Asynchronous testing in Elixir with Mox

29 July, 2018

Testing in Elixir is pretty neat. You can easily test anything written, you have instruments like ExUnit or ESpec, you can practice TDD and more and more. Functional programming paradigm helps you a lot to make your testing even simpler by forcing you to use pure, small functions that will be pleasant to test. Concurrency of Elixir allows you to run all your tests asynchronously and blazingly fast.

However, there can be some pitfalls.

Mocks problem

If you’re programming a big project then I suppose that you will be using third-party services anyway. Imagine that you are sending SMS through you favorite provider services. This code can look like that:

SmsProvider.send_sms(from, to, "message")

In this case you are using some external provider API in order to send SMS. The HTTP request will go from your application to the destination and it will trigger SMS delivery.

Now we need to test this situation. This will be our very first approach:

assert {:ok, %Message{}} = SmsProvider.send_sms(from, to, "message")

As you can see it will behave as if it was on production. Your SMS with the nasty fake test message will be delivered to non-existent users with absent phone numbers. Sad!

The most popular solution is mocking. You’re supposing that call of send_sms function with the expected arguments which will return the expected result:

mock(SmsProvider, :send_sms, fn _, _, _ -> {:ok, %Message{status: :sent}} end)
assert {:ok, %Message{status: :sent}} = SmsProvider.send_sms(from, to, "message")

This approach will work, at least for this test. The problem is simple enough - it’s not the best solution for asynchronous tests. We have mocked SmsProvider so it will be overriden in every next asynchronous test:

use MyApp.DataCase, async: true

# ...

assert {:error, :wrong_phone_number} = SmsProvider.send_sms(wrong_number, to, "message")

This test will fail because send_sms call was already mocked in another test earlier. The entire mocking approach is not suitable for concurrent testing, so we need to find another one to serve this purpose.

Asynchronous testing

Instead of mocking we can try calling the function that will override sms_send/3. Let’s create a TestProvider module with the next content:

defmodule TestProvider do
  def send_sms(from, to, message) do
    {:ok, %Message{status: :sent, from: from, to: to, text: message}}

Now we can try to use this module as an adapter in our SmsProvider module. It will use default adapter in development, production environments and will use TestProvider in testing:

# config/test.exs
config :my_app, SmsProvider, adapter: TestProvider

# config/config.exs
config :my_app, SmsProvider, adapter: SmsApiService

# sms_provider.ex
defmodule SmsProvider do
  @adapter Application.fetch_env(:my_app, :sms_provider, :adapter)

  defdelegate send_sms(from, to, message), to: @adapter

Let’s go straight to the test:

assert {:ok, %Message{}} = SmsProvider.send_sms(from, to, "message") # true

Now it should work even in concurrent tests. Your SMS will not be delivered neither to the real nor fake users, your money will be saved and your tests will not suffer more.

Using Mox

However, there is still a room for improvement.

Mox is a relatively new library that solves the issue of concurrent testing and mocks. It follows the next principles:

Now let’s add it to our dependencies list:

# mix.exs
def deps do
    {:mox, "~> 0.4", only: :test}

It’s easy as a pie to rewrite our tests with Mox. We need to create an SMS provider behaviour and implement it for different environments:

# sms_provider.ex
defmodule SmsProvider do
  @callback send_sms(String.t, String.t, String.t) :: {:ok, %Message{}} | {:error, :wrong_number}

# test_helper.exs
Mox.defmock(SmsProviderMock, for: SmsProvider)

# test.exs

defmodule Test do
  use ExUnit.Case, async: true

  import Mox

  # Make sure mocks are verified when the test exits
  setup :verify_on_exit!

  test "returns message on success" do
    expect SmsProviderMock, :send_sms, fn _, _, _ -> {:ok, %Message{status: :sent}}
    assert {:ok, %{status: :sent}} = SmsProvider.send_sms(from, to, message)

If you don’t need to check expectations in test you can try stub_with/2 function in order to stub entire module:

# sms_provider.ex
defmodule SmsProvider do
  @callback send_sms(String.t, String.t, String.t) :: {:ok, %Message{}} | {:error, :wrong_number}
  @callback sent_sms(String.t) :: [%Message{}]

# test_provider.ex
defmodule TestProvider do
  @behaviour SmsProvider
  def send_sms(_from, _to, message), do: {:ok, %Message{status: :sent, message: message}}
  def sent_sms(number) :: [%Message{}]

# test.exs
defmock(SmsProviderMock, for: SmsProvider)
stub_with(SmsProviderMock, TestProvider)

Viola! Now your tests are all green, fast and shiny thanks to concurrent testing and Mox :)


If you’re definitely interested in concurrent testing, you might want to read excellent article from Jose Valim and dive in into Mox documentation.

Happy hacking, everyone!