ServerSentEvents should return headers immediately?

[cheng93]

using System.Runtime.CompilerServices;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/sse", (HttpContext httpContext) =>
{
    async IAsyncEnumerable<string> GenerateEvents(
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        await Task.Delay(TimeSpan.FromSeconds(30));
        var counter = 0;
        while (!cancellationToken.IsCancellationRequested)
        {
            yield return $"Event {counter++} at {DateTime.UtcNow}";
            await Task.Delay(1000, cancellationToken);
        }
    }

    return TypedResults.ServerSentEvents(GenerateEvents(httpContext.RequestAborted));
});

app.Run();

When subscribing to an SSE endpoint. Events will not always be emitted immediately.

The above example simulates this by waiting 30 seconds before emitting.

Currently, no response is sent until the first item is yielded.

However, should we not emit the headers immediately? and thus returning a response immediately too.

[hamedrabah]

The observed behavior follows the current implementation. ServerSentEventsResult.ExecuteAsync sets the SSE headers and disables buffering, then waits for SseFormatter.WriteAsync. It does not explicitly call Response.StartAsync(), so the transport may not commit the headers until the first event is written.

For the simple case, yield an initial event or heartbeat before the 30-second delay:

async IAsyncEnumerable<string> GenerateEvents()
{
    yield return "connected";
    await Task.Delay(TimeSpan.FromSeconds(30));
    // subsequent events
}

If the connection must start without an event, use a manual endpoint/result: set the headers, disable buffering, call await Response.StartAsync(), then write the SSE frames. Do not call StartAsync() in the handler and then return TypedResults.ServerSentEvents, because that result still needs to set its headers during execution.

[BrennanConroy]

Unfortunately, this behavior is unlikely to change. Applications can set headers at the top of their IAsyncEnumerable, any changes to when we flush headers would be breaking.

[cheng93]

This is a shame, feels unintuitive to not get a response unless an event is emitted.