refactor: Abstract away admission control logic

This commit refactors the admission control logic introduced in the
previous commit.

- Introduces an AdmissionController interface.
- Implements LegacyAdmissionController for existing behavior.
- Implements FlowControlAdmissionController for the new flow control
	path.
- Moves flow control request adaptation into
	FlowControlAdmissionController.
- Updates Director to use the AdmissionController interface.
- Runner now injects the appropriate controller based on the feature
	flag.
- Splits and refactors tests for better focus and isolation.

This improves modularity, testability, and separation of concerns.

Author: LukeAVanDrie

cmd/epp/runner/runner.go

@@ -288,46 +288,43 @@ func (r *Runner) Run(ctx context.Context) error {
 
 	saturationDetector := saturationdetector.NewDetector(sdConfig, setupLog)
 
-	// --- Flow Control Initialization (Experimental) ---
-
+	// --- Admission Control Initialization ---
 	enableFlowControl := env.GetEnvBool(enableExperimentalFlowControlLayer, false, setupLog)
-	var flowController *fccontroller.FlowController
+	var admissionController requestcontrol.AdmissionController
 	if enableFlowControl {
 		setupLog.Info("Initializing experimental Flow Control layer")
-		cfg, err := flowControlConfig.ValidateAndApplyDefaults()
+		fcCfg, err := flowControlConfig.ValidateAndApplyDefaults()
 		if err != nil {
 			setupLog.Error(err, "failed to initialize Flow Control layer")
 			return fmt.Errorf("invalid Flow Control config: %w", err)
 		}
 
-		registry, err := fcregistry.NewFlowRegistry(cfg.Registry, setupLog)
+		registry, err := fcregistry.NewFlowRegistry(fcCfg.Registry, setupLog)
 		if err != nil {
 			return fmt.Errorf("failed to initialize Flow Registry: %w", err)
 		}
 		fc, err := fccontroller.NewFlowController(
 			ctx,
-			cfg.Controller,
+			fcCfg.Controller,
 			registry,
 			saturationDetector,
 			setupLog,
 		)
 		if err != nil {
 			return fmt.Errorf("failed to initialize Flow Controller: %w", err)
 		}
-		flowController = fc
-
 		go registry.Run(ctx)
+		admissionController = requestcontrol.NewFlowControlAdmissionController(saturationDetector, fc)
 	} else {
-		setupLog.Info("Experimental Flow Control layer is disabled")
+		setupLog.Info("Experimental Flow Control layer is disabled, using legacy admission control")
+		admissionController = requestcontrol.NewLegacyAdmissionController(saturationDetector)
 	}
 
 	director := requestcontrol.NewDirectorWithConfig(
 		datastore,
 		scheduler,
-		saturationDetector,
-		flowController,
-		r.requestControlConfig,
-		enableFlowControl)
+		admissionController,
+		r.requestControlConfig)
 
 	// --- Setup ExtProc Server Runner ---
 	serverRunner := &runserver.ExtProcServerRunner{

pkg/epp/requestcontrol/admission.go

@@ -0,0 +1,231 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package requestcontrol
+
+import (
+	"context"
+	"time"
+
+	"sigs.k8s.io/controller-runtime/pkg/log"
+
+	backendmetrics "sigs.k8s.io/gateway-api-inference-extension/pkg/epp/backend/metrics"
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/flowcontrol/types"
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/handlers"
+	errutil "sigs.k8s.io/gateway-api-inference-extension/pkg/epp/util/error"
+	logutil "sigs.k8s.io/gateway-api-inference-extension/pkg/epp/util/logging"
+)
+
+// AdmissionController defines the interface for making admission control decisions.
+// Implementations of this interface determine whether an incoming inference request should be accepted or rejected
+// based on various criteria such as system load, fairness, priority, and available capacity.
+type AdmissionController interface {
+	// Admit determines if a request should be admitted.
+	// It is called by the Director for each incoming request.
+	//
+	// Args:
+	//   ctx: The request context, carrying deadlines, cancellation signals, and logger.
+	//   reqCtx: The handlers.RequestContext containing details about the incoming request.
+	//   candidatePods: A list of potential backend pods that can serve the request.
+	//   priority: The priority level of the request, as determined by the InferenceObjective.
+	//
+	// Returns:
+	//   - nil: If the request is admitted and should proceed to scheduling.
+	//   - errutil.Error: If the request is rejected.
+	Admit(
+		ctx context.Context,
+		reqCtx *handlers.RequestContext,
+		candidatePods []backendmetrics.PodMetrics,
+		priority int,
+	) error
+}
+
+// saturationDetector defines the minimal interface required for checking if the backend pool is saturated.
+type saturationDetector interface {
+	IsSaturated(ctx context.Context, candidatePods []backendmetrics.PodMetrics) bool
+}
+
+// flowController defines the minimal interface required by FlowControlAdmissionController for enqueuing requests and
+// waiting for an admission outcome.
+type flowController interface {
+	EnqueueAndWait(req types.FlowControlRequest) (types.QueueOutcome, error)
+}
+
+// rejectIfSheddableAndSaturated checks if a request should be immediately rejected because it's sheddable
+// (priority < 0) and the system is saturated.
+func rejectIfSheddableAndSaturated(
+	ctx context.Context,
+	sd saturationDetector,
+	reqCtx *handlers.RequestContext,
+	candidatePods []backendmetrics.PodMetrics,
+	priority int,
+) error {
+	if priority < 0 {
+		logger := log.FromContext(ctx)
+		if sd.IsSaturated(ctx, candidatePods) {
+			logger.V(logutil.TRACE).Info("Request rejected: system saturated and request is sheddable",
+				"requestID", reqCtx.SchedulingRequest.RequestId)
+			return errutil.Error{
+				Code: errutil.InferencePoolResourceExhausted,
+				Msg:  "system saturated, sheddable request dropped",
+			}
+		}
+	}
+	return nil
+}
+
+// --- LegacyAdmissionController ---
+
+// LegacyAdmissionController implements saturation-based admission control.
+// It rejects sheddable requests (priority < 0) if the saturationDetector indicates that the system is currently
+// saturated. Non-sheddable requests always bypass the saturation check.
+type LegacyAdmissionController struct {
+	saturationDetector saturationDetector
+}
+
+// NewLegacyAdmissionController creates a new LegacyAdmissionController.
+func NewLegacyAdmissionController(sd saturationDetector) *LegacyAdmissionController {
+	return &LegacyAdmissionController{saturationDetector: sd}
+}
+
+// Admit implements the AdmissionController interface for the legacy strategy.
+// It checks for saturation only for requests with priority < 0.
+func (lac *LegacyAdmissionController) Admit(
+	ctx context.Context,
+	reqCtx *handlers.RequestContext,
+	candidatePods []backendmetrics.PodMetrics,
+	priority int,
+) error {
+	logger := log.FromContext(ctx)
+	logger.V(logutil.TRACE).Info("Executing LegacyAdmissionController",
+		"priority", priority, "fairnessID", reqCtx.FairnessID)
+	if err := rejectIfSheddableAndSaturated(ctx, lac.saturationDetector, reqCtx, candidatePods, priority); err != nil {
+		return err
+	}
+	logger.V(logutil.TRACE).Info("Request admitted", "requestID", reqCtx.SchedulingRequest.RequestId)
+	return nil
+}
+
+// --- FlowControlAdmissionController ---
+
+const (
+	// defaultFairnessID is the default fairness ID used when no ID is provided in the request.
+	// This ensures that requests without explicit fairness identifiers are still grouped and managed by the Flow Control
+	// system.
+	defaultFairnessID = "default-flow"
+)
+
+// FlowControlAdmissionController delegates admission decisions to the Flow Control layer.
+// It first checks if the request is sheddable and the system is saturated, rejecting immediately if both conditions are
+// true. Otherwise, it uses the provided flowController to enqueue the request and await an outcome.
+type FlowControlAdmissionController struct {
+	saturationDetector saturationDetector
+	flowController     flowController
+}
+
+// NewFlowControlAdmissionController creates a new FlowControlAdmissionController.
+// It requires a SaturationDetector and a flowController instance.
+func NewFlowControlAdmissionController(sd saturationDetector, fc flowController) *FlowControlAdmissionController {
+	return &FlowControlAdmissionController{
+		saturationDetector: sd,
+		flowController:     fc,
+	}
+}
+
+// Admit implements the AdmissionController interface by checking for saturation on sheddable requests first, then
+// deferring to the Flow Control system.
+func (fcac *FlowControlAdmissionController) Admit(
+	ctx context.Context,
+	reqCtx *handlers.RequestContext,
+	candidatePods []backendmetrics.PodMetrics,
+	priority int,
+) error {
+	logger := log.FromContext(ctx)
+	logger.V(logutil.TRACE).Info("Executing FlowControlAdmissionController",
+		"requestID", reqCtx.SchedulingRequest.RequestId, "priority", priority, "fairnessID", reqCtx.FairnessID)
+	if err := rejectIfSheddableAndSaturated(ctx, fcac.saturationDetector, reqCtx, candidatePods, priority); err != nil {
+		return err
+	}
+
+	logger.V(logutil.TRACE).Info("Request proceeding to flow control", "requestID", reqCtx.SchedulingRequest.RequestId)
+	fairnessID := reqCtx.FairnessID
+	if fairnessID == "" {
+		logger.V(logutil.TRACE).Info("No fairnessID provided, using default",
+			"requestID", reqCtx.SchedulingRequest.RequestId, "defaultFairnessID", defaultFairnessID)
+		fairnessID = defaultFairnessID
+	}
+
+	fcReq := &flowControlRequest{
+		ctx:             ctx,
+		requestID:       reqCtx.SchedulingRequest.RequestId,
+		fairnessID:      fairnessID,
+		priority:        priority,
+		requestByteSize: uint64(reqCtx.RequestSize),
+		candidatePods:   candidatePods,
+	}
+
+	outcome, err := fcac.flowController.EnqueueAndWait(fcReq)
+	logger.V(logutil.DEBUG).Info("Flow control outcome",
+		"requestID", reqCtx.SchedulingRequest.RequestId, "outcome", outcome, "error", err)
+	return translateFlowControlOutcome(outcome, err)
+}
+
+// flowControlRequest is an adapter that implements the types.FlowControlRequest interface.
+type flowControlRequest struct {
+	ctx             context.Context
+	requestID       string
+	fairnessID      string
+	priority        int
+	requestByteSize uint64
+	candidatePods   []backendmetrics.PodMetrics
+}
+
+var _ types.FlowControlRequest = &flowControlRequest{}
+
+func (r *flowControlRequest) Context() context.Context           { return r.ctx }
+func (r *flowControlRequest) ID() string                         { return r.requestID }
+func (r *flowControlRequest) InitialEffectiveTTL() time.Duration { return 0 } // Use controller default.
+func (r *flowControlRequest) ByteSize() uint64                   { return r.requestByteSize }
+func (r *flowControlRequest) CandidatePodsForScheduling() []backendmetrics.PodMetrics {
+	return r.candidatePods
+}
+func (r *flowControlRequest) FlowKey() types.FlowKey {
+	return types.FlowKey{ID: r.fairnessID, Priority: r.priority}
+}
+
+// translateFlowControlOutcome maps the context-rich outcome of the Flow Control layer to the public errutil.Error
+// contract used by the Director.
+func translateFlowControlOutcome(outcome types.QueueOutcome, err error) error {
+	msg := "request rejected by flow control"
+	if err != nil {
+		msg = err.Error()
+	}
+
+	switch outcome {
+	case types.QueueOutcomeDispatched:
+		return nil
+	case types.QueueOutcomeRejectedCapacity:
+		return errutil.Error{Code: errutil.InferencePoolResourceExhausted, Msg: msg}
+	case types.QueueOutcomeEvictedTTL:
+		return errutil.Error{Code: errutil.ServiceUnavailable, Msg: "request timed out in queue: " + msg}
+	case types.QueueOutcomeEvictedContextCancelled:
+		return errutil.Error{Code: errutil.ServiceUnavailable, Msg: "client disconnected: " + msg}
+	case types.QueueOutcomeRejectedOther, types.QueueOutcomeEvictedOther:
+		return errutil.Error{Code: errutil.Internal, Msg: "internal flow control error: " + msg}
+	default:
+		return errutil.Error{Code: errutil.Internal, Msg: "unhandled flow control outcome: " + msg}
+	}
+}