=None):
          
          # TODO: Support params for services by mapping {servicename: {class,
          # params}}?
          for service in self.services.keys():
              self.services[service] = self.services[service]()
  
          self.server.start(on_exit_callback)
docs: Start the Engel application by initializing all registered services and starting an Autobahn IOLoop.
  
          :param on_exit_callback: Callback triggered on application exit