Circuit Breaker pattern với Resilience4j trong Spring Boot hoạt động thế nào?

resilience4j.circuitbreaker.instances.payment-service:
  slidingWindowSize: 10
  failureRateThreshold: 50       # >50% fail → OPEN
  waitDurationInOpenState: 30s
  permittedCallsInHalfOpenState: 3

@CircuitBreaker(name = "payment-service", fallbackMethod = "paymentFallback")
@Retry(name = "payment-service")
public PaymentResult charge(Long orderId, BigDecimal amount) { ... }

private PaymentResult paymentFallback(Long orderId, BigDecimal amount, Exception ex) {
  pendingOrderQueue.add(orderId);        // fallback khi OPEN
  return PaymentResult.pending(orderId);
}