feat(backend): add per-backend circuit breaker config via BackendLBPolicy

- Add CircuitBreakerConfig to BackendLBPolicy CRD (maxInflightRequests)
- Add CircuitBreakerConfig to IR BackendCluster type
- Add translator mapping: CRD → IR with backendCircuitBreaker()
- Wire into backendLBPolicyIndexes for per-backend assignment

Data plane HttpCircuitBreakerController already supports
backend_max_inflight_requests — this CRD makes it configurable
per backend via BackendLBPolicy.

Author: mahmut-Abi

internal/gatewayapiexperimental/backendlbv1alpha2/types.go

@@ -57,6 +57,11 @@ type BackendLBPolicySpec struct {
 	TargetRefs         []LocalPolicyTargetReference `json:"targetRefs"`
 	SessionPersistence *SessionPersistence          `json:"sessionPersistence,omitempty"`
 	LoadBalancing      *LoadBalancingPolicy         `json:"loadBalancing,omitempty"`
+	CircuitBreaker     *CircuitBreakerConfig        `json:"circuitBreaker,omitempty"`
+}
+
+type CircuitBreakerConfig struct {
+	MaxInflightRequests *int32 `json:"maxInflightRequests,omitempty"`
 }
 
 type BackendLBPolicy struct {

internal/ir/types.go

@@ -211,6 +211,7 @@ type BackendCluster struct {
 	BackendTLSValidation *BackendTLSValidation     `json:"backendTlsValidation,omitempty"`
 	SessionPersistence   *SessionPersistencePolicy `json:"sessionPersistence,omitempty"`
 	LoadBalancing        *LoadBalancingPolicy      `json:"loadBalancing,omitempty"`
+	CircuitBreaker       *CircuitBreakerConfig     `json:"circuitBreaker,omitempty"`
 	Metadata             map[string]string         `json:"metadata,omitempty"`
 	AIService            *AIServiceConfig           `json:"aiService,omitempty"`
 	TokenPolicy          *TokenPolicyConfig         `json:"tokenPolicy,omitempty"`
@@ -274,6 +275,10 @@ type ConsistentHashPolicy struct {
 	HeaderName string `json:"headerName,omitempty"`
 }
 
+type CircuitBreakerConfig struct {
+	MaxInflightRequests int `json:"maxInflightRequests,omitempty"`
+}
+
 type SecretMaterial struct {
 	Namespace string `json:"namespace"`
 	Name      string `json:"name"`

internal/translator/backend_lb_policy.go

@@ -15,12 +15,14 @@ type translatedBackendLBPolicy struct {
 	backendKeys        []string
 	sessionPersistence *ir.SessionPersistencePolicy
 	loadBalancing      *ir.LoadBalancingPolicy
+	circuitBreaker     *ir.CircuitBreakerConfig
 	policy             backendlbv1alpha2.BackendLBPolicy
 }
 
 type backendLBPolicyIndexes struct {
 	sessionPersistence map[string]*ir.SessionPersistencePolicy
 	loadBalancing      map[string]*ir.LoadBalancingPolicy
+	circuitBreaker     map[string]*ir.CircuitBreakerConfig
 }
 
 func buildBackendLBPolicyIndexes(
@@ -48,7 +50,8 @@ func buildBackendLBPolicyIndexesWithIndexes(
 			policy.Spec.SessionPersistence,
 		)
 		loadBalancing := backendLoadBalancing(policy.Spec.LoadBalancing)
-		if sessionPersistence == nil && loadBalancing == nil {
+		circuitBreaker := backendCircuitBreaker(policy.Spec.CircuitBreaker)
+		if sessionPersistence == nil && loadBalancing == nil && circuitBreaker == nil {
 			continue
 		}
 
@@ -61,6 +64,7 @@ func buildBackendLBPolicyIndexesWithIndexes(
 			backendKeys:        backendKeys,
 			sessionPersistence: sessionPersistence,
 			loadBalancing:      loadBalancing,
+			circuitBreaker:     circuitBreaker,
 			policy:             policy,
 		})
 		translationIndex := len(translations) - 1
@@ -74,6 +78,7 @@ func buildBackendLBPolicyIndexesWithIndexes(
 
 	sessionPersistence := make(map[string]*ir.SessionPersistencePolicy, len(owners))
 	loadBalancing := make(map[string]*ir.LoadBalancingPolicy, len(owners))
+	circuitBreaker := make(map[string]*ir.CircuitBreakerConfig, len(owners))
 	for backendKey, ownerIndex := range owners {
 		if item := translations[ownerIndex].sessionPersistence; item != nil {
 			copyItem := *item
@@ -87,11 +92,16 @@ func buildBackendLBPolicyIndexesWithIndexes(
 			}
 			loadBalancing[backendKey] = &copyItem
 		}
+		if item := translations[ownerIndex].circuitBreaker; item != nil {
+			copyItem := *item
+			circuitBreaker[backendKey] = &copyItem
+		}
 	}
 
 	return backendLBPolicyIndexes{
 		sessionPersistence: sessionPersistence,
 		loadBalancing:      loadBalancing,
+		circuitBreaker:     circuitBreaker,
 	}
 }
 
@@ -154,3 +164,12 @@ func backendLBPolicyBackendKeysWithIndexes(
 	sort.Strings(keys)
 	return compactStrings(keys), true
 }
+
+func backendCircuitBreaker(cb *backendlbv1alpha2.CircuitBreakerConfig) *ir.CircuitBreakerConfig {
+	if cb == nil || cb.MaxInflightRequests == nil {
+		return nil
+	}
+	return &ir.CircuitBreakerConfig{
+		MaxInflightRequests: int(*cb.MaxInflightRequests),
+	}
+}

internal/translator/translator_backends.go

@@ -82,11 +82,16 @@ func translateEffectiveBackends(
 					service.logicalName,
 					port.Port,
 				)],
-				LoadBalancing: backendLB.loadBalancing[backendClusterKey(
-					service.namespace,
-					service.logicalName,
-					port.Port,
-				)],
+			LoadBalancing: backendLB.loadBalancing[backendClusterKey(
+				service.namespace,
+				service.logicalName,
+				port.Port,
+			)],
+			CircuitBreaker: backendLB.circuitBreaker[backendClusterKey(
+				service.namespace,
+				service.logicalName,
+				port.Port,
+			)],
 				Metadata: map[string]string{
 					"service": service.logicalName,
 				},