[erlang-questions] how to _cause_ a code_change in a gen_fsm

Knut Nesheim <>
Sat Nov 26 19:06:59 CET 2011


Hi Daniel,

I had to solve a similar problem some time ago. I needed a way to
force supervisors to reload the child specifications, which it does in
supervisor:code_change/3. It is possible to fake what the reltool
does, by manually suspending, forcing code change and then resuming
the process.

I use the following hack:

    ok = sys:suspend(Pid),
    ok = sys:change_code(Pid, Mod, Vsn, []),
    ok = sys:resume(Pid),

Which I admit is very naive as it doesn't take into account something
going wrong in sys:change_code/4.

Regards
Knut

On Sat, Nov 26, 2011 at 6:01 PM, Daniel Dormont
<> wrote:
> Ok. So what is the "right" way to get my code_change callback to fire?
> I see some reference to "appup" and I have looked over that part of
> the manual but don't really understand how it applies to my case.
> Literally all I have is a .beam file I want to install in a running
> system.
>
> dan
>
> On Sat, Nov 26, 2011 at 10:28 AM, Bob Ippolito <> wrote:
>> I don't know how ejabberd likes to do things but l() is the lower level
>> non-OTP way to do things and will immediately purge the old code and load
>> the new code. Nothing else will happen, no magic.
>>
>> On Friday, November 25, 2011, Daniel Dormont <>
>> wrote:
>>> This is a follow-up to my earlier question but the topic is a little
>>> different. Having put together a nice piece of code for handling the
>>> change in my gen_fsm's state record, I discovered to my chagrin that
>>> when I loaded the new code (using l() at first and then using the
>>> "update" command that comes with ejabberd) it did not call the
>>> code_change callback; neither immediately nor before processing the
>>> next message sent to that process. I admit that the details of things
>>> like proc_lib and sys are a little opaque to me, but I had understood
>>> that as long as the module implemented a standard OTP behavior, code
>>> upgrades would "just work" in some sense. I suppose I was wrong :)
>>> This module is fairly self-contained, so I'm not too worried about
>>> having to synchronize other modules, notify supervisors or anything
>>> like that. How do I proceed from here?
>>>
>>> thanks,
>>> dan
>>>
>>>
>>> ---------- Forwarded message ----------
>>> From: Daniel Dormont <>
>>> Date: Mon, Nov 21, 2011 at 10:51 AM
>>> Subject: code_change for gen_fsm: how to actually handle a state change
>>> To: erlang-questions <>
>>>
>>>
>>> Hello all,
>>>
>>> I have a gen_fsm module and would like to take advantage of hot code
>>> deploy. The module uses a record (called "state") and the new version
>>> of the module includes some new fields in the record. What is a nice
>>> clean way to code the my_module:code_change function to deal with
>>> this? Are there any good examples out there on the web?
>>>
>>> Note: for my purposes it would be sufficient to detect that the state
>>> record is out of date, and terminate cleanly. BUT the correct
>>> functionality of the terminate/3 function in my module depends on the
>>> state data, and I would need it to complete cleanly and not crash in
>>> this instance because there are other processes that depend on this
>>> one and need to be notified properly of its exit. The issue is that
>>> the state itself contains the data of which processes those are.
>>>
>>> What's the best approach here?
>>>
>>> thanks,
>>> Dan
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>
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