What can be the reason of "Unable to find subscription with identifier" in Rails ActionCable?

Question

I'm building a messenger application using Rails 5.0.0.rc1 + ActionCable + Redis.

I've single channel ApiChannel and a number of actions in it. There are some "unicast" actions -> ask for something, get something back, and "broadcast" actions -> do something, broadcast the payload to some connected clients.

From time to time I'm getting RuntimeError exception from here: https://github.com/rails/rails/blob/master/actioncable/lib/action_cable/connection/subscriptions.rb#L70 Unable to find subscription with identifier (...).

What can be a reason of this? In what situation can I get such exception? I spent quite a lot of time on investigating the issue (and will continue to do so) and any hints would be greatly appreciated!

Answer 1

It looks like it's related to this issue: https://github.com/rails/rails/issues/25381

Some kind of race conditions when Rails reply the subscription has been created but in fact it hasn't been done yet.

As a temporary solution adding a small timeout after establishing the subscription has solved the issue.

More investigation needs to be done, though.