[erlang-questions] beginning erlang/OTP - spawning gen_servers and supervisors

[AJ Heller]

Thank you, Geoff. That is the best explanation I've read of why the
supervisor dies after being spawned in this way.

Don't you find it odd, then, that a gen_server can be spawned the same
way, and it continues to run after its parent process exits?

On Sat, May 8, 2010 at 5:19 PM, Geoff Cant <nem@REDACTED> wrote:
> If you do spawn(echo_sup, start_link, []), you'll spawn a process (A) that will spawn and link to an echo_sup supervisor process (B) which will spawn your gen_server (C). A will then exit because it has no more code to run (ala exit(normal)), causing B to exit (due to the link to A), causing C to exit (due to the link to B).
>
> You should either run echo_sup:start_link() at the shell (and then unlink the pid returned as otherwise mistakes you type in the shell will kill echo_sup) or you want to start echo_sup underneath an already running supervisor. (Or you want to write an application callback echo_app, and an echo.app file i.e. build an echo_server OTP application and have the application_controller deal with the top level of supervision).
>
> Cheers,
> -Geoff
>
> On 2010-05-08, at 16:38 , AJ Heller wrote:
>
>> I am reading through the OTP documentation, stopping to work with and
>> understand every piece as it is introduced. I've written a very simple
>> gen_server and a simpler supervisor, and they both work as I expect
>> them to, but I've had to stop there. I'd like to understand them each
>> before I move on to building an application, but I'm confused about
>> the behavior of the system when I spawn each of these pieces
>> independently.
>>
>> (For reference, my gen_server (echo_server) and supervisor (echo_sup)
>> modules are at the end of this post)
>>
>> I am able to spawn my simple gen_server from my erl shell using
>> `spawn(echo_server, start_link, []).` I can then interact with it
>> using its public api (`echo_server:echo(...)`).
>>
>> However, I cannot spawn a supervisor the same way. By calling
>> `spawn(echo_sup, start_link, []).`, my supervisor and gen_server both
>> appear to exit immediately after they have finished initializing.
>> Everything looks fine until I try to interact with my gen_server and
>> get
>>
>> "** exception exit: {noproc,{gen_server,call,[echo_server,{echo,hi}]}}
>>    in function  gen_server:call/2"
>>
>> Why is it that I can spawn a gen_server but not a supervisor? Thank you much.
>>
>>
>> ---------------------------------------------------------------------------------------------------------
>> echo_server:
>>
>> % -- begin echo_server.erl -- %
>>
>> -module(echo_server).
>> -behaviour(gen_server).
>> -export([start_link/0]).
>> -export([echo/1, crash/0]).
>> -export([init/1, handle_call/3, handle_cast/2]).
>>
>> start_link() ->
>>   gen_server:start_link({local, echo_server}, echo_server, [], []).
>>
>> %% public api
>> echo(Text) ->
>>   gen_server:call(echo_server, {echo, Text}).
>>
>> %% crash this gen_server to test its supervisor
>> crash() ->
>>   gen_server:call(echo_server, crash).
>>
>> %% behaviours
>> init(_Args) ->
>>   {ok, none}.
>> handle_call({echo, Text}, _From, State) ->
>>   {reply, Text, State};
>> handle_call(crash, _From, State) ->
>>   X=1,
>>   {reply, X=2, State}.
>> handle_cast(_, State) ->
>>   {noreply, State}.
>>
>> % -- end echo_server.erl -- %
>>
>>
>>
>>
>> echo_sup:
>>
>> % -- begin echo_sup.erl -- %
>>
>> -module(echo_sup).
>> -behaviour(supervisor).
>> -export([start_link/0]).
>> -export([init/1]).
>>
>> start_link() ->
>>   supervisor:start_link({local, echo_sup}, echo_sup, []).
>> init(_Args) ->
>>   {ok,  {{one_for_one, 5, 60},
>>          [{echo_server, {echo_server, start_link, []},
>>                permanent, brutal_kill, worker, [echo_server]}]}}.
>>
>>
>> % -- end echo_sup.erl -- %
>>
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