[erlang-questions] Understanding supervisor / start_link behaviour

[Mazen Harake]

Steve,

I wouldn't say that you are wrong. I think that you are reasoning good
about not putting the gen_event module under a supervisor because
*that is what links are for*. Just because you have a supervisor
doesn't mean the you shove everything underneath there! If the
gen_server and the gen_event are truly linked (meaning: gen_server
doesn't act as a "supervisor" keeping track of its gen_event process
and restarts it all the time but rather that they really are linked
and they crash together) then your approach, in my opinion, is good.

There are great benefits in doing it in that way. Many will claim that
it is best practice to put *everything* under a supervisor but this is
simply not true. 90% of cases it *is* the best thing to do and many
times it is more about how you designed your application rather than
where to put the supervisors and their children but doing it the way
you did is not necessarily wrong.

The only problem I see with your approach is that you have registered
the gen_event process which clearly isn't useful (since only the
gen_server should know about it, after all, it started it). Other than
that, this approach is extremely helpful and a nice way to clean up
things after they die/shutdown (Again: assuming truly linked).

There is a big misconception in the community that everything
should/must look like the supervisor-tree model which shows how
gen_servers are put under supervisors and more supervisors under the
"top" supervisor but that is not enforced and the design principles
doesn't take many cases into account where this setup actually brings
more headache to the table than to just exit and clean up using linked
processes (because they do exist).

/M

On 1 June 2011 21:26, Steve Strong <steve@REDACTED> wrote:
> Hi,
>
> I've got some strange behaviour with gen_event within a supervision tree
> which I don't fully understand.  Consider the following supervisor
> (completely standard, feel free to skip over):
> <snip>
> -module(sup).
> -behaviour(supervisor).
> -export([start_link/0, init/1]).
> -define(SERVER, ?MODULE).
> start_link() ->
>     supervisor:start_link({local, ?SERVER}, ?MODULE, []).
> init([]) ->
>     Child1 = {child, {child, start_link, []}, permanent, 2000, worker,
> [child]},
>     {ok, {{one_for_all, 1000, 3600}, [Child1]}}.
> </snip>
> and corresponding gen_server (interesting code in bold):
> <snip>
> -module(child).
> -behaviour(gen_server).
> -export([start_link/0, init/1, handle_call/3, handle_cast/2,
> handle_info/2, terminate/2, code_change/3]).
> start_link() ->
>     gen_server:start_link({local, child}, child, [], []).
> init([]) ->
>     io:format("about to start gen_event~n"),
>     X = gen_event:start_link({local, my_gen_event}),
>     io:format("gen_event started with ~p~n", [X]),
>     {ok, _Pid} = X,
>     {ok, {}, 2000}.
> handle_call(_Request, _From, State) ->
>     {reply, ok, State}.
> handle_cast(_Msg, State) ->
>     {noreply, State}.
> handle_info(_Info, State) ->
>     io:format("about to crash...~n"),
>     1 = 2,
>     {noreply, State}.
> terminate(_Reason, _State) ->
>     ok.
> code_change(_OldVsn, State, _Extra) ->
>     {ok, State}.
> </snip>
> If I run this from an erl shell like this:
> <snip>
> --> erl
> Erlang R14B01 (erts-5.8.2) [source] [64-bit] [smp:2:2] [rq:2]
> [async-threads:0] [hipe] [kernel-poll:false]
> Eshell V5.8.2  (abort with ^G)
> 1> application:start(sasl), supervisor:start_link(sup, []).
> </snip>
>
> Then the supervisor & server start as expected.  After 2 seconds the server
> gets a timeout message and crashes itself; the supervisor obviously spots
> this and restarts it.  Within the init of the gen_server, it also does a
> start_link on a gen_event process.  By my understanding, whenever the
> gen_server process exits, the gen_event will also be terminated.
> However, every now and then I see the following output (a ton of sasl trace
> omitted for clarity!):
> <snip>
> about to crash...
> about to start gen_event
> gen_event started with {error,{already_started,<0.79.0>}}
> about to start gen_event
> gen_event started with {error,{already_started,<0.79.0>}}
> about to start gen_event
> </snip>
> What is happening is that the gen_server is crashing but on its restart the
> gen_event process is still running - hence the gen_server fails in its init
> and gets restarted again.  Sometimes this loop clears after a few
> iterations, other times it can continue until the parent supervisor gives
> up, packs its bags and goes home.
> So, my question is whether this is expected behaviour or not.  I assume that
> the termination of the linked child is happening asynchronously, and that
> the supervisor is hence restarting its children before things have cleaned
> up correctly - is that correct?
> I can fix this particular scenario by trapping exits within the gen_server,
> and then calling gen_event:stop within the terminate.  Is this type of
> processing necessary whenever a process is start_link'ed within a supervisor
> tree, or is what I'm doing considered bad practice?
> Thanks for your time,
> Steve
> --
> Steve Strong, Director, id3as
> twitter.com/srstrong
>
>
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