Fl4m3Ph03n1x
Supervisor.start_link/2 documentation incomplete?
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,term} | [term]}
Problem
The problem is that even though the documentation specifies when start_link returns {:ok, pid} and {:error, any} it doesn’t specify when it returns :ignore, leaving me to think that it can never return :ignore.
Question
In which cases does Supervisor.start_link/2 return :ignore?
Marked As Solved
LostKobrakai
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
@impl true
def init(_init_arg) do
:ignore
end
end
Supervisor.start_link(MyApp.Supervisor, [])
# :ignore
Also Liked
LostKobrakai
It surely can if at least for the Supervisor.start_link/3 version, but skipping the optional options, which are the 3rd parameter.
LostKobrakai
https://hexdocs.pm/elixir/Supervisor.html#c:init/1
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 https://hexdocs.pm/elixir/GenServer.html#c:init/1
peerreynders
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.178
The
init/1callback function normally returns the whole tuple comprising the restart tuple and a list of child specifications. But if it instead returnsignore, the supervisor terminates with reasonnormal.
LostKobrakai
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.
sasajuric
I believe you actually got it right initially. It seems that Supervisor.start_link/2 can’t return :ignore (unless I’m missing some nuance, since I didn’t bother to read the code of Supervisor). Even the docs for start_link/2 don’t mention it.
You can consider making a PR which would tighten the specification for start_link/2.
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