In modern web applications, sometimes mere request for data is not enough for the client, the server has to push the data to the client as soon as an event happens. Think about live dashboards, chat notifications or any other real time notifications. In these cases, waiting for the user to refresh the page or constantly polling the server just won’t cut it. We need a way to push updates from the server into the browser in real time. This is where Server-Sent Events (SSE) fill the gap. They offer an efficient, lightweight, and browser-supported method of server-to-client streaming updates over a standard HTTP connection. Before technologies like WebSockets became popular, developers often used long polling to simulate real-time updates. And while long polling still works, it’s not always the best choice, especially when you’re building something that only requires one-way communication from the server to the client. That’s exactly the sweet spot at which SSE thrives: it’s lightweight, easy to deploy, and ideal for pushing text-based updates without the baggage of WebSocket protocols or the inefficiencies of constant HTTP requests.

SSE vs Long Polling: What’s the Difference?

Long polling’s been in use for years. It’s a technique where the client makes requests to the server and has that request hang open until the server receives new information. The client makes the request again when the server returns and the cycle continues.

While it’s effective, long polling has some drawbacks:

  • It creates many HTTP requests, which is overhead.
  • There is a tiny lag between messages because of the opening and closing of connections taking time.
  • It is more difficult to scale, particularly under heavy traffic.

Server-Sent Events make this easier. With SSE:

  • The client opens a single, long-lived HTTP connection.
  • The server sends data to the client as it arrives.
  • You don’t need to recreate the link each time.

In brief: SSE is a cleaner, more streamlined alternative to long polling if you only need one-way communication from server to client.

Implementing SSE in .NET

Now that we understand why SSE can be a great fit, let’s build a simple example in .NET using minimal APIs. Here’s how you can set up an SSE endpoint using top-level statements:

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Hosting;

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

app.MapGet("/sse", async (HttpContext ctx, ItemService service, CancellationToken ct) =>
{
    ctx.Response.Headers.Add("Content-Type", "text/event-stream");
    
    while (!ct.IsCancellationRequested)
    {
        var item = await service.WaitForNewItem();
        
        await ctx.Response.WriteAsync($"data: ");
        await JsonSerializer.SerializeAsync(ctx.Response.Body, item);
        await ctx.Response.WriteAsync($"\n\n");
        await ctx.Response.Body.FlushAsync();
            
        service.Reset();
    }
});

app.Run();

In summary, this SSE endpoint:

  • Waits for new data to become available (WaitForNewItem()).
  • Sends it to the client as a Server-Sent Event.
  • Flushes the stream so the client receives it immediately.
  • Then waits again, continuing until the client disconnects.

Implementing an SSE Client

Creating a client for Server-Sent Events is straightforward thanks to the built-in EventSource API available in all modern browsers. Here’s a simple HTML + JavaScript example that connects to the SSE endpoint we created in .NET:

<!DOCTYPE html>
<html>
<head>
    <title>SSE Client Example</title>
</head>
<body>
    <h2>Live Updates</h2>
    <pre id="output">Waiting for events...</pre>

    <script>
        const eventSource = new EventSource("/sse");

        eventSource.onmessage = function (event) => {
            const data = JSON.parse(event.data);
            console.log("New item received:", data);
        };
    </script>
</body>
</html>

When to Use SSE in .NET

Server-Sent Events (SSE) are not a one-size-fits-all solution, but in the right context, they can be a perfect fit. The key is to understand what kind of communication your app needs and how much complexity you’re willing to manage.

Here’s when SSE becomes a great choice in .NET applications:

You Only Need One-Way Communication (Server → Client)

If your app only needs to send data from the server to the browser, SSE is probably your simplest and most efficient option.

Use cases:

  • Live logs: Streaming server or application logs in real time to a web dashboard.
  • Notifications: Sending alerts or system messages to the user without requiring them to refresh.
  • Progress updates: Long-running background tasks that update the user when something changes.

You Want Simplicity Over Full-Duplex Control

Compared to WebSockets, SSE is much easier to implement. You don’t need to worry about handshakes, binary protocols, or managing bidirectional channels. You just open a connection and push plain text updates.

This is great when:

  • You don’t need the client to reply or send data back.
  • You like to keep your infrastructure light.
  • You’re building a read-only feed or similar.

You’re Working in an HTTP-Friendly Environment

SSE runs on standard HTTP/1.1, which means:

  • It works behind most proxies and firewalls without extra configuration.
  • It’s easy to secure via HTTPS.

Unlike WebSockets, which can be blocked by intermediaries or require fallback logic, SSE is often the most reliable option in corporate or enterprise environments.

Conclusion

Building features live in web applications doesn’t necessarily always have to require strong infrastructure or chunky protocols. Server-Sent Events (SSE) give one a nice-looking, light-weighted, effective method to broadcast from server to client. In the .NET ecosystem, SSE fits seamlessly because of low-level APIs, async programming, and native HTTP streaming support. Whether sending live notifications, streaming logs, or updating dashboards in real time, SSE offers a lightweight, scalable solution without WebSockets overhead or inefficiency of long polling.

In short:

  • Easy to implement with .NET.
  • Ideal for server-to-client scenarios.
  • Less resource-intensive than long polling.
  • More available and reliable than WebSockets in gated networks.

Every app is different and SSE’s not the answer to everything. But when your application needs to push text-based alerts in real time and ease of use is a factor, Server-Sent Events are definitely worth considering.

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