Problem with starting supervisor through script -

Question

I have made an OTP compliant application where i have a gen_server and a supervisor. Also i have a script to start them.

My script contains something like this. erl -pa module_name/ebin -name abc@hostname -setcookie test -s module_sup start_link()

This does not start the supervisor. But when i do module_sup:start_link() inside the shell, it works.

Also when i do erl -pa module_name/ebin -name abc@hostname -setcookie test -s module_srv start_link() i.e the server alone without the supervisor, the server gets started.

So, what am i doing wrong here. Are we not allowed to start supervisor in such a way.

Any help would be highly appriciated.

Thanx, Wilson

Answer 1

supervisor:start_link/2 creates a link to its calling process. when that calling process exits, the supervisor is taken down with it.

erl -s module_sup start_link is starting the supervisor but it is killed because your start function runs inside its own process which dies once the function exits.

you can observe similar behavior with spawn(module_sup, start_link, []). the supervisor starts and gets killed immediately. when you manually start the supervisor, the calling process is the shell. when the shell exits, it will kill the supervisor.

generally the top-level supervisor is meant to be started by an application.

Answer 2

This is very similar to How do I start applications by command line as a daemon? In short, you can't use -s to start a supervisor unless use unlink/1, which is a total kludge. Your time is better spent learning how to package your code as an application. I'd recommend doing this with rebar.

Answer 3

It is important to notice that a process only dies if the linked process is terminating with a reason other than 'normal', which means that a process that simply finishes its execution does not kill the processes linked to it. (source http://www.erlang.org/doc/reference_manual/processes.html#id204170) I think that is an important aspect of Erlang that should not be misinterpreted.

The following source code shows this:

1> spawn(
1>   fun() ->
1>      io:format("outer ~p~n", [self()]),
1>      spawn_link(
1>         fun () ->
1>            io:format("inner ~p~n", [self()]),
1>            receive
1>               Msg -> io:format("received ~p~n", [Msg])
1>            end
1>         end)
1>   end).
outer <0.37.0>
<0.37.0>
inner <0.38.0>
2> is_process_alive(pid(0,37,0)).
false
3> pid(0,38,0) ! test.
received test
test
4>

You can see that the caller <0.37.0> is not running, but the process <0.38.0> is still there, waiting for a message.

Anyway, the supervisor will not terminate when the caller terminates since the supervisor traps exit signals. Of course, unless it is programmed to do so. But I examined the source code and couldn't find this, but alas, my analysis may have been too superficial.

Have you had any luck with that? I will try to run some tests and see if I can figure out what is happening.