Check if a machine is 'settled'

[doender]

In our backend we have a machine that can go through several states based on a single triggering event, e.g. waiting -> adding info -> ready for submission -> submitting -> submitted. Within the controller we basically only wait for the first transitions to happen (waiting -> adding info -> ready for submission`) and the other transitions happen after the response has been sent.

My concern is that since we never stop the actor (because submitted is not a final state and it could take weeks before the final state is hit) this will cause memory to increase. I think the solution would be to check whether the machine is 'settled', that is, there are no actors running and no automatic transitions going to happen through always or after. In that case we could stop the machine and when a new event comes in, we just create a new machine and rehydrate it.

Is this the right approach, and is there a way to check whether a machine is settled?

[tamlyn]

Yes, I have exactly this problem. In order to know when to serialise a long-running actor in a backend context, I need to distinguish when a machine actor is waiting for an external event vs waiting for a child actor to complete.

[davidkpiano]

The transition API that's coming up soon (#4954) may help with this, since it allows you to have full control over how your state machines run.