How to unit test?

[tigerinus]

I've build a simple app using fx. It's a simple New-Provide-Invoke structure:

	app := fx.New(
		fx.Provide(service.NewServiceA),
		fx.Provide(service.NewServiceB),
		fx.Invoke(runHTTPListener),
	)

...where ServiceB depends on ServiceA.

I find that https://github.com/uber-go/fx/tree/master/fxtest is somewhat hard to follow, but I'd like to know that in the unit test for ServiceB, how do I inject ServiceA properly?

Thanks.

[abhinav]

To unit test just ServiceB, you can write a unit test that calls NewServiceB directly—like a regular function.

func TestServiceB(t *testing.T) {
  mockServiceA := // ...
  NewServiceB(mockServiceA)
  // ...
}

However, to test ServiceB integration with Fx, you can use fxtest.
For this, you need to either duplicate the list of arguments to fx.New, or extract them into a helper function.
Internally, most of our applications have a helper opts() fx.Option function that looks like this:

func opts() fx.Option {
  return fx.Options(
    fx.Provide(service.NewServiceA),
    fx.Provide(service.NewServiceB),
    fx.Invoke(runHTTPListener),
  )
}

func main() {
  app := fx.New(opts()...)
  // ...
}

With this, you can test a few different things.

Verify dependencies

You can test just that all dependencies are supplied and present with the help of fx.ValidateApp.

func TestDependenciesAreSatisfied(t *testing.T) {
  if err := fx.ValidateApp(opts()); err != nil {
    t.Error(err)
  }
}

We include a similar test in most of our services internally.

Integration test

If you want to write an integration test for any component, you can use opts() directly, and use fx.Replace and fx.Decorate to replace components of the application with mocks or fakes for the integration test.

For example,

func TestServiceB(t *testing.T) {
  app := fxtest.New(t,
    opts(),
    fx.Decorate(newFakeServiceA), // returns ServiceA
  )

  // Starts the app right away, and defers a stop when the test ends.
  defer app.RequireStart().RequireStop()

  // TODO: make a request to the app
}

In this example, we replace the ServiceA consumed by ServiceB with a fake so that the real ServiceA is not instantiated.

[tigerinus]

Great. Do I replace fx.Invoke(runHTTPListener) with some test method? Where do I put the asserts?

[abhinav]

You cannot replace invokes. If there's need of replacing invokes, the service may be too tightly coupled to the invoke, in which case you should consider unit testing the function directly.

Asserts should go after the defer app.RequireStart().RequireStop() line if you're using fxtest. Depending on what you're testing, that's where you'll verify the behavior of your system. We usually use fxtest with replace for integration tests (with fakes for external systems, storage, etc.) and then test the behavior of the running service.