Real-Time Data Streaming in Spring Boot: A Deep Dive into Server-Sent Events (SSE) and Long-Polling

Introduction

Real-time data streaming is essential for interactive applications that require live updates, such as dashboards, stock tickers, and notifications. While WebSockets are a popular choice, alternatives like Server-Sent Events (SSE) and Long-Polling provide effective solutions where full-duplex communication isn't necessary.

This article explores how SSE and Long-Polling work in Spring Boot, their scalability considerations, and implementation examples tailored for different use cases.

1. Server-Sent Events (SSE): A Deep Dive

1.1 Understanding SSE

SSE is a lightweight protocol based on HTTP/1.1 that allows the server to push real-time updates to the client. It is natively supported in modern browsers via the EventSource API.

Key Characteristics:

• Uses HTTP streaming (single persistent connection)
• Server can send multiple updates without the client requesting each
• Auto-reconnection is built-in

Example SSE Response:

HTTP/1.1 200 OK
Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive

id: 101
event: notification
data: {"message": "New update available!"}

id: 102
data: {"message": "Real-time event received"}        

1.2 Implementing SSE in Spring Boot

A simple SSE endpoint in Spring Boot using SseEmitter:

@RestController
@RequestMapping("/sse")
public class SSEController {

    private final List<SseEmitter> emitters = new CopyOnWriteArrayList<>();

    @GetMapping("/subscribe")
    public SseEmitter subscribe() {
        SseEmitter emitter = new SseEmitter(30_000L);
        emitters.add(emitter);

        emitter.onCompletion(() -> emitters.remove(emitter));
        emitter.onTimeout(() -> emitters.remove(emitter));
        return emitter;
    }

    @PostMapping("/send")
    public ResponseEntity<String> sendEvent(@RequestParam String message) {
        for (SseEmitter emitter : emitters) {
            try {
                emitter.send(SseEmitter.event()
                        .id(UUID.randomUUID().toString())
                        .name("message")
                        .data(message));
            } catch (IOException e) {
                emitters.remove(emitter);
            }
        }
        return ResponseEntity.ok("Event sent!");
    }
}        

1.3 Handling Resilience in SSE

To ensure resilience, SSE provides automatic reconnection and supports the Last-Event-ID header for recovering missed events.

@GetMapping("/events")
public SseEmitter streamEvents(@RequestHeader(value = "Last-Event-ID", required = false) String lastEventId) {
    SseEmitter emitter = new SseEmitter();
    if (lastEventId != null) {
        // Fetch and send missed events from database
    }
    return emitter;
}        

2. Long-Polling: Enhancing Performance

2.1 How Long-Polling Works

Long-Polling is a request-response model where the client repeatedly requests updates. The server holds the request until new data is available or a timeout occurs.

Key Considerations:

• Increased server load due to frequent requests
• Requires optimized polling intervals
• Suitable for environments where SSE or WebSockets are not supported

2.2 Implementing Long-Polling in Spring Boot

Controller Implementation:

@RestController
@RequestMapping("/polling")
public class LongPollingController {

    private final Queue<String> messageQueue = new ConcurrentLinkedQueue<>();

    @GetMapping("/updates")
    public DeferredResult<ResponseEntity<String>> getUpdates() {
        DeferredResult<ResponseEntity<String>> deferredResult = new DeferredResult<>(10_000L);

        String message = messageQueue.poll();
        if (message != null) {
            deferredResult.setResult(ResponseEntity.ok(message));
        } else {
            deferredResult.onTimeout(() -> deferredResult.setResult(ResponseEntity.noContent().build()));
        }
        return deferredResult;
    }

    @PostMapping("/send")
    public ResponseEntity<String> sendUpdate(@RequestParam String message) {
        messageQueue.add(message);
        return ResponseEntity.ok("Message added to queue");
    }
}        

3. SSE vs WebSockets: Which One Should You Choose?

4. Practical Use Cases

4.1 Real-Time Dashboards

• Best Choice: SSE (Efficient server-to-client updates)
• Alternative: WebSockets (If bidirectional communication is needed)

4.2 Chat Applications

• Best Choice: WebSockets (Two-way communication)
• Fallback: Long-Polling (If WebSockets are unavailable)

4.3 Notification Systems

• Best Choice: SSE (Lightweight and efficient push mechanism)
• Fallback: Long-Polling (If SSE/WebSockets aren't supported)

5. Conclusion

SSE and Long-Polling are viable alternatives to WebSockets for real-time data streaming. While SSE is ideal for scenarios where the server pushes updates to clients, Long-Polling serves as a reliable fallback when WebSockets and SSE are not feasible.

Selecting the right approach depends on application needs, scalability requirements, and environmental constraints. By leveraging these technologies effectively, developers can build robust real-time communication systems in Spring Boot.