Good taste design question about nested FSMs

Jani Launonen <>
Fri Dec 17 20:10:57 CET 2004

> On Fri, 17 Dec 2004 07:45:05 +0200 (EET), Jani Launonen <> 
> wrote:
>> So --- finally --- the question is; apparently I should ditch the
>> gen_fsm and make my custom fsm that can be attached to the supervisor
>> tree (by using sys and proc_lib), so that I can make consistent
>> receive loop, but the inherent "nestedness" is still a mess. Should
>> I make a process for each substate machine (which would make
>> communicating with other players difficult as the "active" process
>> is changing) or should I make nested receive loops that handle the
>> substate machines?
> This sort of thing is a bit difficult to do well in gen_fsm, as you
> point out. One one hand, it provides built-in support for the OTP
> system messages, which is good, but on the other hand, it provides
> no good support for nested state machines.
> Whether you should break up your state machine in multiple processes
> is difficult to answer in general. Sometimes, it's a good idea, but
> other times it makes things more complex. You have to go through the
> different models in your mind (or on paper) and try to get a feeling
> for it.
> Reasonably simple sub states are nicely done with nested receives.
> The gen_fsm model has a problem if you may want to handle system
> messages (suspend, resume, change_code, get_status) in sub states -
> gen_fsm only does that on the top level. If this is a real problem,
> you can either use multiple gen_fsms (which may buy you other
> problems, like synchronization bugs), or you might try out plain_fsm,
> on jungerl. It tries to help you out with system messages without
> forcing you to leave the textbook Erlang fsm programming model.
> Discussions about plain_fsm on erlang-questions include:

Hey, the plain_fsm looks great! How come it didn't register in my mind as I 
was scanning the mailinglist... perhaps because it didn't straight out 
spell nes-ted FSM. But it really looks something that I've been looking 
for. Thanks!

> /Uffe
> -- 
> Ulf Wiger

Jani Launonen 
Student. . . . . . . . . .University of Oulu, Dept. of El. & Inf. Eng.

"Life is what happens to you while you're busy making future plans."
  - Alfred E. Neuman

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