Capstone A — Reactive Order Platform bằng WebFlux

1. Architecture

Controller → OrderService(Mono/Flux)
              ├─ R2DBC
              ├─ Inventory
              └─ WebClient→Payment
                     ↓
                 Event Flux→SSE

2. Create flow

Mono<OrderResponse> create(CreateOrder cmd, String key) {
    return idempotency.find(key)
        .switchIfEmpty(
            userService.requireActive(cmd.userId())
                .then(inventory.reserve(cmd.items()))
                .then(payment.charge(cmd, key))
                .flatMap(p -> orderRepository.save(toEntity(cmd, p, key)))
                .flatMap(order ->
                    eventPublisher.publishCreated(order)
                        .thenReturn(OrderResponse.from(order)))
        )
        .timeout(Duration.ofSeconds(3));
}

3. Idempotency

Lưu key → request fingerprint → outcome/orderId. Cùng key nhưng payload khác trả conflict.

4. SSE

Demo có thể dùng Reactor Sink; production nên cân nhắc durable event source.

5. Tests

• StepVerifier service flows.
• WebTestClient HTTP contract.
• PostgreSQL test container.
• Mock payment.
• Timeout/error.
• Concurrent idempotency.

6. Observability

• Correlation ID.
• Create-order timer.
• Payment timeout counter.
• DB pool metrics.
• Tracing context.

7. Checklist

• ☑ No block() in request path.
• ☑ R2DBC.
• ☑ WebClient timeout.
• ☑ Safe retry only.
• ☑ Idempotency.
• ☑ SSE.
• ☑ Tests.

8. Challenge

Thêm transactional outbox để xử lý failure “DB commit thành công nhưng publish event thất bại”.