Supervisors behaviour

Question

when implementing a supervisor.. If, in the supervisor module, I do something like

init([_]) ->
{ok,
{{one_for_one, 5, 60},
[{reverese, {reverse, start_reverse, []}, permanent, brutal_kill, worker,[]}]}}.

and the reverse function is:

start_reverse() ->
Pid=spawn(?MODULE,reverse,[]).

It will not work since the start_reverse function exits normally in every case. However, when I add a line like this:

start_reverse() ->
Pid=spawn(?MODULE,reverse,[]),
{ok,Pid}.

It works, even when the function exits normally as well. Can someone explain why?

score 5 · Answer 1 · answered May 03 '11 at 10:45

5

Easily,

The problem is that the supervisor needs a specific calling convention to support the shutdown and initialization of processes. Your code with a low-level spawn ignores that convention. You should either

Use a gen_something behaviour, gen_server is most common.
Spawn the process using proc_lib
Use a supervisor_bridge

Otherwise, your code will not take advantage of OTP at all. And you want it to.

answered May 03 '11 at 10:45

I GIVE CRAP ANSWERS

18,739
3
42
47

1

Can you tell me why I need or should want OTP? – Daniel May 03 '11 at 11:07
@dkk: That would be a good topic for another question? Search for it, it has probably been answered already... – Adam Lindberg May 03 '11 at 11:33
FYI I have created a new question: http://stackoverflow.com/questions/5880499/why-use-otp-with-erlang – Daniel May 04 '11 at 08:17

score 1 · Accepted Answer · answered May 03 '11 at 16:16

Actually.. It isnt really required for the supervisor child process to be a gen_server. The supervisor documentation specifically mentions that

The start function must create and link to the child process, and should return {ok,Child} or {ok,Child,Info} where Child is the pid of the child process and Info an arbitrary term which is ignored by the supervisor.

which is the reason why when you returned {ok, Pid} it worked..

Supervisors behaviour

2 Answers2