def start(self, on_exit_callback=None):
<EOL>for service in self.services.keys():<EOL><INDENT>self.services[service] = self.services[service]()<EOL><DEDENT>self.server.start(on_exit_callback)<EOL>
Start the Engel application by initializing all registered services and starting an Autobahn IOLoop.

:param on_exit_callback: Callback triggered on application exit