Any way Channels can read existing published messages from another app using Redis?

[cdnsteve]

I'm using Channels to power a real-time dashboard from Redis, where the data is published from another app. Trying to share messages between two apps, via Redis.

Architecture:

• Redis (shared between two apps)
• Flask, publisher
• Django with Channels, subscriber

When the Flask app publishes a message onto Redis I'd like to be able to have Django Channels read that message from Redis.
Code to publish:
current_app.redis.publish('dashboard', message)

I've tried setting the Django channel to dashboard as well but it doesn't seem to be getting messages in a way it expects.

How I managed to get it working:

In the Django app, I've created a worker.py file that subscribes to Redis channel dashboard.
It reads the message from Redis, then re-publishes it using Django Channels on a default channel.
Then the client side JS uses websockets get the messages/data in real-time.

I'd like to see if there's a better way of doing this. Can Django Channels be customized/configured to support getting data from an existing Redis instance, outside of Channels creating it, so I don't have to use this custom worker?

[andrewgodwin]

Channels doesn't use Redis pub/sub, but instead Redis list keys with expires and some other stuff, and Lua running a lot of it for efficiency. It's not a format you're meant to be able to publish into directly in Redis, really.

In this scenario you have two options:

• Publish directly to Channels from Flask. You can publish onto Channels from any Python code; either you import your project's asgi.py file and use the channel_layer object inside, which presents an interface matching the ASGI spec (see http://channels.readthedocs.io/en/latest/asgi.html); in particular, you can just do channel_layer.send(channel, message). If you don't want to import the Django project you can also import and set up an instance of RedisChannelLayer with matching settings directly too, from the asgi_redis package; everything in Channels is backed by a non-Django-specific thing.
• As you have done, make a new process that bridges the two, receiving from one and subscribing to the other. This is commonly called an "interface server" in the docs; Daphne bridges HTTP and WebSockets into Channels, for example, so it would not be crazy to have something that bridged Redis pub/sub over too.