Return a WSGI application for subdomain.  The subdomain is
        passed to the create_application constructor as a keyword argument.

        :param subdomain: Subdomain to get or create an application with

def _get_application(self, subdomain):
        """ Return a WSGI application for subdomain.  The subdomain is
        passed to the create_application constructor as a keyword argument.

        :param subdomain: Subdomain to get or create an application with
        """
        with self.lock:
            app = self.instances.get(subdomain)
            if app is None:
                app = self.create_application(subdomain=subdomain)
                self.instances[subdomain] = app
            return app