Server Side events consumer from spring boot

Question 1-> can this be done using restTemplate, and somehow SseEmitter (to stream to UI) class to stream the events from service-A.

Question 1: Yes, it's possible to consume an SSE (Server-Sent Events) stream from service-A using RestTemplate, but it's not the most efficient approach. RestTemplate is synchronous and blocking, while SSE is an asynchronous and non-blocking protocol. However, you can still use RestTemplate with a workaround to handle SSE. You would need to create a separate thread to continuously read from the SSE stream and process the events. SseEmitter is not directly compatible with RestTemplate, as it's a part of Spring WebFlux, which is designed for reactive programming.

Given that this service-B is a normal MVC, with spring security integration. can i also bring webflux and have both working ?

Question 2: Yes, you can bring in Spring WebFlux alongside Spring MVC with Spring Security integration. Spring WebFlux is designed to work seamlessly with Spring MVC, allowing you to gradually migrate towards a reactive architecture. You can have both working together in the same application.

is there any other potential solutions for that scenario.

Question 3: Potential solutions for your scenario include:

a. Use WebClient (Reactive Approach): Instead of RestTemplate, you can use WebClient from Spring WebFlux, which is non-blocking and reactive. It's better suited for consuming reactive streams like SSE.

b.Proxy Service: Create a separate Spring WebFlux service that consumes the SSE stream from service-A and exposes a REST API. Then, your Spring MVC service-B can consume this REST API using RestTemplate.

NON-BLOCKING - Reactive -SSE

@RestController
public class SSEController {

    @GetMapping(value = "/events", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<ServerSentEvent<String>> streamEvents() {
        return Flux.interval(Duration.ofSeconds(1))
                .map(sequence -> ServerSentEvent.<String>builder()
                        .id(String.valueOf(sequence))
                        .event("event")
                        .data("SSE Event #" + sequence + " at " + LocalTime.now())
                        .build());
    }
}

DEPENDENCY

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

SERLVET BLOCKING-MODEL SSE

@RestController
public class SSEController {

    @GetMapping("/events")
    public SseEmitter streamEvents() {
        SseEmitter emitter = new SseEmitter();
        
        // Using a scheduled executor service to simulate periodic events
        ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
        executorService.scheduleAtFixedRate(() -> {
            try {
                // Send event data
                emitter.send(SseEmitter.event().data("SSE Event at " + LocalTime.now()));
            } catch (IOException e) {
                // Log or handle the exception
            }
        }, 0, 1, TimeUnit.SECONDS);

        // Complete the emitter after a fixed delay
        executorService.schedule(() -> {
            emitter.complete();
            executorService.shutdown();
        }, 10, TimeUnit.SECONDS);

        return emitter;
    }
}

Besides spring.main.web-application-type: servlet , you can code like following to start up different web service container .

 SpringApplication app = new SpringApplication(Application.class);
        app.setWebApplicationType(WebApplicationType.SERVLET);
        app.run(args);

public enum WebApplicationType {
    NONE,
    SERVLET,
    REACTIVE;
}