SseFormatter.WriteAsync does not flush after writing an event

[erik-kallen]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When running my application in our production environment, our SSE endpoints did not work. The main difference between my dev environment (which works) and prod (which does not) is that in prod we are hosting in IIS.

It can be made to work by doing:

static async IAsyncEnumerable<T> FlushingIterator<T>(IAsyncEnumerable<T> inner, HttpResponse response)
{
    await foreach (var item in inner)
    {
        yield return item;
        await response.Body.FlushAsync();
    }
}
await SseFormatter.WriteAsync(FlushingIterator(GetMyEvents(), Response), Response.Body, cancellationToken);

but I find it unexpected that it does not do this out of the box.

Expected Behavior

The output stream should be flushed immediately after an event is written by the SseFormatter.

Steps To Reproduce

await SseFormatter.WriteAsync(GetMyEvents(), Response.Body, cancellationToken);

Exceptions (if any)

No response

.NET Version

No response

Anything else?

No response

[martincostello]

Looks like this issue should be transferred to dotnet/runtime as it doesn't appear to be specific to ASP.NET Core.

[BrennanConroy]

SseFormatter.WriteAsync calls Stream.WriteAsync, which in the case of Kestrel and IIS should call Pipe.FlushAsync.

Since this is working in Kestrel and not in IIS, that indicates an IIS/ASP.NET Core issue.

Do you have a minimal repro so it'd be easy for us to test this? Does it work in IIS Express but not full IIS?