Question

In the examples provided on the Cowboy github, and in some of the other examples I have found on-line, there is a one-to-one supervisor that does not seem to do anything. I even believe that I saw an example that had the following comment, “like a real supervisor doesn’t do anything”.

What is the purpose of the supervisor module that seems to be part of so many cowboy examples?

From the echo_get example:

%% Feel free to use, reuse and abuse the code in this file.

%% @private-module(echo_get_sup).
-behaviour(supervisor).

%% API.
-export([start_link/0]).

%% supervisor.
-export([init/1]).

%% API.
-spec start_link() -> {ok, pid()}.
 start_link() ->    
     supervisor:start_link({local, ?MODULE}, ?MODULE, []).

%% supervisor.
init([]) ->
    Procs = [], 
    {ok, {{one_for_one, 10, 10}, Procs}}.

Answer 1

From erlang application behavior documentation:

start is called when starting the application and should create the supervision tree by starting the top supervisor. It is expected to return the pid of the top supervisor and an optional term State, which defaults to []. This term is passed as-is to stop.

He has this dummy supervisor, so he can call it at the end of the start function here. I think it has no practical purpose except to satisfy this condition.