WebSockets & Real-Time Systems with Spring · Lesson

Server-Sent Events (SSE) vs. WebSockets

Compare SSE and WebSockets, understanding when to choose each technology for specific real-time needs.

Server-Sent Events (SSE) vs. WebSockets is a free WebSockets & Real-Time Systems with Spring lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Real-Time Systems with Spring learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Real-Time: Beyond Request-Response

Modern web applications often need instant updates. Think live scores, chat messages, or stock price changes. Traditional HTTP's request-response model isn't always ideal for these scenarios.

We need ways for the server to push data to the client without the client constantly asking for it.

WebSockets: A Quick Refresher

You've learned about WebSockets. They establish a persistent, full-duplex connection between a client and server over a single TCP connection.

Full-duplex: Both client and server can send and receive messages independently at any time.
Great for highly interactive applications like chat or multiplayer games.

Introducing Server-Sent Events (SSE)

Server-Sent Events (SSE) offer another way for servers to push data to clients. Unlike WebSockets, SSE is unidirectional.

The server sends data to the client.
The client cannot send data back to the server using the same SSE connection.
It's built on standard HTTP, making it simpler for certain use cases.

SSE Server: Spring Boot Example

Let's see how a Spring Boot server can send events. This controller streams a new message every second to any connected client.

Notice the text/event-stream media type.

import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Flux;

import java.time.Duration;

@RestController
public class SseController {

    @GetMapping(path = "/stream-events", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<String> streamEvents() {
        return Flux.interval(Duration.ofSeconds(1))
                .map(sequence -> "data: Event " + sequence + "\n\n");
    }

    public static void main(String[] args) {
        // This is a minimal Spring Boot application setup.
        // In a real app, you'd have @SpringBootApplication
        // and SpringApplication.run(YourApplication.class, args);
        // For this snippet, we just show the controller.
        System.out.println("SSE Controller is defined. Run a Spring Boot app to test.");
    }
}

SSE Client: JavaScript Example

Here's how a simple web page can connect to the SSE stream from our Spring server. The EventSource API handles reconnections automatically!

<!DOCTYPE html>
<html>
<head>
    <title>SSE Client</title>
</head>
<body>
    <h1>SSE Events</h1>
    <div id="events"></div>
    <script>
        const eventSource = new EventSource('/stream-events');

        eventSource.onmessage = function(event) {
            const p = document.createElement('p');
            p.textContent = event.data;
            document.getElementById('events').appendChild(p);
        };

        eventSource.onerror = function(err) {
            console.error("EventSource failed:", err);
            eventSource.close();
        };

        console.log("Connected to SSE stream...");
    </script>
</body>
</html>

Key Difference: Communication Flow

The core distinction lies in how data flows:

WebSockets: Offer two-way (full-duplex) communication. Both client and server can initiate sending messages.
SSE: Provides one-way (unidirectional) communication. The server pushes data to the client; the client cannot push data back using the same connection.

For client-to-server data with SSE, a separate HTTP request (e.g., POST) is needed.

When to Choose WebSockets

Opt for WebSockets when your application requires:

Real-time interactivity: Both client and server need to send messages frequently.
Low latency: Interactive games, collaborative editing tools, live video/audio streaming.
Bidirectional communication: Chat applications where users send and receive messages.
Binary data exchange: WebSockets handle both text and binary data efficiently.

When to Choose Server-Sent Events (SSE)

SSE is an excellent choice for scenarios where:

Server-to-client updates are primary: Stock tickers, news feeds, social media updates, system notifications.
Simplicity is key: SSE uses standard HTTP, making it simpler to implement for one-way data push.
Automatic reconnection: The EventSource API handles connection drops and retries automatically.
Browser compatibility: Widely supported, with robust polyfills for older browsers if needed.

Choosing the Right Tool

Consider your application's needs carefully:

If you only need to push data from server to client, SSE is often simpler and more efficient.
If you need a true two-way, low-latency, interactive channel, WebSockets are the way to go.
Don't overcomplicate; start with the simplest solution that meets your requirements.

Quick Check: SSE vs. WebSockets

Which of the following scenarios is best suited for Server-Sent Events (SSE)?

Recap: SSE vs. WebSockets

We explored two powerful technologies for real-time communication:

WebSockets: Offers full-duplex (two-way) communication, best for highly interactive, bidirectional applications.
Server-Sent Events (SSE): Provides unidirectional (server-to-client) communication, ideal for dashboards, notifications, and continuous data feeds.

Choosing between them depends on whether your application primarily needs to push data from the server or requires interactive two-way messaging.

Frequently asked questions

Is the “Server-Sent Events (SSE) vs. WebSockets” lesson free?

Yes — the full text of “Server-Sent Events (SSE) vs. WebSockets” is free to read here on the web, and the WebSockets & Real-Time Systems with Spring course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Real-Time Systems with Spring course, upgrade to CoddyKit PRO.

What will I learn in “Server-Sent Events (SSE) vs. WebSockets”?

Compare SSE and WebSockets, understanding when to choose each technology for specific real-time needs. You practise WebSockets & Real-Time Systems with Spring with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start WebSockets & Real-Time Systems with Spring?

No prior experience is required. WebSockets & Real-Time Systems with Spring on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Server-Sent Events (SSE) vs. WebSockets” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this WebSockets & Real-Time Systems with Spring lesson?

Yes. Every WebSockets & Real-Time Systems with Spring lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to WebSockets & Real-Time Systems with Spring