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<  Advanced Erlang/OTP  ~  How to get to know that supervisor restarted a process?
qoocku
Posted: Wed Jul 01, 2009 1:22 pm Reply with quote
User Joined: 31 Mar 2008 Posts: 10 Location: Poland
Hi list,

A problem:

A process wants to be informed that a supervisor has just restarted one of its child processes. Is it possible (AFAIK the supervisor code does not have any callbacks for this purposes)?

D.
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qoocku
Posted: Thu Jul 02, 2009 10:13 am Reply with quote
User Joined: 31 Mar 2008 Posts: 10 Location: Poland
I have found a solution, but this hack afraids me a bit.

Having looked at the supervisor's source code I've noticed that just after a child process restart the supervisor calls
error_logger:info_report using progress report type with value:
Code:

[{supervisor, SupName}, {started, ChildInfo}]

which gave me an example of the solution:


    * create event handler module x with functions:
    Code:

      init ([SupName]) ->
          {ok, SupName}.
      handle_event ({info_report, _, {_, progress, Report}}, State) -> ... ;
      handle_event (_, State) ->
         {ok, State}.
     

    Now your init/1 function returns the event handler state value as your specific supervisor name/pid.
    * handle_event(...) should contain a code to filter
    reports which are related to your supervisor's children restars, for example:
    Code:

      case Report of
        [SupInfo, StartInfo] ->
           case SupInfo of
             {supervisor, {State, module_of_your_supervisor}} ->
               {started, ChildInfo} = StartInfo,
               {pid, ChildPid} = hd(ChildInfo),
               ... here do something with this Pid ...
             _ -> ok
           end;
      _ -> ok
      end,
      {ok, State}     

    Personaly, I'm using the extracted child pid to inform another process about the new child. ChildInfo contains several other interesting items which may be useful.

    * The last thing is to register the event handler. The best place is in module_of_your_supervisor:start_link function when the brand new supervisor process pid is available:
    Code:

    error_logger:add_report_handler(x, [SupPid])



I'm looking for more elegant ways, but this worked so far.
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baryluk
Posted: Tue Aug 18, 2009 11:48 am Reply with quote
User Joined: 05 Aug 2009 Posts: 48
This is very elegant solution. Only question is if this event will change in next releases of OTP.

Evetually you can use erlang:monitor(process, pid()), for knowing when process dies. (or use link(pid()) and process_flag(trap_exit, true), but i prefer monitor). This will only give you information about processes destroyed/killed/crashed/exited/ended, but not about new ones (created).
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satishbhawra37
Posted: Fri Sep 25, 2009 6:18 pm Reply with quote
Joined: 25 Sep 2009 Posts: 2
to be informed that a supervisor has just restarted one of its child processes.

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absolute
Posted: Mon Dec 14, 2009 7:08 pm Reply with quote
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Very sorry,I don't know about it.I can't give you a hand.

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