Hey there! Let's break down why you're hitting this error after upgrading Phoenix and Phoenix PubSub. The core issue here is how your application is trying to initialize the PubSub process—Phoenix PubSub 2.0 (paired with Phoenix 1.5+) expects a different setup than older versions.

What's Causing the Error? #

The error message tells us your AppName.Application.start/2 function is returning {Phoenix.PubSub, [name: AppName.PubSub, adapter: Phoenix.PubSub.PG2]} instead of a valid return value (like {:ok, pid} or :ok). That's because Phoenix PubSub 2.0 needs to be started as part of your application's supervision tree, not directly returned from the start function.

Step-by-Step Solution #

1. Update your lib/app_name/application.ex file
   Open your application module and adjust the start/2 function to use a Supervisor to manage all child processes (including PubSub). Here's the correct structure:

   defmodule AppName.Application do
     use Application
   
     def start(_type, _args) do
       # List all child processes to be supervised
       children = [
         # Start the PubSub system
         {Phoenix.PubSub, name: AppName.PubSub, adapter: Phoenix.PubSub.PG2},
         # Include your Phoenix Endpoint (if present)
         AppNameWeb.Endpoint
         # Add other child processes here as needed
       ]
   
       # Start the supervisor with the child processes
       opts = [strategy: :one_for_one, name: AppName.Supervisor]
       Supervisor.start_link(children, opts)
     end
   
     # Rest of your application module code...
   end
   

   The key fix is wrapping your PubSub configuration in the children list and starting it via Supervisor.start_link/2, which returns the valid {:ok, pid} value Mix expects.

2. Double-check your dependencies
   Confirm your mix.exs has the correct version specifications:

   defp deps do
     [
       {:phoenix, "~> 1.5.9"},
       {:phoenix_pubsub, "~> 2.0"}
     ]
   end
   

   Run mix deps.get to ensure all dependencies are properly installed and updated.

3. Clean and restart your server
   Old build artifacts can sometimes cause unexpected issues. Run these commands to reset things:

   mix clean
   mix deps.compile
   mix phx.server
   

Why This Works #

Phoenix PubSub 2.0 shifted to using Elixir's built-in Supervisor system for process management, whereas older versions might have allowed direct initialization. By adding PubSub to your supervision tree, you ensure it's started and managed correctly alongside your other application processes, which resolves the invalid return value error.

内容的提问来源于stack exchange，提问作者Ajay Anarse