I trust you’ve solved this issue by now (or given up!!).

For anyone else that finds this, I’ve written up the complete steps to deploy to Google Cloud Run + Cloud SQL here: https://www.stibbard.io/elixir-phoenix-deploy-google-cloud-run-and-cloud-sql

The answer to your specific question:

• What was previously contained in config/prod.secret.exs was moved into config/runtime.exs
• The default Ecto connection in a fresh mix phx.new project_name is in the form of ecto://USER:PASS@HOST/DATABASE however Cloud SQL expects a socket connection, not a domain (e.g., HOST) so you need to update your config/runtime.exs. Refer to Ecto.Adapters.Postgres — Ecto SQL v3.14.0 – the one you want is :socket. An example updated config could be like below, which includes moving the values into environment variables/secrets.

# do this for each secret. e.g., socket, db, user, pass
database_password =
  System.get_env("DATABASE_PASSWORD") ||
    raise """
    environment variable DATABASE_PASSWORD is missing.
    """

config :insight, Insight.Repo,
  socket: database_socket,
  database: database_name,
  username: database_user,
  password: database_password,
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
  socket_options: maybe_ipv6

• In your deployment you will need to populate those environment variables. In my writeup I did this in Google Secrets Manager
• You will also need to sort IAM permissions to allow your server to talk to Cloud SQL, and this will vary depending on what service you deployed your app to AND what Service Account you used. In my writeup I created a new Service Account, and we deployed to Google Cloud Run