Supervision tree failing to start - erlang

Question

I'm trying to implement something like what is described in this answer, but I am getting errors like what I've included below when I compile the application.
** (Mix) Could not start application workers: Workers.Application.start(:normal, []) returned an error: shutdown: failed to start child: {Workers.UrlSupervisor, 2}
** (EXIT) already started: #PID<0.1034.0>
I'm not sure if I am inherently doing something I'm not allowed to here, or I've just made a little mistake.
For some context here are the supervisors:
defmodule Workers.Application do
# See http://elixir-lang.org/docs/stable/elixir/Application.html
# for more information on OTP Applications
#moduledoc false
use Application
def start(_type, _args) do
import Supervisor.Spec, warn: false
url_workers = 1..100 |> Enum.map(fn (i) -> supervisor(Workers.UrlSupervisor, [i], [id: {Workers.UrlSupervisor, i}, restart: :temporary]) end)
domain_workers = 1..100 |> Enum.map(fn (i) -> supervisor(Workers.DomainSupervisor, [i], [id: {Workers.DomainSupervisor, i}, restart: :temporary]) end)
opts = [strategy: :one_for_one, name: Workers.Supervisor]
Supervisor.start_link(url_workers ++ domain_workers, opts)
end
end
defmodule Workers.UrlSupervisor do
def start_link(id) do
import Supervisor.Spec, warn: false
children = [worker(Task, [&Workers.Url.worker/0], [id: {Workers.Url, id}, restart: :permanent])]
opts = [strategy: :one_for_one, name: Workers.UrlSupervisor]
Supervisor.start_link(children, opts)
end
end
defmodule Workers.DomainSupervisor do
def start_link(id) do
import Supervisor.Spec, warn: false
children = [worker(Task, [&Workers.Domain.worker/0], [id: {Workers.Domain, id}, restart: :permanent])]
opts = [strategy: :one_for_one, name: Workers.DomainSupervisor]
Supervisor.start_link(children, opts)
end
end
And here is one of the workers (they look largely the same).
defmodule Workers.Domain do
def worker do
case Store.Domains.pop do
:empty ->
IO.puts "[Domain] none found, waiting..."
:timer.sleep(1000)
{crawl_id, domain} ->
IO.puts "[Domains] found a domain to check: #{domain}"
case Core.check_domain(domain) do
:error ->
Utils.insert(crawl_id, domain, false)
:registered ->
Utils.insert(crawl_id, domain, false)
:available ->
Utils.insert(crawl_id, domain, true)
end
end
worker()
end
end

Answer 1

In your Workers.Application when starting Supervisors, you'r providing unique ids, but they also should have unique names.
Try adding another keyword, something like name: :"url_supervisor_#{i}":
def start(_type, _args) do
import Supervisor.Spec, warn: false
url_workers = 1..100 |> Enum.map(fn (i) ->
supervisor(Workers.UrlSupervisor, [i],
[id: {Workers.UrlSupervisor, i},
name: :"url_supervisor_#{i}", # Name added here
restart: :temporary])
end)
domain_workers = 1..100 |> Enum.map(fn (i) ->
supervisor(Workers.DomainSupervisor, [i],
[id: {Workers.DomainSupervisor, i},
name: :"domain_supervisor_#{i}", # Name added here
restart: :temporary])
end)
opts = [strategy: :one_for_one, name: Workers.Supervisor]
Supervisor.start_link(url_workers ++ domain_workers, opts)
end