Best practices for unit testing with SSE events?

[GabrielBianconi]

Summary

I'm trying to write unit tests for a function that writes axum::response::sse::Event's to a channel used by Sse. However, Event.buffer is private and I can't find another way to retrieve the event's contents to ensure it was generated correctly.

What is the best practice for testing this?

Here's a toy example - I want to write a unit test for mini_worker that checks that the events are sent correctly:

async fn mini_handler() -> Result<Sse<impl Stream<Item = Result<Event, Infallible>>>, Error> {
    let (client_sender, client_receiver) = mpsc::unbounded_channel();

    tokio::spawn(mini_worker(client_sender));

    Ok(Sse::new(UnboundedReceiverStream::new(client_receiver))
        .keep_alive(axum::response::sse::KeepAlive::new()))
}

async fn mini_worker(
    client_sender: mpsc::UnboundedSender<Result<Event, Infallible>>,
) -> Result<(), Error> {
    let event = Event::default()
        .json_data(json!({"some": "stuff"}))
        .unwrap();

    client_sender.send(Ok(event)).unwrap();

    let event2 = Event::default()
        .json_data(json!({"more": "things"}))
        .unwrap();

    client_sender.send(Ok(event2)).unwrap();

    Ok(())
}

axum version

0.7.4

[jplatte]

The SSE example includes tests, are thise helpful to answer your question maybe?

[GabrielBianconi]

Hi @jplatte,

In practice that's how we did it. But the example is more like an integration test since you have to launch the server and call the entire handler. I was hoping to test just the spawned task (mini_worker in the example above) in isolation with a unit test, and ensure the stream or channel is constructed correctly in isolation. But that doesn't seem possible since Event.buffer is private?

Thanks!

[GabrielBianconi]

@jplatte Would you consider a PR that somehow exposes an Event's buffer and possibly flags? Happy to look into it.

axum/axum/src/response/sse.rs

Lines 160 to 163 in b03f6c1

	pub struct Event {
	buffer: BytesMut,
	flags: EventFlags,
	}

[jplatte]

Hey, this has been sitting in my notifications backlog for a while. I know next to nothing about SSE myself, but if you still care about this and don't want it to be forgotten, can you make a new issue for this feature request? Thanks.

[GabrielBianconi]

Hi @jplatte - this isn't a blocker for us as we are able to test the events by instantiating a dummy Router and parsing the streamed events from the response. That said, that adds a lot of boilerplate and making those fields public would simplify that a lot, so I think that'd be helpful DX for others in the future as well!