Execute prepare data plugins in topological order of data dependencies

cmd/epp/runner/runner.go

@@ -459,6 +459,10 @@ func (r *Runner) parseConfigurationPhaseTwo(ctx context.Context, rawConfig *conf
 
 	// Add requestControl plugins
 	r.requestControlConfig.AddPlugins(handle.GetAllPlugins()...)
+	// Check prepare data plugins for cycles.
+	if r.requestControlConfig.ValidatePrepareDataPlugins() != nil {
+		return nil, errors.New("failed to load the configuration - prepare data plugins have cyclic dependencies")
+	}
 
 	// Handler deprecated configuration options
 	r.deprecatedConfigurationHelper(cfg, logger)

pkg/epp/requestcontrol/dag.go

@@ -0,0 +1,99 @@
+/*
+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 "errors"
+
+// buildDAG builds a dependency graph among data preparation plugins based on their
+// produced and consumed data keys.
+func buildDAG(plugins []PrepareDataPlugin) map[string][]string {
+	dag := make(map[string][]string)
+	for _, plugin := range plugins {
+		dag[plugin.TypedName().String()] = []string{}
+	}
+	// Create dependency graph as a DAG.
+	for i := range plugins {
+		for j := range plugins {
+			if i == j {
+				continue
+			}
+			// Check whether plugin[i] produces something consumed by plugin[j]. In that case, j depends on i.
+			if plugins[i].Produces() != nil && plugins[j].Consumes() != nil {
+				// For all the keys produced by plugin i, check if plugin j consumes any of them.
+				// If yes, then j depends on i.
+				for producedKey := range plugins[i].Produces() {
+					// If plugin j consumes the produced key, then j depends on i. We can break after the first match.
+					if _, ok := plugins[j].Consumes()[producedKey]; ok {
+						iPluginName := plugins[i].TypedName().String()
+						jPluginName := plugins[j].TypedName().String()
+						dag[jPluginName] = append(dag[jPluginName], iPluginName)
+						break
+					}
+				}
+			}
+		}
+	}
+	return dag
+}
+
+// prepareDataGraph builds a DAG of data preparation plugins and checks for cycles.
+// If there is a cycle, it returns an error.
+func prepareDataGraph(plugins []PrepareDataPlugin) (map[string][]string, error) {
+	dag := buildDAG(plugins)
+
+	// Check for cycles in the DAG.
+	if cycleExistsInDAG(dag) {
+		return nil, errors.New("cycle detected in data preparation plugin dependencies")
+	}
+
+	return dag, nil
+}
+
+// cycleExistsInDAG checks if there are cycles in the given directed graph represented as an adjacency list.
+func cycleExistsInDAG(dag map[string][]string) bool {
+	visited := make(map[string]bool)
+	recStack := make(map[string]bool)
+
+	var dfs func(string) bool
+	dfs = func(node string) bool {
+		if recStack[node] {
+			return true // Cycle detected
+		}
+		if visited[node] {
+			return false
+		}
+		visited[node] = true
+		recStack[node] = true
+
+		for _, neighbor := range dag[node] {
+			if dfs(neighbor) {
+				return true
+			}
+		}
+		recStack[node] = false
+		return false
+	}
+
+	for pluginName := range dag {
+		if !visited[pluginName] {
+			if dfs(pluginName) {
+				return true
+			}
+		}
+	}
+	return false
+}

pkg/epp/requestcontrol/dag_test.go

@@ -0,0 +1,146 @@
+/*
+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"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/stretchr/testify/assert"
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/plugins"
+	"sigs.k8s.io/gateway-api-inference-extension/pkg/epp/scheduling/types"
+)
+
+type mockPrepareRequestDataP struct {
+	name     string
+	produces map[string]any
+	consumes map[string]any
+}
+
+func (m *mockPrepareRequestDataP) TypedName() plugins.TypedName {
+	return plugins.TypedName{Name: m.name, Type: "mock"}
+}
+
+func (m *mockPrepareRequestDataP) Produces() map[string]any {
+	return m.produces
+}
+
+func (m *mockPrepareRequestDataP) Consumes() map[string]any {
+	return m.consumes
+}
+
+func (m *mockPrepareRequestDataP) PrepareRequestData(ctx context.Context, request *types.LLMRequest, pods []types.Pod) error {
+	pods[0].Put(mockProducedDataKey, mockProducedDataType{value: 42})
+	return nil
+}
+
+func TestPrepareDataGraph(t *testing.T) {
+	pluginA := &mockPrepareRequestDataP{name: "A", produces: map[string]any{"keyA": nil}}
+	pluginB := &mockPrepareRequestDataP{name: "B", consumes: map[string]any{"keyA": nil}, produces: map[string]any{"keyB": nil}}
+	pluginC := &mockPrepareRequestDataP{name: "C", consumes: map[string]any{"keyB": nil}}
+	pluginD := &mockPrepareRequestDataP{name: "D", consumes: map[string]any{"keyA": nil}}
+	pluginE := &mockPrepareRequestDataP{name: "E"} // No dependencies
+
+	// Cycle plugins
+	pluginX := &mockPrepareRequestDataP{name: "X", produces: map[string]any{"keyX": nil}, consumes: map[string]any{"keyY": nil}}
+	pluginY := &mockPrepareRequestDataP{name: "Y", produces: map[string]any{"keyY": nil}, consumes: map[string]any{"keyX": nil}}
+
+	testCases := []struct {
+		name        string
+		plugins     []PrepareDataPlugin
+		expectedDAG map[string][]string
+		expectError bool
+	}{
+		{
+			name:        "No plugins",
+			plugins:     []PrepareDataPlugin{},
+			expectedDAG: map[string][]string{},
+			expectError: false,
+		},
+		{
+			name:    "Plugins with no dependencies",
+			plugins: []PrepareDataPlugin{pluginA, pluginE},
+			expectedDAG: map[string][]string{
+				"A/mock": {},
+				"E/mock": {},
+			},
+			expectError: false,
+		},
+		{
+			name:    "Simple linear dependency (A -> B -> C)",
+			plugins: []PrepareDataPlugin{pluginA, pluginB, pluginC},
+			expectedDAG: map[string][]string{
+				"A/mock": {},
+				"B/mock": {"A/mock"},
+				"C/mock": {"B/mock"},
+			},
+			expectError: false,
+		},
+		{
+			name:    "DAG with multiple dependencies (A -> B, A -> D)",
+			plugins: []PrepareDataPlugin{pluginA, pluginB, pluginD, pluginE},
+			expectedDAG: map[string][]string{
+				"A/mock": {},
+				"B/mock": {"A/mock"},
+				"D/mock": {"A/mock"},
+				"E/mock": {},
+			},
+			expectError: false,
+		},
+		{
+			name:        "Graph with a cycle (X -> Y, Y -> X)",
+			plugins:     []PrepareDataPlugin{pluginX, pluginY},
+			expectedDAG: nil,
+			expectError: true,
+		},
+		{
+			name:        "Complex graph with a cycle",
+			plugins:     []PrepareDataPlugin{pluginA, pluginB, pluginX, pluginY},
+			expectedDAG: nil,
+			expectError: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			dag, err := prepareDataGraph(tc.plugins)
+
+			if tc.expectError {
+				assert.Error(t, err)
+				assert.Nil(t, dag)
+				assert.Contains(t, err.Error(), "cycle detected")
+			} else {
+				assert.NoError(t, err)
+
+				// Normalize the slices in the maps for consistent comparison
+				normalizedDAG := make(map[string][]string)
+				for k, v := range dag {
+					normalizedDAG[k] = v
+				}
+				normalizedExpectedDAG := make(map[string][]string)
+				for k, v := range tc.expectedDAG {
+					normalizedExpectedDAG[k] = v
+				}
+
+				if diff := cmp.Diff(normalizedExpectedDAG, normalizedDAG); diff != "" {
+					t.Errorf("prepareDataGraph() mismatch (-want +got):\n%s", diff)
+				}
+			}
+		})
+	}
+}

pkg/epp/requestcontrol/director.go

@@ -353,13 +353,9 @@ func (d *Director) runPreRequestPlugins(ctx context.Context, request *scheduling
 	}
 }
 
-// TODO: Execute plugins in parallel once DAG execution is supported.
-// runPrepareDataPlugins executes PrepareDataPlugins sequentially.
 func (d *Director) runPrepareDataPlugins(ctx context.Context,
 	request *schedulingtypes.LLMRequest, pods []schedulingtypes.Pod) error {
-	return prepareDataPluginsWithTimeout(
-		prepareDataTimeout, d.requestControlPlugins.prepareDataPlugins, ctx, request, pods)
-
+	return prepareDataPluginsWithTimeout(prepareDataTimeout, d.requestControlPlugins.prepareDataPlugins, ctx, request, pods)
 }
 
 func (d *Director) runAdmissionPlugins(ctx context.Context,

pkg/epp/requestcontrol/plugin_executor.go

@@ -19,25 +19,84 @@ package requestcontrol
 import (
 	"context"
 	"errors"
+	"fmt"
 	"time"
 
 	schedulingtypes "sigs.k8s.io/gateway-api-inference-extension/pkg/epp/scheduling/types"
 )
 
+// executePluginsAsDAG executes PrepareData plugins as a DAG based on their dependencies asynchronously.
+// So, a plugin is executed only after all its dependencies have been executed.
+// If there is a cycle or any plugin fails with error, it returns an error.
+func executePluginsAsDAG(plugins []PrepareDataPlugin, ctx context.Context, request *schedulingtypes.LLMRequest, pods []schedulingtypes.Pod) error {
+	// Build the DAG
+	// The error validation happens on startup when loading the config. So, here there should not be any error.
+	dag, err := prepareDataGraph(plugins)
+	if err != nil {
+		return err
+	}
+	// Create a readonly map of plugin name to plugin instance.
+	nameToNode := map[string]PrepareDataPlugin{}
+	for _, plugin := range plugins {
+		nameToNode[plugin.TypedName().String()] = plugin
+	}
+	// Execute the DAG
+
+	// Channels to signal plugin execution completion.
+	pluginExecuted := make(map[string]chan error)
+	// The capacity of the channel is equal to the number of dependents + 1 (for itself).
+	capacityMap := make(map[string]int)
+	for pluginName := range dag {
+		capacityMap[pluginName]++
+		for _, dep := range dag[pluginName] {
+			capacityMap[dep]++
+		}
+	}
+	for pluginName, capacity := range capacityMap {
+		pluginExecuted[pluginName] = make(chan error, capacity)
+	}
+	for pluginName, dependents := range dag {
+		// Execute plugins based on dependencies.
+		//  Wait for the dependencies to complete before executing a plugin.
+		go func(pN string, depds []string) {
+			for _, dep := range depds {
+				// Wait for the dependency plugin to signal completion.
+				if err, closed := <-pluginExecuted[dep]; closed {
+					if err != nil {
+						for range cap(pluginExecuted[pN]) {
+							// Notify all dependents about the failure.
+							pluginExecuted[pN] <- fmt.Errorf("dependency plugin %s failed: %w", dep, err)
+						}
+						// Do not execute this plugin as one of its dependencies failed.
+						return
+					}
+				}
+			}
+			res := nameToNode[pN].PrepareRequestData(ctx, request, pods)
+			for range cap(pluginExecuted[pN]) {
+				// Notify all dependents about the completion.
+				pluginExecuted[pN] <- res
+			}
+		}(pluginName, dependents)
+	}
+
+	// Check for errors in plugin execution.
+	// This will also ensure that all plugins have completed execution before returning.
+	for pluginName := range dag {
+		err := <-pluginExecuted[pluginName]
+		if err != nil {
+			return fmt.Errorf("prepare data plugin %s failed: %v", pluginName, err)
+		}
+	}
+	return nil
+}
+
 // prepareDataPluginsWithTimeout executes the PrepareRequestData plugins with retries and timeout.
 func prepareDataPluginsWithTimeout(timeout time.Duration, plugins []PrepareDataPlugin,
 	ctx context.Context, request *schedulingtypes.LLMRequest, pods []schedulingtypes.Pod) error {
 	errCh := make(chan error, 1)
-	// Execute plugins sequentially in a separate goroutine
 	go func() {
-		for _, plugin := range plugins {
-			err := plugin.PrepareRequestData(ctx, request, pods)
-			if err != nil {
-				errCh <- errors.New("prepare data plugin " + plugin.TypedName().String() + " failed: " + err.Error())
-				return
-			}
-		}
-		errCh <- nil
+		errCh <- executePluginsAsDAG(plugins, ctx, request, pods)
 	}()
 
 	select {