How to properly implement Server-Sent Events with ASP.NET?

[adamsitnik]

I am back from the leave and trying to catch up with most recent changes in the A2A project. While doing that, I've focused on exception handling.

I've noticed that A2AEventStreamResult.A2AEventStreamResult does not perform any exception handling.

https://github.com/a2aproject/a2a-dotnet/blob/47061171e14510b0b9676b2f26095963436ee1e0/src/A2A.AspNetCore/A2AHttpProcessor.cs#L412-L432

Since it was the first time I was using SSE ever, I've asked Copilot about the best practices:

It has suggested to add a bunch of headers and I can see that some of our repos use it:

https://github.com/dotnet/WatsonWebserver/blob/d3d1c5d3f93ea3be6dda3b56a2da2b6977078b2c/src/WatsonWebserver/HttpResponse.cs#L410-L412

aspnetcore/src/SignalR/common/Http.Connections/src/Internal/Transports/ServerSentEventsServerTransport.cs

Lines 33 to 39 in b568259

	context.Response.ContentType = "text/event-stream";
	context.Response.Headers.CacheControl = "no-cache,no-store";
	context.Response.Headers.Pragma = "no-cache";
	// Make sure we disable all response buffering for SSE
	var bufferingFeature = context.Features.GetRequiredFeature<IHttpResponseBodyFeature>();
	bufferingFeature.DisableBuffering();

@DamianEdwards should we also do that? And moreover, should we also ignore OperationCanceledException and treat it as end of streaming as suggested by LLM? And emit error and close events as it suggested below?

@DamianEdwards I was not able to find any high quality docs about best practices for that which makes it impossible for me to say whether LLM is hallucinating or telling the truth.

[adamsitnik]

cc @SergeyMenshykh @darrelmiller

[BrennanConroy]

You should consider just using TypedResults.ServerSentEventsResult(...). Or if you don't want to for some reason, you can at least look at the code:
https://source.dot.net/#Microsoft.AspNetCore.Http.Results/ServerSentEventsResult.cs,f5ecd8fa478c4d9b,references

[adamsitnik]

You should consider just using TypedResults.ServerSentEventsResult(...). Or if you don't want to for some reason, you can at least look at the code: https://source.dot.net/#Microsoft.AspNetCore.Http.Results/ServerSentEventsResult.cs,f5ecd8fa478c4d9b,references

We target .NET 8 and 9, but it's exactly what I was looking for. Thank you @BrennanConroy !