Re: Channels 2.0 - Exception when sending a message to client

Thank you so much for the detailed explanation.

1) I now understand where my error was - the custom channel I used was passing events to a background worker that was not related to the websocket consumer. This also answers my question about how channel names must be included in the runworker command explicitly, similar to celery.

2) To fix my own issue, all I had to do was add the group to the right channel (self.channel_name in this case). I did read the multichat example a few times but missed the reference to the implicit channel name. 

I can now send messages to my front end app perfectly.

Andrew/Ken - Thank you for taking the time to assist and helping me solve the issue.

On Sunday, February 25, 2018 at 2:56:45 AM UTC+2, muha...@swordfish.co.za wrote:

I'm still trying to find my way around channels 2.0, so I'm not sure if my implementation is incorrect or if this is a valid issue. I'm going to post as much info as possible in the hope that it will assist with finding the problem.

I have a single page app which opens a JS websocket connection - with channels 1 I used to add a session_key to the querystring and that used to handle the authentication.

I see this is no longer the case, so I now have a custom middleware class that sets the user object on the scope:

from django.contrib.sessions.models import Session

from users.models import User


class QueryAuthMiddleware:

    def __init__(self, inner):
        # Store the ASGI application we were passed
        self.inner = inner

    def __call__(self, scope):
        # Look up user from query string (you should also do things like
        # check it's a valid user ID, or if scope["user"] is already populated)
        if scope.get("user", None) is None:
            session_obj = Session.objects.get(session_key=scope["query_string"].decode("utf-8").split("=")[1])
            session_decoded = session_obj.get_decoded()

            scope["user"] = User.objects.get(id=session_decoded.get("_auth_user_id"))

            # Return the inner application directly and let it run everything else
        return self.inner(scope)

This is in turn added to my routing (channels.py):

from django.conf.urls import url
from django.conf import settings
from channels.routing import ProtocolTypeRouter, URLRouter, ChannelNameRouter

from notifications.consumer import TestWebsocketConsumer, TestConsumer
from notifications.middleware.query_auth_middleware import QueryAuthMiddleware

ROOT_PATH = "" if settings.DEBUG else "/ws/"


application = ProtocolTypeRouter({

    "websocket": QueryAuthMiddleware(
        URLRouter([
            url(f"^{ROOT_PATH}(?P<user_id>[-\w]+)/$", TestWebsocketConsumer),

        ])
    ),

    "channel": ChannelNameRouter({
         "user-notifications": TestConsumer,
    })


})

Here's my consumers.py:

from asgiref.sync import async_to_sync
from channels.consumer import SyncConsumer
from channels.generic.websocket import WebsocketConsumer


class TestWebsocketConsumer(WebsocketConsumer):
    def websocket_connect(self, message):
        async_to_sync(self.channel_layer.group_add)(str(self.scope["user"].id), "user-notifications")
        self.connect()


class TestConsumer(SyncConsumer):
    def notification_handler(self, message):

        self.send(
            {
             "type": "websocket.send",
             "text": message["text"]
            }
        )

The idea of the app is that each user that logs in on the front end is able to receive messages meant only for them sent by the back end.  I have been trying to test it like this:

>>> channel_layer = get_channel_layer()
>>> async_to_sync(channel_layer.send)("user-notifications", {"type": "notification.handler", "text": "My Message"})

Here's the traceback in the runworker output:

2018-02-25 02:34:14,002 - INFO - runworker - Running worker for channels ['user-notifications']

ERROR:root:Exception inside application: You must implement application_send()

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/channels/consumer.py", line 54, in __call__

    await await_many_dispatch([receive, self.channel_receive], self.dispatch)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/channels/utils.py", line 48, in await_many_dispatch

    await dispatch(result)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/asgiref/sync.py", line 110, in __call__

    return await asyncio.wait_for(future, timeout=None)

  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/asyncio/tasks.py", line 333, in wait_for

    return (yield from fut)

  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/thread.py", line 55, in run

    result = self.fn(*self.args, **self.kwargs)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/channels/db.py", line 13, in thread_handler

    return super().thread_handler(loop, *args, **kwargs)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/asgiref/sync.py", line 125, in thread_handler

    return self.func(*args, **kwargs)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/channels/consumer.py", line 99, in dispatch

    handler(message)

  File "/Users/muhammed/projects/xxx/my-app/app/notifications/consumer.py", line 18, in notification_handler

    "text": message["text"]

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/channels/consumer.py", line 107, in send

    self.base_send(message)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/asgiref/sync.py", line 64, in __call__

    return call_result.result()

  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 405, in result

    return self.__get_result()

  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 357, in __get_result

    raise self._exception

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/asgiref/sync.py", line 78, in main_wrap

    result = await self.awaitable(*args, **kwargs)

  File "/Users/muhammed/projects/xxx/lib/python3.6/site-packages/asgiref/server.py", line 71, in application_send

    raise NotImplementedError("You must implement application_send()")

  You must implement application_send()

OS: MacOS High Sierra

Python: 3.6.1

Django==2.0.2

channels==2.0.2

channels-redis==2.1.0

asgiref==2.1.6

daphne==2.0.4

Config from settings.py:

# -- Channels Details
CHANNEL_LAYERS = {
    "default": {
        "BACKEND": "channels_redis.core.RedisChannelLayer",
        "CONFIG": {
            "hosts": [("localhost", 6379)]
        }
    }
}

ASGI_APPLICATION = "myapp.settings.channels.application"

Any help would be appreciated.

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