TL;DR; Elixir is a compiled language and as such, every information needed for the code to compile needs to be available at, well…, compilation time. If your application depends on the configuration to run, you’ll need to make it available before releasing it.

Env vars why you no work

The way to env vars

I’m writing a small, barebones, web services using elixir-plug and here are the main mistakes I made trying to specify the Router’s HTTP port using environment variables

Mistake #1: Using the wrong key in the configuration file

My application is called Closeconnection and that was my configuration file.

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use Mix.Config

config Closeconnection, port: 3000

Whenever I tried to access the value of port using, Application.get_env(:closeconnection, :port) I’d get back nil as return. Once I changed to config :closeconnection, port: 3000 it worked 🎉!

Mistake #2: Env vars are always String

That’s the code to start the server:

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Plug.Adapters.Cowboy.child_spec(
  :http,
  Router,
  [],
  port: Application.get_env(:closeconnection, :port)
)

The Application.get_env would get the information from the config and the config would get the information from the system with System.get_env, so far so good.

$ PORT=4444 mix run
💥

The problem here is that the function Plug.Adapters.Cowboy.child_spec/4 expects port to be an integer but System.get_env (and in turn Application.get_env) return String. After casting it to integer with String.to_integer(Application.get_env(:closeconnection, :port)) it worked 🎉!

Mistake #3: Declaring the Env variable only at Run time

I wanted to get the value of port from the command line and not “hard coded” in a configuration file. Luckily, that’s not so hard to do, I just changed my configuration file to the following:

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use Mix.Config

config :closeconnection, port: System.get_env("PORT")

I tested the app, ran it locally with PORT=4444 mix run and all seemed to work. Life was good again. I, then, went ahead and bundled the app in a binary using distillery which allows the app to run in any machine with erlang installed. Cool!

$ mix release
...
$ ./build/dev/rel/closeconnection start
💥

The app wouldn’t start. After some debugging I found the PORT was nil. But why!? That’s when remembering that Elixir is a compiled language comes in handy. My app relied in that piece of information (the port) to run, but not to compile, so I needed to pass it during compilation time.

$ PORT=4444 mix release
...
$ ./build/dev/rel/closeconnection start
🎉

To recap

  1. Check if config you specified has the same key in the configuration file and in the Application.get_env
  2. Values coming from System.get_env are always String (or nil if not found)
  3. Env variables must be available at compilation time

Did you have a similar experience with Elixir? How do you manage your env variables? Let me know in the comments below!