Why simple_one_for_one workers can share the same Child Spec Id?

Question

I define simple_one_for_one worker spec for one supervisor named band_supervisor， and the child spec id is jam_musician:

  init([]) ->
    {ok, {{simple_one_for_one, 3, 60},
    [{jam_musician,
    {musicians, start_link, []},
    temporary, 1000, worker, [musicians]}
    ]}};

musicians module is:

-module(musicians).
-behaviour(gen_server).

-export([start_link/2, stop/1]).
-export([init/1, handle_call/3, handle_cast/2,
handle_info/2, code_change/3, terminate/2]).

-record(state, {name="", role, skill=good}).
-define(DELAY, 750).

start_link(Role, Skill) ->
  gen_server:start_link({local, Role}, ?MODULE, [Role, Skill], []).

stop(Role) -> gen_server:call(Role, stop).

and I can create many workers by:

3> supervisor:start_child(band_supervisor, [drum, good]).
Musician Arnold Ramon, playing the drum entered the room
{ok,<0.696.0>}
3> supervisor:start_child(band_supervisor, [guitar, good]).
Musician Wanda Perlstein, playing the guitar entered the room
{ok,<0.698.0>}

I notice that all workers have the same Child spec Id: jam_musician

You know other type workers must has the unique child id, Right ?

Could you post your code for the musicians module, please? It looks like you are registering the child processes with the same name. — Isac
Yes, your code please for the supervisor module as well. Something is wrong as you cannot get an already_started from a simple_one_for_one supervisor. — rvirding

user601836 user601836 · Accepted Answer · 2013-04-02T13:36:38

Most likely you wrote the start_link function of the child process (I assume it is a gen_server) as:

start_link() ->
    gen_server:start_link({local,Name}, ?MODULE, [], []).

This not only calls the init/1 function but also registers the process using the atom Name.

Therefore any new child started in a second moment will try to register inside the erlang node using a name already taken by the first child.

In order to avoid such name clashing you should use something like:

start_link() ->
    gen_server:start_link(?MODULE, [], []).

So that no child will have a registered name and you will have no clashing.

If you really need to register every child an option may consist in the use of gproc.

Why simple_one_for_one workers can share the same Child Spec Id?

2 Answers