What is Spring WebFlux and reactive programming in Spring Boot 4?

// WebFlux reactive controller:
@RestController
@RequestMapping("/api/orders")
public class OrderReactiveController {

    private final OrderReactiveService service;

    // Mono<T>: 0 or 1 item (like Optional)
    @GetMapping("/{id}")
    public Mono<ResponseEntity<OrderDto>> getOrder(@PathVariable String id) {
        return service.findById(id)
            .map(ResponseEntity::ok)
            .defaultIfEmpty(ResponseEntity.notFound().build());
    }

    // Flux<T>: 0 to N items (like Stream)
    @GetMapping
    public Flux<OrderDto> listOrders() {
        return service.findAll();
    }

    // Server-Sent Events (SSE) streaming:
    @GetMapping(value = "/stream", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<OrderDto> streamOrders() {
        return service.streamNewOrders()  // infinite stream of new orders
            .delayElements(Duration.ofSeconds(1));
    }

    // R2DBC (reactive database):
    // Replace JpaRepository with ReactiveCrudRepository
}

// Starter for WebFlux:
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
// WebFlux native API versioning (Boot 4):
@GetMapping(path = "/{id}", version = "2.0")
public Mono<OrderDtoV2> getOrderV2(@PathVariable String id) {
    return service.findByIdV2(id);
}