Testing ExAws.stream! with Mox

Questions / Help
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1

I’ve been working with a function that looks similar to the one that I’ve described below. It works adequately in practice but trying to unit test it has been a challenge to this point.

lib/my_module.ex

defmodule MyApp.MyModule do
  def my_func do
    "my-s3-bucket"
    |> ExAws.S3.list_objects_v2(prefix: "path/to/files")
    |> ExAws.stream!()
    |> Enum.map(&do_work/1)
  end
end

I’ve currently set up ExAws like the code samples below for mocking out ExAws.request! calls.

config/test.exs

config :my_app, :ex_aws,
  http_client: ExAws.Request.HttpMock

test/support/mocks.ex

Mox.defmock(ExAws.Request.HttpMock, for: ExAws.Request.HttpClient)

If anyone has suggestions for how to make the Mox.expect/4 call here properly set up the ExAws.stream! call, that would be greatly appreciated.

test/my_module_test.exs

defmodule MyApp.MyModuleTest do
  use ExUnit.Case, async: true

  import Mox

  setup :verify_on_exit!

  describe "my_func/0" do
    test "mocks api call(s)" do
      # how do we mock the stream?
      expect(ExAws.Request.HttpMock, ..., fn ..., ... ->
        # unsure
      end)

      assert [_ | _] = my_func()
    end
  end
end
2

To mock at the HTTP layer you need to return a response shape that is the same as what AWS itself returns. the stream function is just making repeated object list calls with a cursor, so you’ll need to return that shape.

I don’t however recommend mocking at this layer. You’ll spend more time trying to make ExAws internals work. Rather I would make an internal FileStore behavior in your app and then have a mock of that which can return a stream directly.

2 Likes
3

That – but also if you eventually want to run integration tests you can consider pointing it against minio or localstack instances.

2 Likes
4

Thank you for the feedback. Very much appreciated!