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

Robert Virding <>
Thu Jun 28 14:49:23 CEST 2012


I agree on the documentation. It is also tricky to teach. :-)

I honestly have no idea how often simple_one_for_one is actually used. Or if people use the other restart types and use start_child/2 with a child spec. It would be interesting to find out.

Robert

----- Original Message -----
> 
> How much is used this feature? How many people use it ?
> 
> I dont think so many, after  a couple of greps along my installed OTP
> packages i would say i dont think anyone  is using
> this simple_one_for_one feaure as all MFA's in child specs seem to
> have the A part as [ ] so the args count in fixed by what you
> put on start_child/2. The best i cant say in my limited knowledge is
> that this feature is nullified buy this fact so apparently almost
> nobody is using it, hence the dumb patch i did.
> 
> However i didnt meant having all callbacks as init/1 but in this case
> provided that start_child can have arbitrary args list
> this would make your posible callbacks arity any, at any time, upon
> what you put on the args.
> 
> this is error prone at least
> 
> In the best case scenario you can pass both args surrounded in list
> so you get a Module:init/2 callback to be called.... better
>  than nothing.
> 
> Do in the end maybe it is worth check if spec args are [] (the usual
> case) and keep doing a simple ++ but in other case do a better list
> joint
> and keep your callback arity sane as documented...
> 
> I think it deserve at least be documented so other guys dont get
> stuck with this on the future .
> 
> 
> /Angel
> 
> On Miércoles, 27 de Junio de 2012 14:41:58 usted escribió:
> > 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
> 
> Ok ok, at least im not completely wrong, ill try to overcome this
> with other
> approach like passing a pid of something else so children can ask
> some adult
> whatever they know about their new playground.
> 
> Maybe things like this deserve a place in a TODO list for a future
> rewrite or
> a OTP next-generation saga....
> 
> As always the good side of the history is a better understanding on
> the inner
> working of the OTP beats.
> 
> Thanks Angel
> > 
> > ----- 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,[arg
> > > 1,arg2,arg3,arg4],[]},{supervisor,do_start_child_i,3,[{file,"supervisor.e
> > > rl"},{line,319}]},{supervisor,handle_call,3,[{file,"supervisor.erl"},{lin
> > > e,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]}
> > > 
> > > _______________________________________________
> > > erlang-questions mailing list
> > > 
> > > http://erlang.org/mailman/listinfo/erlang-questions
> 



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