Supervisors and Dynamic Supervisors : Do they interoperate?

eksperimental · May 22, 2022, 6:56pm

Inagurating this section.

While working with the DynamicSupervisor library, it is not clear to me whether I can as supervisor a supervisor create by Supervisor in any of the DynamicSupervisor functions and vice-versa.

The specs in DynamicSupervisor mention that supervisor is a t:Supervisor.supervisor/0, but I have the feeling we should have a t:DynamicSupervisor.supervisor/0 of its own, as they don’t interoperate.

eksperimental · May 22, 2022, 6:58pm

Error message does not help as it returns {:error, {:invalid_child_spec, child_spec}} while the spec is valid, but the issue stems from the interop issue.

al2o3cr · May 23, 2022, 1:05pm

If it existed, a DynamicSupervisor.supervisor() type would have the same values as Supervisor.supervisor() - the possible values are ways to identify an supervisor process, not specific details about the kind of process.

To use a telecom analogy, the type says “this is what patterns of numbers you can dial to reach a remote phone” but doesn’t specify who answers.

Many of the functions in the two modules are nearly identical, but handle arguments a little differently - for instance, start_child:

github.com

elixir-lang/elixir/blob/v1.13.4/lib/elixir/lib/supervisor.ex#L821-L837

      
        
            def start_child(supervisor, {_, _, _, _, _, _} = child_spec) do
              call(supervisor, {:start_child, child_spec})
            end
            
            
def start_child(supervisor, args) when is_list(args) do
              IO.warn_once(
                {__MODULE__, :start_child},
                "Supervisor.start_child/2 with a list of args is deprecated, please use DynamicSupervisor instead",
                _stacktrace_drop_levels = 2
              )
            
            
  call(supervisor, {:start_child, args})
            end
            
            
def start_child(supervisor, child_spec) do
              call(supervisor, {:start_child, Supervisor.child_spec(child_spec, [])})
            end

vs

github.com

elixir-lang/elixir/blob/v1.13.4/lib/elixir/lib/dynamic_supervisor.ex#L317-L330

      
        
            def start_child(supervisor, {_, _, _, _, _, _} = child_spec) do
              validate_and_start_child(supervisor, child_spec)
            end
            
            
def start_child(supervisor, child_spec) do
              validate_and_start_child(supervisor, Supervisor.child_spec(child_spec, []))
            end
            
            
defp validate_and_start_child(supervisor, child_spec) do
              case validate_child(child_spec) do
                {:ok, child} -> call(supervisor, {:start_child, child})
                error -> {:error, error}
              end
            end

Both ultimately result in a GenServer.call with a message shaped like {:start_child, child_spec}.

eksperimental · May 23, 2022, 3:38pm

Yes, but internally, the child spec for DynamicSupervisor is a five-element tuple, and for Supervisor is like the old erlang child spec, with 6 elements IIRC.
The error is cryptic, it says ‘:invalid_child_spec’ and if you call the same function with the same child spec but your suppervisor goes to the local supervisor the the function works fine, since the chld spec was valid.

Having a new type is a way to document that DynamicSupervisor functions expect a DynamicSupervisor and not a regular Supervisor.