Hi!

I’m writing an Elixir application with a DynamicSupervisor. Once the application has started, I’d like to add some children to this dynamic supervisor, but I receive an error when calling start_child():

** (Mix) Could not start application backend: exited in: Backend.Application.start(:normal, [])
    ** (EXIT) exited in: GenServer.call(Backend.Crawler.CrawlerSupervisor, {:start_child, {{Backend.Crawler.Server, :start_link, [[domain: "mastodon.social"]]}, :permanent, 5000, :worker, [Backend.Crawler.Server]}}, :infinity)
        ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started

The same error occurs when I run through the steps one at a time in iex.

I’m confused by this error because I know that the dynamic supervisor CrawlerSupervisor is alive (I can see it in the supervision tree). The GenServer I’m trying to start, Backend.Crawler.Server, does exist – but why would it need to be alive before I call start_child? Can someone clear this up for me?

Here’s the full code, if it’s helpful.

The application

defmodule Backend.Application do
  @moduledoc false

  use Application
  alias Backend.Crawler.CrawlerSupervisor
  alias Backend.{Instance, Repo}
  import Ecto.Query

  def start(_type, _args) do
    children = [
      Backend.Repo,
      BackendWeb.Endpoint,
      CrawlerSupervisor
    ]

    opts = [strategy: :one_for_one, name: Backend.Supervisor]

    case Supervisor.start_link(children, opts) do
      ok = {:ok, _pid} ->
        start_instance_servers()
        ok

      other ->
        other
    end
  end

  defp start_instance_servers() do
    domains = ["mastodon.social"]
    domains
    |> Enum.each(fn domain -> CrawlerSupervisor.start_child(domain) end)
  end
end

The dynamic supervisor

defmodule Backend.Crawler.CrawlerSupervisor do
  use DynamicSupervisor
  alias Backend.Crawler.Server

  def start_link(_init_arg) do
    DynamicSupervisor.start_link(__MODULE__, name: __MODULE__)
  end

  @impl true
  def init(_opts) do
    DynamicSupervisor.init(strategy: :one_for_one)
  end

  def start_child(domain) do
    spec = {Server, domain: domain}
    DynamicSupervisor.start_child(__MODULE__, spec)
  end
end

The GenServer child process

defmodule Backend.Crawler.Server do
  use GenServer
  import Backend.Crawler.Util
  alias Backend.Crawler.Crawler

  # Client
  def start_link(domain) do
    GenServer.start_link(__MODULE__, domain, name: domain)
  end

  # Server
  @impl true
  def init(domain) do
    schedule_crawl()
    {:ok, domain}
  end

  @impl true
  def handle_cast(:crawl, state) do
    Crawler.run(state)
    schedule_crawl()
    {:noreply, state}
  end

  defp schedule_crawl() do
    interval = get_config(:crawl_interval_mins) * 60_000
    Process.send_after(self(), :crawl, interval)
  end
end