Return ReadableStream from form action

[robertutterback]

Describe the problem

We can return a ReadableStream from an endpoint in +server.ts, but trying to do the same from a form action in +page.server.ts makes Sveltekit complain that the data returned is not serializable.

I might be missing a reason why this isn't a good idea, but it would be convenient for me. In my case I allow the user to upload many files, some of which might be quite large. I would like to push status messages back to the user as the files are written on the server.

Describe the proposed solution

Allow returning ReadableStream from form actions.

Alternatives considered

I could just use an API route, but then to handle the streamed response I need to use browser-side fetch to submit, which means users without JS can't upload files (I think?).

In an ideal world, users without JS can still upload files, they just can't see any progress updates.

Importance

nice to have

Additional Information

No response

[LorisSigrist]

If I understand you correctly, you want to return a ReadableStream in order to start sending a response before the request has fully arrived. Full duplex http. By the spec that should work, but unfortunately none of the current browser support it (Source).
Even if you manage to send the response before the request has finished, the client side JS will not receive it until it has. I really wish they supported this.

However, if you want to send incremental status updates after the request has fully arrived, that should work.
The question then is, how would SvelteKit handle the ReadableStream on the client, specifically with the form ActionData? I don't have a good answer to that. And I don't think anything should be implemented here unless we have a good answer to that.

If you want to use streams, it's probably best to use a POST endpoint in a +server.js, point the form to that and manually handle the request & response.

[robertutterback]

Thanks for the response!

Full duplex http. By the spec that should work, but unfortunately none of the current browser support it (Source).

Interesting, I didn't realize that wasn't supported. I'll have to adjust my approach. I might end up doing separate POSTs for each file.

However, if you want to send incremental status updates after the request has fully arrived, that should work.

Not as useful for me, but there are probably other use cases where that is useful. Overall it's just a convenience feature.