Server-Sent Events: Simple Real-Time Updates

Server-Sent Events (SSE) provide a simple way for servers to push updates to clients over HTTP. They are ideal for live feeds, notifications, and dashboards.

SSE Endpoint in ASP.NET

[ApiController]
[Route("api/[controller]")]
public class EventsController : ControllerBase
{
    [HttpGet("stream")]
    public async Task StreamEvents(CancellationToken cancellationToken)
    {
        Response.Headers.Add("Content-Type", "text/event-stream");
        Response.Headers.Add("Cache-Control", "no-cache");
        Response.Headers.Add("Connection", "keep-alive");

        while (!cancellationToken.IsCancellationRequested)
        {
            var data = await GetLatestDataAsync();

            await Response.WriteAsync($"event: update\n");
            await Response.WriteAsync($"data: {JsonSerializer.Serialize(data)}\n\n");
            await Response.Body.FlushAsync();

            await Task.Delay(TimeSpan.FromSeconds(5), cancellationToken);
        }
    }
}

Client Implementation

const eventSource = new EventSource('/api/events/stream');

eventSource.addEventListener('update', (event) => {
  const data = JSON.parse(event.data);
  console.log('Update received:', data);
  updateUI(data);
});

eventSource.onerror = (error) => {
  console.error('SSE error:', error);
  eventSource.close();
};

// Cleanup
window.addEventListener('beforeunload', () => {
  eventSource.close();
});

SSE vs WebSocket

Feature	SSE	WebSocket
Direction	Server to client	Bidirectional
Protocol	HTTP	WebSocket
Reconnection	Automatic	Manual
Binary data	No	Yes

Summary

SSE offers a simple, HTTP-based solution for server-to-client streaming, with automatic reconnection and broad browser support.

---

References:

• Server-Sent Events
• EventSource API