Environment Variables on Heroku and Mailgun Problems with Phoenix Framework

Question

I was following this guide on deploying to Heroku and this one for sending email.

Everything works fine in development. My variables are set in Heroku:

heroku config
...
MAILGUN_DOMAIN:  https://api.mailgun.net/v3/xxxxxx.mailgun.org
MAILGUN_KEY:     key-3-xxxxxx
...

And loaded from the config files like so:

config :take_two, Mailer,
    domain: System.get_env("MAILGUN_DOMAIN"),
    key: System.get_env("MAILGUN_KEY")

However when I try to send email on Heroku when the Mailgun config is set from environment variables I get this error:

** (FunctionClauseError) no function clause matching in IO.chardata_to_string/1
    (elixir) lib/io.ex:346: IO.chardata_to_string(nil)
    (elixir) lib/path.ex:467: Path.join/2
    (elixir) lib/path.ex:449: Path.join/1
             lib/client.ex:44: Mailgun.Client.send_without_attachments/2

This happens when the domain is not set for the Mailgun Client. But it is supposed to be set from the environment variable. I made a simple module to test:

defmodule TakeTwo.Mailer do
    require Logger

    use Mailgun.Client,
        Application.get_env(:take_two, Mailer) 

    def blank_shot do
        Logger.info Application.get_env(:take_two, Mailer)[:domain]
        Logger.info Application.get_env(:take_two, Mailer)[:key]
        send_email from: "[email protected]", to: "[email protected]", subject: "Hello", text: "This is a blank shot"
    end

When I run TakeTwo.Mailer.blank_shot I see the correct domain/key variables logged followed by the error. I am not sure how to debug the Mailgun client remotely.

Finally, if I recreate the above module in the shell (after running heroku run iex -S mix) it works just fine!?

I feel like when the original module is being loaded perhaps the environment variables have yet to be loaded??

Answer 1

The answer was a little buried in a comment so I wanted to make it easier to find. As the other answer mentions, the environment variables aren't available, but the buildpack lets you configure them to be:

I created a elixir_buildpack.config file and added the following:

config_vars_to_export=(DATABASE_URL MAILGUN_DOMAIN MAILGUN_KEY SECRET_KEY_BASE)

Answer 2

The environment variables aren't available at build time. I had the same issue and decided to get rid of the macro carrying the configuration. You can use this patch to move on.