How to test with application env in async: true

It is quite common for Elixir application to be configured via application env (which is set in config/ files and accessed with Application.get_env and such). However, this application env is global and therefore is very difficult to get right when testing with use ExUnit.Case, async: true.

This article describes how to overcome this problem with Repatch library

Problem

You have a configuration which controls a feature-flag in config/config.exs

import Config
config :my_app, :features,
  lucky_feature_enabled: System.get_env("LUCKY_FEATURE") == "1"

Which controls this simple code

defmodule EightBall do
  def ask(_question) do
    if Application.get_env(:my_app, :features)[:lucky_feature_enabled] do
      Enum.random([:absolutely, :sure, :definitely])
    else
      Enum.random([:no, :no_way, :definitely_not])
    end
  end
end

And you have these tests

test "Enabled feature" do
  Application.put_env(:my_app, :features, lucky_feature_enabled: true)
  assert EightBall.ask("Is it awesome?") in [:absolutely, :sure, :definitely]
end

And

test "Disabled feature" do
  assert EightBall.ask("Is it awesome?") in [:no, :no_way, :definitely_not]
end

Testing with async: true won’t be possible, because if the test of enabled feature test is executed before the disabled test, env will be set and the second test will fail. And executing tests with async: false reduces performance of testing and requires manual cleanup of application env after the test completion.

Solution

Repatch library provides a simple solution to the problem.

1. First, install it with this code in mix.exs file:

   def deps do
     [
       {:repatch, "~> 1.2"}
     ]
   end
   

   Then mix deps.get

2. Setup Repatch with this code in test/test_helper.exs file:

   ExUnit.start()
   Repatch.setup()
   Repatch.Application.setup()
   

3. And just add this into your test file and env will be isolated for each test.

   use Repatch.ExUnit, isolate_env: :local
   

Voila

Now these tests are safe to be used with async: true testing mode. Repatch is a powerful and small library which makes testing in Elixir very simple. Please check out the docs for all available features: