[erlang-questions] gen_statem: Join states?

[Oliver Korpilla]

Hello.

In the system we're building we're using gen_statem processes to keep track of signalling scenarios, essentially series of different types of messages from various sources. For linear or branching scenarios gen_statem is well prepared. But there's one thing that seemed to be missing and that is support for joins?

I recently reviewed a developer's code that implemented receiving two different messages that could arrive in any order as four state functions - receiving A first (1), then receiving B (2), then entering an end state; or receiving B first (3), then receiving A (4), then entering an end state. The code entering the end state was duplicated in both (2) and (4). What essentially was needed was a "check list" of which messages had already arrived to determine the transition into the end state.

Such "check list" scenarios are quite common in our work (and can grow to be more complex) and I couldn't find a direct support for it (though I was limiting myself to state_function mode), so we're now starting a separate "join process" - a gen_server that keeps and maintain the join list - and do the decision now from a single wait state. Given how light-weight Erlang processes are this works fine.

What prevented me from doing this through data maintained between calls was that I would have to partition the state into information for tracking the join and the actual information kept to process the signals. This seemed less generic and clean.

I'm also aware that had I rewritten the whole state machine to handle_event functions and utilized a complex state this could have helped achieve the same. What is essentially is needed, from my POV, is a way to track information pertaining to state transitions separate from information for processing incoming events, and complex states seem to do that...?

What is your preferred way of doing this? Is it possible to somehow support this at the library level? Are handle_event/complex states the way to go in such situations?

Thank you and regards,
Oliver