tornado_server_inside_blender.py

import bpy

import tornado.ioloop
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")

def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),
    ])

class ModalTimerOperator(bpy.types.Operator):
    """Operator which runs its self from a timer"""
    bl_idname = "wm.modal_timer_operator"
    bl_label = "Modal Timer Operator"

    _timer = None

    def modal(self, context, event):
        if event.type in {'RIGHTMOUSE', 'ESC'}:
            self.cancel(context)
            return {'CANCELLED'}

        if event.type == 'TIMER':
            tornado.ioloop.IOLoop.current().stop()
            tornado.ioloop.IOLoop.current().start()

        return {'PASS_THROUGH'}

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(0.1, window=context.window)
        
        self._app = make_app()

        self._server = tornado.httpserver.HTTPServer(self._app)
        self._server.listen(3636)

        print(self._app)
        wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}

    def cancel(self, context):
        wm = context.window_manager
        print('stopping')
        self._server.stop()
        tornado.ioloop.IOLoop.current().stop()
        wm.event_timer_remove(self._timer)


def register():
    bpy.utils.register_class(ModalTimerOperator)


def unregister():
    bpy.utils.unregister_class(ModalTimerOperator)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.wm.modal_timer_operator()

This is a skeleton example of how a Tornado web server can be run inside a Modal loop in Blender