How do I send a websocket message in Tornado at will?

Question

I'm very new to Tornado and wanted to know if it is possible to send a message (write_message) at will from within my Python program to all clients? For example, say my program is monitoring a directory to see if a file appears/exists. When it appears, I want to send a web socket message to a browser client that the file exists. I can't seem to understand how to invoke the "write_message" method without first receiving a websocket message (on_message handler.)

Even if I use the "PeriodicCallback" method, I'm still unclear as to how I actually call the "write_message" method. Are there any examples out there of how to invoke "write_message" without doing it within the "on_message" handler?

Answer 1

You need to keep a collection of openned websockets and iterate through that collection at will to send messages.

As an example, I'll send a message anytime a client connect to your.domain.example/test/ but the idea is the same whenever you want to send something:

import os.path
import logging

from tornado import ioloop, web, websocket


SERVER_FOLDER = os.path.abspath(os.path.dirname(__file__))
LOGGER = logging.getLogger('tornado.application')


class TestHandler(web.RequestHandler):
    def get(self):
        server = ioloop.IOLoop.current()
        data = "whatever"
        server.add_callback(DefaultWebSocket.send_message, data)
        self.set_status(200)
        self.finish()


class DefaultWebSocket(websocket.WebSocketHandler):
    live_web_sockets = set()

    def open(self):
        LOGGER.debug("WebSocket opened")
        self.set_nodelay(True)
        self.live_web_sockets.add(self)
        self.write_message("you've been connected. Congratz.")

    def on_message(self, message):
        LOGGER.debug('Message incomming: %s', message)

    def on_close(self):
        LOGGER.debug("WebSocket closed")

    @classmethod
    def send_message(cls, message):
        removable = set()
        for ws in cls.live_web_sockets:
            if not ws.ws_connection or not ws.ws_connection.stream.socket:
                removable.add(ws)
            else:
                ws.write_message(message)
        for ws in removable:
            cls.live_web_sockets.remove(ws)


def serve_forever(port=80, address=''):
    application = web.Application([
            (r"/test/", TestHandler),
            (r"/websocket/", DefaultWebSocket),
            ...
        ],
        static_path=os.path.join(SERVER_FOLDER, ...),
        debug=True,
    )
    application.listen(port, address)
    LOGGER.debug(
            'Server listening at http://%s:%d/',
            address or 'localhost', port)
    ioloop.IOLoop.current().start()


if __name__ == "__main__":
    serve_forever()

You'll obviously need to create a websocket in the browser using the following JavaScript:

socket = new WebSocket('ws://your.domain.example:80/websocket/');

And manage it accordingly.