Circuit Breaker Pattern

// Circuit breaker states
//
//  ┌────────────────────────────────────────┐
//  │              CLOSED                     │
//  │  (requests flow normally)               │
//  │  Failure counter incremented on error   │
//  └──────────────┬─────────────────────────┘
//                 │ failure threshold reached
//                 ▼
//  ┌────────────────────────────────────────┐
//  │               OPEN                      │
//  │  (all requests fail immediately)        │
//  │  Return fallback / error                │
//  │  Timer starts (e.g. 30 seconds)         │
//  └──────────────┬─────────────────────────┘
//                 │ timeout expires
//                 ▼
//  ┌────────────────────────────────────────┐
//  │           HALF-OPEN                     │
//  │  (allow N trial requests)               │
//  │  If trials succeed → CLOSED             │
//  │  If trials fail    → OPEN               │
//  └────────────────────────────────────────┘

// application.yml — Resilience4j circuit breaker config
resilience4j:
  circuitbreaker:
    instances:
      paymentService:
        slidingWindowSize: 10
        failureRateThreshold: 50        # open after 50% failures
        waitDurationInOpenState: 30s     # stay open for 30s
        permittedNumberOfCallsInHalfOpenState: 3
        slowCallRateThreshold: 80
        slowCallDurationThreshold: 2s

// Java service with circuit breaker
@Service
public class PaymentService {
    private final PaymentClient client;

    @CircuitBreaker(name = "paymentService", fallbackMethod = "paymentFallback")
    public PaymentResult charge(PaymentRequest req) {
        return client.charge(req);  // remote call
    }

    // Fallback when circuit is open or call fails
    private PaymentResult paymentFallback(PaymentRequest req, Throwable t) {
        log.warn("Payment circuit open, queuing for retry: {}", req.getOrderId());
        retryQueue.enqueue(req);
        return PaymentResult.pending(req.getOrderId());
    }
}

// Istio DestinationRule — circuit breaking + outlier detection
apiVersion: networking.istio.io/v1beta1
kind: DestinationRule
metadata:
  name: payment-service
spec:
  host: payment-service
  trafficPolicy:
    connectionPool:
      tcp:
        maxConnections: 100
      http:
        h2UpgradePolicy: DEFAULT
        maxRequestsPerConnection: 10
    outlierDetection:
      consecutive5xxErrors: 5       # eject after 5 consecutive 5xx
      interval: 10s                  # check every 10s
      baseEjectionTime: 30s          # eject for 30s
      maxEjectionPercent: 50         # never eject more than 50% of pods

// No code changes — Envoy sidecar handles circuit breaking
// Metrics available in Prometheus/Grafana for monitoring