[erlang-questions] code_change for gen_fsm: how to actually handle a state change

Barco You <>
Wed Nov 23 04:37:48 CET 2011


And then a new process would be restarted by the supervisor with new
version code?
If in that way, how do you recover the state data?

Barco

On Tue, Nov 22, 2011 at 10:46 PM, Daniel Dormont
<>wrote:

> Thanks, that makes sense.
>
> Here's another crazy idea, that I don't imagine is possible: can I
> give code_change some kind of return value that says to the system "go
> ahead and terminate this process, but call terminate/3 with the _old_
> version of the code"? I know it sounds weird, but that would also be a
> solution here.
>
> dan
>
> On Tue, Nov 22, 2011 at 5:35 AM, Richard Carlsson
> <> wrote:
> > On 11/22/2011 08:40 AM, Bengt Kleberg wrote:
> >>
> >> Is it correct to say that the old definition of #state{} is
> >> -record(state, {item1, item2}).
> >> and that the new is
> >> -record(state, {item1, item2, newfield}).
> >>
> >> I think you will find it difficult to get both these records to work in
> >> the same file at the same time (during code change).
> >
> >
> > Possible solution: let code_change call a separate module that transforms
> > the old definition to an intermediate format that the gen_fsm module can
> use
> > to initialize the new state. That way, the different record declarations
> are
> > never present in the same module.
> >
> > code_change(#state{}=State) ->
> >    State;  % already the new state
> > code_change(OldState) ->
> >    init_state(state_converter:old_to_proplist(OldState)).
> >
> > init_state([{foo, Foo} | Ps], State) ->
> >    init_state(Ps, State#state{foo=Foo});
> > ...
> > init_state([], State) ->
> >    State.
> >
> > A more quick-and-dirty solution is to re-tag the old state so you can
> use it
> > with a different state declaration:
> >
> > -record(old_state, {item1, item2}).
> > -record(state, {item1, item2, newfield}).
> >
> > code_change(#state{}=State) ->
> >    State;  % already the new state
> > code_change(OldState) ->
> >    update_state(setelement(1, State, 'old_state'), #state{}).
> >
> > update_state(OldState, State) ->
> >    State#state{item1 = OldState#old_state.item1,
> >                item2 = OldState#old_state.item2,
> >                newfield = 42}.
> >
> >
> >    /Richard
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://erlang.org/mailman/listinfo/erlang-questions
> >
> _______________________________________________
> erlang-questions mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20111123/5bb626bf/attachment.html>


More information about the erlang-questions mailing list