Tornado websocket- send data based on an event

Question

I have a python program which on certain event (for example on curl request) would calculate the function value. What I need is the moment the function executes, some data needs to be posted to tornado websocket. I have looked around internet and found examples on how to create websocket but all these examples cover scenarios where the data is invoked inside the websocket handler

Referring to this code for example: https://github.com/benjaminmbrown/real-time-data-viz-d3-crossfilter-websocket-tutorial/blob/master/rt-data-viz/websocket_server.py

Can someone guide me on how can I post message on websocket. Basically I have tornado API where if user do a curl request I would like to log that message to websocket

Answer 1

You can do it by creating a registry of all active websockets and use it to send messages on a certain event.

class WebsocketRegistry:
    def __init__(self):
        self._active_websockets = []

    def add_listener(self, listener):
        self._active_websockets.append(listener)

    def remove_listener(self, listener):
        self._active_websockets.remove(listener)

    def send_messages(self, msg_txt):
        for ws in self._active_websockets:
            ws.write_message(msg_txt)

registry = WebsocketRegistry()

class WSHandler(tornado.websocket.WebSocketHandler):
    def open(self, *args, **kwargs):
        super(WSHandler, self).open(*args, **kwargs)
        registry.add_listener(self)

    def on_close(self):
        super(WSHandler, self).on_close()
        registry.remove_listener(self)

P.S. Take note that if you plan to scale your app with 2+ instances, this won't work and you would have to use, for example, a message queue (RabbitMQ is good) to deliver events to the all opened websockets. But overall approach would be the same: MQ would be a registry and websockets subscribe on messages (and unsubscribe on closing) on connection.