Background I am trying to build an OTP compliant app using supervisors. However I am having trouble interpreting a part of the documentation. Docs The documentation states that Supervisor.start_link/2 returns the following: {:ok, pid} | :ignore | {:error, {:already_started, pid} | {:shutdown,ter…

When the supervisor itself (it’s init/1) returns :ignore then Supervisor.start_link returns :ignore. E.g. defmodule MyApp.Supervisor do # Automatically defines child_spec/1 use Supervisor def start_link(init_arg) do Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__) end …

If I understand it correctly in module-based supervisors .

Yes, but those use Supervisor.start_link/3 and Supervisor.init/2. In short, they don’t use Supervisor.start_link/2.

Supervisor — Elixir v1.20.2 If init/1 returns :ignore then start_link will return :ignore. This is useful if you want to selectively start certain parts of your supervision tree / workers. It’s explicitly documented here GenServer — Elixir v1.20.2

The init/1 callback is allowed to return {:ok, {:supervisor.sup_flags(), [:supervisor.child_spec()]}} | :ignore [Erlang] Designing for Scalability with Erlang/OTP (O'Reilly) p.81: If the startup fails but you do not want to affect other processes started by the same supervisor, return ignore. p…

Yes, but how is any of this related to Supervisor.start_link/2? I don’t care what the children may or may not return. It is specified how the Supervisor.start_link/2 function behaves for those scenarios. What I really want to know is when Supervisor.start_link/2 returns :ignore. In what circumsta…

A supervisor has an init/1 callback as well as workers have. So both actually work the same in regards to returning :ignore from that callback.

Forgive me for my ignorance, but I still don’t see how any of this affects the return value of Supervisor.start_link/2 If a child returns an error, the supervisor returns one. If a child returns :ok or :ignore, the supervisor returns :ok. So … when really?

Excellent example! But where is that code is the usage of Supervisor.start_link/2 ? EDIT: ignore me, I get it now. Thanks for the help!

In my opinion it doesn’t really belong to start_link. It’s a behaviour for the init/1 callback, as you can see in the GenServer module, where it’s even mentioned in the documentation text.

Elixir Programming Language Forum

Supervisor.start_link/2 documentation incomplete?

Questions & Help Questions

peerreynders February 28, 2019, 11:44am 9

A supervisor can be a child of a supervisor.