Implementing Reactive Programming

Mono<User> one = userRepo.findById(id);          // 0..1
Flux<Order> many = orderRepo.findByUser(id);     // 0..N

Flux.range(1, 5)
    .map(i -> i * 2)
    .filter(i -> i > 4)
    .subscribe(System.out::println);

public interface OrderRepository extends ReactiveCrudRepository<Order, UUID> {
    Flux<Order> findByCustomerId(UUID customerId);
    Mono<Long> countByStatus(String status);
}

@RestController @RequestMapping("/orders")
public class OrderController {
    @GetMapping("/{id}")
    public Mono<Order> get(@PathVariable UUID id) { return service.findById(id); }
    @GetMapping(produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<Order> stream() { return service.streamAll(); }
}

Mono<OrderDetails> details = Mono.zip(
        orderService.findById(id),
        customerService.findByOrderId(id),
        shippingService.estimate(id))
    .map(t -> new OrderDetails(t.getT1(), t.getT2(), t.getT3()));

StepVerifier.create(service.findById(id))
    .expectNextMatches(o -> o.id().equals(id))
    .verifyComplete();

StepVerifier.create(failing)
    .expectError(NotFoundException.class)
    .verify();