Session affinity scorer

pkg/epp/scheduling/max_score_picker.go

@@ -0,0 +1,65 @@
+/*
+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 scheduling
+
+import (
+	"fmt"
+	"math/rand/v2"
+
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/scheduling/types"
+	logutil "sigs.k8s.io/gateway-api-inference-extension/pkg/epp/util/logging"
+)
+
+type maxScorePicker struct {
+}
+
+func (p *maxScorePicker) Name() string {
+	return "max score picker"
+}
+
+func (p *maxScorePicker) Pick(ctx *types.Context, pods []types.Pod) (*types.Result, error) {
+	ctx.Logger.V(logutil.DEBUG).Info(fmt.Sprintf("Selecting a pod with maximum score from %d candidates: %+v", len(pods), pods))
+
+	// select pod with maximum score, if more than one with the max score - use random pods from the list
+	var highestScoreTargets []types.Pod
+	// score weights cound be negative
+	maxScore := 0.0
+	isFirst := true
+
+	for _, pod := range pods {
+		if isFirst {
+			maxScore = pod.Score()
+			highestScoreTargets = []types.Pod{pod}
+			isFirst = false
+		} else {
+			if pod.Score() > maxScore {
+				maxScore = pod.Score()
+				highestScoreTargets = []types.Pod{pod}
+			} else if pod.Score() == maxScore {
+				highestScoreTargets = append(highestScoreTargets, pod)
+			}
+		}
+	}
+
+	// single pod with max score
+	if len(highestScoreTargets) == 1 {
+		return &types.Result{TargetPod: highestScoreTargets[0]}, nil
+	}
+
+	// select random pod from list of pods with max score
+	return &types.Result{TargetPod: highestScoreTargets[rand.IntN(len(highestScoreTargets))]}, nil
+}

pkg/epp/scheduling/scheduler.go

@@ -72,9 +72,9 @@ func NewScheduler(datastore Datastore) *Scheduler {
 		datastore:           datastore,
 		preSchedulePlugins:  []types.PreSchedule{},
 		postSchedulePlugins: []types.PostSchedule{},
-		scorers:             []types.Scorer{},
+		scorers:             []types.Scorer{&SessionAffinityScorer{1.0, datastore}},
 		filters:             []types.Filter{defaultPlugin},
-		picker:              defaultPlugin,
+		picker:              &maxScorePicker{},
 	}
 }
 

pkg/epp/scheduling/session_affinity_scorer.go

@@ -0,0 +1,56 @@
+/*
+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 scheduling
+
+import (
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/scheduling/types"
+)
+
+// sessionAffinity is a routing scorer that routes subsequent
+// requests in a session to the same pod as the first request in the
+// session was sent to, by giving that pod the specified weight and assigning
+// zero score to the rest of the targets
+type SessionAffinityScorer struct {
+	weight    float64
+	datastore Datastore
+}
+
+func NewSessionAffinityScorer(weight float64, datastore Datastore) types.Scorer {
+	return SessionAffinityScorer{
+		weight:    weight,
+		datastore: datastore,
+	}
+}
+
+func (s SessionAffinityScorer) Name() string {
+	return "session affinity scorer"
+}
+
+func (s SessionAffinityScorer) Score(ctx *types.Context, pod types.Pod) (float64, error) {
+	score := 0.0
+
+	if ctx.Req.SessionID != "" {
+		podForSession := s.datastore.GetPodForSession(ctx.Req.SessionID)
+		if podForSession != nil {
+			if podForSession.NamespacedName.String() == pod.GetPod().NamespacedName.String() {
+				score = 1.0
+			}
+		}
+	}
+
+	return score, nil
+}