[erlang-questions] More on this: simple_one_by_one trouble and rare start_link/2

Gustav Simonsson <>
Wed Jun 27 16:28:27 CEST 2012


This is a post talking about why you should not name things "Data" :P
http://petdance.com/2012/04/the-worlds-two-worst-variable-names/

Perhaps a better name for what you send to Mod:init/1 in gen_XXX is "Argument"
so it's more clear from the variable name that it is a single term.

// Gustav

Sent from my PC

----- Original Message -----
> From: "Robert Virding" <>
> To: 
> Cc: 
> Sent: Wednesday, June 27, 2012 2:41:58 PM
> Subject: Re: [erlang-questions] More on this: simple_one_by_one trouble and	rare	start_link/2
> 
> I don't really think that there is an inconsistency here.
> 
> In all the cases where something is to be started, supervisor,
> application and proc_lib, the specifier for this is the Mod,Fun,Args
> of apply and spawn. Though I will admit that in the application case
> the function name is predefined, almost like a call back function.
> And for callbacks all the arguments, number and meaning, are
> predefined.
> 
> I personally think the main problems are:
> 
> - Inconsistency in the documentation caused by using "Args" to mean
> different things. So in supervisor. application and proc_lib "Args"
> really is a list of arguments in apply/spawn style where the call is
> built using the length of the list to determine the arity of the
> called function. In gen_XXX the ONE argument to Mod:init/1 is also
> called "Args" where it can be anything and even if it is a list it
> will only become one argument. Calling it "Data" would have made it
> easier to understand.
> 
> - The second argument supervisor:start_child/2 to completely
> different, both type and meaning, depending on whether the
> supervisor is a simple_one_for_one or anything else. Yes, I know
> that it is documented but it IS inconsistent.
> 
> I think that in general making all the start functions arity 1 and
> calling them with a list of arguments is of no real benefit. Apart
> from simple_one_for_one supervisors the arguments to the start
> function are anyway fixed so there would be no real gain here
> anyway.
> 
> Also it would be a completely backwards incompatible change which
> would cause a lot of rewriting, so I don't see it being applied.
> 
> We will just have to live with it. If it is of any consolation there
> are changes I would like to make which would also never be made. :-)
> 
> Robert
> 
> ----- Original Message -----
> > 
> > i did'nt find any example of mixing args from the supervirsor
> > childspec
> > and start_child(...) so im going to make my own digging in this
> > issue:
> > 
> > Let see How the simple_one_by_one works...
> > 
> > handle_call({start_child, EArgs}, _From, State) when
> > ?is_simple(State) ->
> >     Child = hd(State#state.children),
> >     #child{mfargs = {M, F, A}} = Child,
> >     Args = A ++ EArgs,
> >     case do_start_child_i(M, F, Args) of
> > <------>{ok, undefined} when Child#child.restart_type =:= temporary
> > ->
> > <------>    {reply, {ok, undefined}, State};
> > <------>{ok, Pid} ->
> > <------>    NState = save_dynamic_child(Child#child.restart_type,
> > Pid, Args, State),
> > <------>    {reply, {ok, Pid}, NState};
> > <------>{ok, Pid, Extra} ->
> > <------>    NState = save_dynamic_child(Child#child.restart_type,
> > Pid, Args, State),
> > <------>    {reply, {ok, Pid, Extra}, NState};
> > <------>What ->
> > <------>    {reply, What, State}
> >     end;
> > 
> > 
> > Shouldnt be do_start_child_i(M,F,EArgs) of the form :
> > 
> > do_start_child(M,F,[EArgs]) ??
> > 
> > or the inner call apply(M,F,[A])?
> > 
> > Provided  the docs state that your unique exported funcion must be
> > of
> > arity one you should
> > expect that code will take measures in order to enforce arity one,
> > enclosing whatever you pass it in a List on the apply phase.
> > 
> > The semanctics of apply use a list to KNOW how many args the target
> > call has, while the
> > semantics of sup childspec + sup start_child use a list ++ op to
> > know
> > how many args will be passed
> > to your callback of arity ONE.
> > 
> > So for the apply part all your functions are arity ONE
> > 
> > Without the ability to join Args from the ChildSpec and from the
> > start child call i would concur that you should
> > enclose in a list whatever you want to be as a sole argument and
> > this
> > code be ok, but as soon as you can put
> > two or more args one on the ChildSpec and one on the call Sup code
> > must enforce that only one arg APPLY
> > call is made so you end calling start_link/1 whatever you pass on
> > the
> > args...
> > 
> > Even the DOCS state "[ term() ]"  as the MFA on the ChildSpec
> > 
> > 
> > My stdlib-1.18.1.pdf says (Pag 369)
> > 
> > "start_child(SupRef, ChildSpec) -> startchild_ret()
> > Types:
> > SupRef = sup_ref()
> > ChildSpec = child_spec() | (List :: [term()])
> > ...
> > ...
> > 
> > ...If the case of a simple_one_for_one supervisor, the child
> > specification defined in Module:init/1 will be
> > used and ChildSpec should instead be an arbitrary list of terms
> > List.
> > The child process will then be started by
> > appending List to the existing start function arguments, i.e. by
> > calling apply(M, F, A++List) where {M,F,A}
> > is the start function defined in the child specification."
> > 
> > Here IMHO lies the error, as A is a list we have  [any()...] ++
> > [any()...] is a list [any()...,any()...] and thats get us an apply
> > call of arity > 1 ALWAYS even when you chilkdspec has a {M,F,[]}
> > and
> > you try provide a [..] in your start_child it will fail
> > unless both argslists are void.
> > 
> > So a combination of:
> > 
> > init(Opts) ->
> >   io:format("[GROUP SUP] Ready to manage MuC with opts:
> >   ~p\'s\n",[Opts]),
> >                                                                                                                                                                                                                                  
> >    {ok, {
> >          {simple_one_for_one, 1, 60},
> >          [
> >             {mucfsm,      {sim_group_fsm,      start_link,
> >             [arg1,arg2]},  transient, 60, worker, [sim_group]}
> >          ]
> >          }
> >    }.
> > 
> > 
> > and a client that makes a call like:
> > ...
> >     {ok,ChildPid} = supervisor:start_child(SupPid,[arg3,arg4]),
> > ...
> > 
> > Gets you a final apply call {M,F,A} where A is in the form
> > [arg1,arg2,arg3,arg4]
> > not [[arg1,arg2,argf3,arg4]] at is should be when you intent to
> > call
> > a start_link/1....
> > 
> > 
> > 
> > Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4]
> > [async-threads:0] [hipe] [kernel-poll:false]
> > 
> > [QuickCheck] [PropEr] [Erl0MQ]
> > Starting simple_one_by_one group supervisor...
> > [GROUP SUP] staring with argument: [{option1,40}]
> > Eshell V5.9.1  (abort with ^G)
> > 1> [GROUP SUP] Ready to manage MuC with opts: [{option1,40}]'s
> > Adding a child dynamically...
> > {"init terminating in
> > do_boot",{{badmatch,{error,{'EXIT',{undef,[{sim_group_fsm,start_link,[arg1,arg2,arg3,arg4],[]},{supervisor,do_start_child_i,3,[{file,"supervisor.erl"},{line,319}]},{supervisor,handle_call,3,[{file,"supervisor.erl"},{line,344}]},
> > {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,588}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]}}}},[{test,main,0,[{file,"test.erl"},{line,11}]},{init,start_it,1,[]},{init,start_em,1,[]}]}}
> > 
> > 
> > /Angel
> > 
> > On Jueves, 21 de Junio de 2012 09:30:15 Angel J. Alvarez Miguel
> > escribió:
> > > That's right...
> > > 
> > > But start_link it is expected to be or arity 3 and Opts is a
> > > property list.
> > > 
> > > but then on test:main/0 when i do
> > > supervisor:start_child(SupPid,Opts2)
> > > 
> > > being Opts2 another property list, then suppervisor should call:
> > > 
> > > apply(sim_group_fsm,start_link,Opts ++ Opts2) on its line 319
> > > which
> > > in turn
> > > calls gen_fsm:start_link, thus providing a Pid in the end...
> > > 
> > > but what it really does is apply(sim_group_fsm,start_link,Opts ++
> > > Opts2,[
> > > ]) and then crashes unless i declare a start_link/2 on my
> > > gen_fsm...
> > > 
> > > {
> > > 	{badmatch,
> > > 		{error,
> > > 			{'EXIT',
> > > 				{undef,
> > > 					[
> > > 					{sim_group_fsm,start_link,[[{option1,40}],[{option29,100}]],[]},
> > > 					{supervisor,do_start_child_i,3,[
> > > 												{file,"supervisor.erl"},
> > > 												{line,319}
> > > 												]}......
> > > 
> > > i dont see why i finish having a apply/4 call in supervisor: 319
> > > instead of
> > > an apply/3...
> > > 
> > > I want the sup to propagate one property list to children specs
> > > while i
> > > will provide another during children spawns providing that sup
> > > combines
> > > two in any way children will manage to demunge ...both property
> > > lists
> > > 
> > > So cant just see what im doing wrong....
> > > 
> > > /angel
> > > 
> > > 
> > > PS: i changed a bit the test code to force sup crash
> > > 
> > > On Jueves, 21 de Junio de 2012 06:22:54 Vance Shipley escribió:
> > > > On Thu, Jun 21, 2012 at 12:30:08AM +0200, Angel J. Alvarez
> > > > Miguel
> > > > wrote:
> > > > }  While calling a simple_one_by_one supervisor to start a gen
> > > > fsm I
> > > > found }  it causes calling a start_link/2 function on the
> > > > callback
> > > > module of the }  gen_fsm.
> > > > 
> > > > You are specifying that function to be called to start the
> > > > worker
> > > > yourself. In your module group_supervisor.erl you provide a
> > > > child_spec()
> > > > in the
> > > > 
> > > > return value of init/1:
> > > >   {mucfsm, {sim_group_fsm, start_link, [Opts]}, transient, 60,
> > > >   worker,
> > > > 
> > > > [sim_group]}
> > > > 
> > > > The form of which is:
> > > >    {Id, StartFunc, Restart, Shutdown, Type, Modules}
> > > > 
> > > > The second argument, StartFunc, has the form:
> > > >    {Module, Function, Arguments}
> > > > 
> > > > You provided:
> > > > 	{sim_group_fsm, start_link, [Opts]}
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> > 
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