I’m not that experienced with Elixir and I’m totally new to Gmail API. Also the post is pretty long but I tried to give as much info as possible.

My situation is the following: I have an Elixir application (MyFirstApp), written by a previous developer, and it works fine and it successfully sends emails via Gmail API using Bamboo (). Now I need to add email sending logic to another Elixir application (MySecondApp) and I’m trying to understand how to migrate the email sending code from the first to the second application.

The way I understand how the email works with MyFirstApp is this:

In mix.exs Bamboo and Bamboo gmail adapter are listed as dependencies

      {:bamboo, "~> 1.3"},
      {:bamboo_gmail, "0.2.0"},

…and bamboo app is listed as an “extra_application”

def application do
    [
      extra_applications: [
         ...
        :bamboo,
         ...
      ],
      ...
    ]
  end

There is a Mailer module which uses Bamboo:

defmodule MyFirstApp.Mailer do
  use Bamboo.Mailer, otp_app: :myfirstapp
end

There is a goth configuration that looks like this:

config :goth,
  json: "priv/email/service.json" |> File.read!()

…and the above goth.json file looks like this:

{
  "type": "service_account",
  "project_id": ***,
  "private_key_id": ***,
  "private_key": ***,
  "client_email": ***,
  "client_id": ***,
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": ***,
  "universe_domain": "googleapis.com"
}

…and then the emails are sent using MyFirstApp.Mailer.deliver_now.

My understanding is that Bamboo starts :goth app, which does all the Oauth work and provides the access token to the Bamboo Gmail adapter, the email gets sent and everybody is happy.

Please, correct me if I’m wrong!

Now I have to add email sending logic that uses exactly the same configuration to a new application, MySecondApp. The problem is that this new application is built with a different set of libraries and if I add Bamboo as a dependency I’m getting some library conflicts because of the Bamboo Gmail adapter version depending on an older version of some library (I don’t remember exactly which one). Please note that Bamboo Gmail Adapter doesn’t seem to be actively maintained and the last commit was 4 years ago. For this reason I decided to migrate the email sending code to use Swoosh so I did the following:

In mix.exs I listed Swoosh ( GitHub - swoosh/swoosh: Compose, deliver and test your emails easily in Elixir) as a dependency:

      {:swoosh, "~> 1.17"}

I defined the mailer module:

defmodule MySecondApp.Mailer do
  use Swoosh.Mailer, otp_app: :mysecondapp
end

…and I’m trying to send the emails (as explained here: Swoosh.Adapters.Gmail — Swoosh v1.18.2)

new()
|> to("test_email@example.com")
|> subject("Hello!")
|> text_body("some email body")
|> Mailer.deliver(access_token: gmail_access_token)

Now I guess I should obtain the gmail_access_token. Remember that I have a configuration file that works fine with Bamboo (see goth.json above) and I need to use the same account so I guess I should use Goth (GitHub - peburrows/goth: Elixir package for Oauth authentication via Google Cloud APIs) in MySecondApp and I add goth to my deps:

      {:goth, "~> 1.4"}

…and start Goth in my supervisor:

  scopes = [
      "https://www.googleapis.com/auth/gmail.send",
      "https://www.googleapis.com/auth/gmail.compose",
      "https://www.googleapis.com/auth/gmail.readonly"
    ]
    |> Enum.join(" ")

    credentials = "/path/to/goth.json" |> File.read!() |> Jason.decode!()

    source =
      {
        :service_account,
        credentials,
        [
          claims: %{
            "sub" => ***,
            "scope" => scopes
          }
        ]
      }

    [
     ...
      {Goth, name: MySecondApp.Goth, source: source}
    ]
    |> Supervisor.start_link([strategy: :one_for_one, name: MySecondApp.Supervisor])

…and then obtain the token like this, in order to use it in the above:

    {:ok, gmail_access_token} = Goth.fetch(MySecondApp.Goth)

…but when I do that I get the following error:

{:error, %RuntimeError{message: "unexpected status 401 from Google\n\n{\n  \"error\": \"unauthorized_client\",\n  \"error_description\": \"Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested.\"\n}\n"}}

The credentials are certainly fine because I’m using exactly the same goth.json from MyFirstApp so I don’t understand what’s the difference between the way I’m using the Goth configuration in the two applications.

Now I’m completely stuck and I would highly appreciate any pointers.

Thanks a lot for your help!