django channels group subscriptions

Hello,

I would like to be able to create a Django Channels group such that:

1. Websockets can can subscribe and unsubscribe (or disappear) to group that the client specifies.
   
2. When the first client subscribes to a group, it starts some process (e.g. via celery task) that feeds this group with real time notifications about a particular event.
3. When the last client unsubscribes (or expires) from the group, it kills the process or marks the process to self-destruct when next convenient.

Step 1 is reasonable with this is groups. However not sure about 2 or 3. I can't see anyway with the existing groups to find out when the first client subscribes or the last client disappears.

I could replace/override ChannelLayer with my own code, but this seems unnecessarily intrusive. I could also write my own groups code, this also seems maybe excessive.

Any ideas or recommendations?

Regards

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