Re: [Boost-users] [MSM] a timer/timeout for the whole state machine (including submachines)

Subject: Re: [Boost-users] [MSM] a timer/timeout for the whole state machine (including submachines)
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-04-04 16:30:07

  >Hi,
  >

  >I've been working for a while with the MSM and I'm very impressed about it... congratulations for your work!!
  Thanks :)

  >I've to say that I'm not an expert on UML nor state machines and I think that my needs are a very common user-case... so I'm probably >missing something!

  >Basically what I need is a (reusable) Timer that, after a number of Clocks, it sends a TimeOut to the state machine, and when the Timer
  >receives a Reset it restarts the counting of clocks.
  >(I want to also add a Pause or similar but this doesn't matter right now...)

  Yes, you could also use a region with 1 state and internal transitions but it's about the same.

  >So, the goal is that any (or all) of my states could be controlled by the Timer and if no events are received for a while a TimeOut will
  > be (always) received/processed.

  >AFAIK there is no concept of timer/timeout in the MSM, but I found very straightforward to implement my idea with an Orthogonal
  >Region with a single state Timer that receives Clock and Reset events and sends a TimeOut.
  >And it worked like a charm!!
  >

  >But I found a problem with one of my states become a submachine...
  >The Timer::Reset action executes a fsm.process_event(Timer::Reset()), but this event is not processed.
  >I realized that "fsm" is the submachine and AFAIK there is no way to access to the parent(est) fsm...

  Yes it's something which has been requested a few times, it will be done, but I didn't come to it yet. In the meantime you have 2 solutions:
  - exit the submachine using an exit pseudo state, it's UML conform and visible in your diagram.
  - keep a pointer to the parent-est machine. Not very beautiful but it'll work.

  >So, is there a better or more standard way to implement a Timer/Timeout in UML/MSM?

  There is no timer in MSM for 2 reasons:
  - it's out of the scope of the library and I'm probably not the ideal person for this (too system-dependent).
  - it would have to be done in another thread generating tick events => possible race condition.

  The only solution is outside the fsm. You can use asio, which will cost you a thread and you'll have to prevent the race condition. You can use a posix function to this aim too. To my knowledge, there is no other timer in boost (I looked but I might have missed it).

  >And, although "fsm" is the submachine, why the no_transition function is not executed (this last seems a bug to me...).

  I don't think so. If the submachine doesn't handle the event, the upper one could, so it gets a chance to do it, etc. until the top-level fsm. If no fsm including the top-level fsm brings a no_transition, then it's a bug.

  >

  >Albert
  >

  >PD: Do you think that has meaning that the MSM could have an "on_timeout" function for the states (like on_entry/exit?), or to have a
  >timeout event like "none"... ok may be this sounds huge to implement and not so generic...

  And system-dependent. I have the hope someone in boost will provide this one day. Then I'd be happy to use it in MSM.

  HTH,
  Christophe