I’m working my way through “Phoenix in Action 1.4” for the second time. The instructions have you create an umbrella app with two apps: auction and auction_web, and following the instructions I created the auction_web app with --no-ecto. Unfortunately, --no-ecto seems to be the root cause of continuing PubSub warnings/errors. I was successful squelching the PubSub warnings/errors for a few days while I was working on getting the auction_web app to display a list of items on a web page, but then I moved inside the auction app to create a Postgres database, and when I tried to execute iex -S mix, then PubSub, like a Phoenix, rose from the ashes to bite me again:

...phoenix_apps/second_auction/auction_umbrella $ iex -S mix
...
...
...
> 14:25:30.946 [notice] Application eex exited: :stopped
> ** (Mix) Could not start application auction: Auction.Application.start(:normal, []) returned an error: shutdown: failed to start child: Phoenix.PubSub.Supervisor
>     ** (EXIT) shutdown: failed to start child: Phoenix.PubSub.PG2
>         ** (EXIT) shutdown: failed to start child: Auction.PubSub.Adapter
>             ** (EXIT) exited in: :gen_server.call(Phoenix.PubSub, {:join_local, Auction.PubSub.Adapter, #PID<0.4916.0>}, :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

Am I in the wrong directory? If I change into apps/auction/ and try again, I get:

auction_umbrella/apps/auction$ iex -S mix
Generated auction app
15:13:27.742 [notice] Application auction exited: exited in: Auction.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Phoenix.PubSub was given as a child to a supervisor but it does not exist
            (elixir 1.16.0-rc.1) lib/supervisor.ex:797: Supervisor.init_child/1
            (elixir 1.16.0-rc.1) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.16.0-rc.1) lib/supervisor.ex:783: Supervisor.init/2
            (elixir 1.16.0-rc.1) lib/supervisor.ex:707: Supervisor.start_link/2
            (kernel 9.1) application_master.erl:293: :application_master.start_it_old/4
15:13:27.747 [notice] Application ecto_sql exited: :stopped
15:13:27.747 [notice] Application postgrex exited: :stopped
15:13:27.747 [notice] Application db_connection exited: :stopped
15:13:27.747 [notice] Application ecto exited: :stopped
15:13:27.748 [notice] Application decimal exited: :stopped
15:13:27.748 [notice] Application telemetry exited: :stopped
15:13:27.748 [notice] Application eex exited: :stopped
** (Mix) Could not start application auction: exited in: Auction.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Phoenix.PubSub was given as a child to a supervisor but it does not exist
            (elixir 1.16.0-rc.1) lib/supervisor.ex:797: Supervisor.init_child/1
            (elixir 1.16.0-rc.1) lib/enum.ex:1700: Enum."-map/2-lists^map/1-1-"/2
            (elixir 1.16.0-rc.1) lib/supervisor.ex:783: Supervisor.init/2
            (elixir 1.16.0-rc.1) lib/supervisor.ex:707: Supervisor.start_link/2
            (kernel 9.1) application_master.erl:293: :application_master.start_it_old/4

And, if I switch into apps/auction_web/ I get:

auction_umbrella/apps/auction_web$ iex -S mix
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Interactive Elixir (1.16.0-rc.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> 

I’m using:

Phoenix 1.7.10
Erlang/OTP 26.1.2
Elixir 1.16.0-rc.1-otp-26

Here are all the places where the name PubSub occurs in my umbrella app:

1. apps/auction/lib/auction/application.ex:

defmodule Auction.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  @impl true
  def start(_type, _args) do
    children = [
      # Starts a worker by calling: Auction.Worker.start_link(arg)
      # {Auction.Worker, arg}
      {Phoenix.PubSub, name: Auction.PubSub},
      {Auction.Repo, []}
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Auction.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

2. apps/auction_web/lib/auction_web/application.ex:

defmodule AuctionWeb.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  @impl true
  def start(_type, _args) do
    children = [
      {Phoenix.PubSub, name: AuctionWeb.PubSub},
      AuctionWeb.Telemetry,
      # Start a worker by calling: AuctionWeb.Worker.start_link(arg)
      # {AuctionWeb.Worker, arg},
      # Start to serve requests, typically the last entry
      AuctionWeb.Endpoint
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: AuctionWeb.Supervisor]
    Supervisor.start_link(children, opts)
  end

  # Tell Phoenix to update the endpoint configuration
  # whenever the application is updated.
  @impl true
  def config_change(changed, _new, removed) do
    AuctionWeb.Endpoint.config_change(changed, removed)
    :ok
  end
end

3. auction_umbrella/config/config.exs:

# This file is responsible for configuring your umbrella
# and **all applications** and their dependencies with the
# help of the Config module.
#
# Note that all applications in your umbrella share the
# same configuration and dependencies, which is why they
# all use the same configuration file. If you want different
# configurations or dependencies per app, it is best to
# move said applications out of the umbrella.
import Config

# Stuff I added:

config :auction, ecto_repos: [Auction.Repo]

config :auction, Auction.Repo,
  database: "auction",
  username: "7stud",
  password: "",
  hostname: "localhost",
  port: "5432"

##############################

config :auction_web,
  generators: [context_app: false]

# Configures the endpoint
config :auction_web, AuctionWeb.Endpoint,
  url: [host: "localhost"],
  adapter: Phoenix.Endpoint.Cowboy2Adapter,
  render_errors: [
    formats: [html: AuctionWeb.ErrorHTML, json: AuctionWeb.ErrorJSON],
    layout: false
  ],
  pubsub_server: AuctionWeb.PubSub,
  live_view: [signing_salt: "cOmLEVHn"]

# Configure esbuild (the version is required)
config :esbuild,
  version: "0.17.11",
  default: [
    args:
      ~w(js/app.js --bundle --target=es2017 --outdir=../priv/static/assets --external:/fonts/* --external:/images/*),
    cd: Path.expand("../apps/auction_web/assets", __DIR__),
    env: %{"NODE_PATH" => Path.expand("../deps", __DIR__)}
  ]

# Configure tailwind (the version is required)
config :tailwind,
  version: "3.3.2",
  default: [
    args: ~w(
      --config=tailwind.config.js
      --input=css/app.css
      --output=../priv/static/assets/app.css
    ),
    cd: Path.expand("../apps/auction_web/assets", __DIR__)
  ]

# Sample configuration:
#
#     config :logger, :console,
#       level: :info,
#       format: "$date $time [$level] $metadata$message\n",
#       metadata: [:user_id]
#
import Config

# Configures Elixir's Logger
config :logger, :console,
  format: "$time $metadata[$level] $message\n",
  metadata: [:request_id]

# Use Jason for JSON parsing in Phoenix
config :phoenix, :json_library, Jason

# Import environment specific config. This must remain at the bottom
# of this file so it overrides the configuration defined above.
import_config "#{config_env()}.exs"