Listening to a django channel from outside a consumer

[Alex]

Hi,

The docs explain that it is possible to publish to the channel layer from outside of a consumer: https://channels.readthedocs.io/en/latest/topics/channel_layers.html#using-outside-of-consumers I need to do the opposite. I have a fairly complex python script that reads live data from pubnub, processes it, and pushes it to consumers via groups on the channel_layer. This works fine, but I need consumers to be able to announce their presence to this script so that it can push them data (it currently pushes to the channel layer only when it gets new data from pubnub, which could be every 24 hours).

I've decided to solve this by having the consumers publish to a 'presence' channel on connect. I now need the pubnub source script to listen to this channel. I've tried adding the below to the script, and it no longer throws errors, but it doesn't actually respond to messages. It successfully joins the channel layer, but the message handler 'receive_json', never fires.

This is the class, and then the initiation further down in the code:

from channels.generic.websocket import JsonWebsocketConsumer


class channelConsumer(JsonWebsocketConsumer):

    def __init__(self):
        self.channel_name = 'source'
        def join(self):
        async_to_sync(channel_layer.group_add)('presence', self.channel_name)
    def receive_json(self, message):
        print("Presence Detected")
        # do some stuff

global channel_layer
channel_layer = get_channel_layer()

global listener
listener = channelConsumer()
listener.join()

[Andrew Godwin]

Hi Alex,

Consumers aren't designed to be run outside of a Channels or ASGI environment. If you read the ASGI spec (https://github.com/django/asgiref/blob/master/specs/asgi.rst), you'll see that to run a consumer - which is an ASGI app - you need to do quite a bit more than you did there, in particular making an event loop and running its coroutine.

Instead, if you want to receive on a channel layer channel externally, I recommend calling channel_layer.receive directly. It takes a channel name to receive on and blocks until there's a message. Ideally you should do this all within an async context rather than using async_to_sync, but I believe both should work.

Andrew

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