You may find these interesting...
- "The Onion Layer Theory" https://learnyousomeerlang.com/building-applications-with-ot...
- "On Erlang, State and Crashes" http://jlouisramblings.blogspot.com/2010/11/on-erlang-state-...
- "Why Restarting Works" https://ferd.ca/the-zen-of-erlang.html (search for "Heisenbug")
> you should store the state in the external system
Disk works too, but if you're multi-node this means you now have a distributed database embedded in your system, which may or may not be your goal :)
RabbitMQ does this, they developed a library for "persistent, fault-tolerant and replicated state machines" based on Raft: https://github.com/rabbitmq/ra.