[erlang-questions] Hot code replacement issues ?

Taavi Talvik <>
Wed Dec 14 11:34:57 CET 2016


Switch to new version of code happens, when fully qualified 
function is called:

1) Switch to new code version happens:
loop(..) ->
   modulename:loop().

2) Old code will remain in use:
loop(..) ->
   loop().

best regards,
taavi

> On 14 Dec 2016, at 12:28, Alex Feng <> wrote:
> 
> Hi Erlangers,
> 
> I am trying to understand how hot patch upgrade is done in Erlang, I got this piece of code from stackoverflow, but I ran into 2 issues when I played around with it.
> 
> Issue 1)  In the piece of code, I suppose to send a message 'upgrade' to the running process, then it will finish the upgrade. It is working fine with "?MODULE:loop();", but if I remove the prefix ?MODULE in the code, then the upgrade will be failed, I mean when I send message 'run' after upgrade neither the new message nor the old message are printed.
> 
> Issue 2) I was told if I recompile and reload the updated file from shell, then it will be done as well.
> So, I did this from shell, update the source file -> compile the file -> reload it ->send msg 'run' to the same process -> 'new version' is not printed out.   What could be the mistake? 
>   
> 
> 
> -module(test_hot_swap).
> 
> -export([loop/0]).
> 
> loop() ->
>     receive
>         upgrade ->
>             code:purge(?MODULE),
>             compile:file(?MODULE),
>             code:load_file(?MODULE),       
>             ?MODULE:loop();                                    %%without ?MODULE, it has problem.
>         run ->
>             io:format("This is a new version ~n"),       %% before upgrade, the text is  'old version'.
>             loop();
>         _ ->
>             loop()
>     end.
> 
> Thank you.
> 
> Br,
> Alex
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