epp: add multicluster feature gate and endpoint attribute

Multi-cluster mode is declared at the EPP level, behind the multicluster feature
gate, rather than per endpoint. When the gate is on, the datalayer Runtime marks
every endpoint it creates with a multicluster attribute, so the mark covers all
discovery sources including endpoints that are not pods. Plugins read the mark
through IsMultiCluster, holding only the endpoint and never the global config.

It is a feature gate rather than a config section because multi-cluster routing is
still maturing and most EPPs never use it. Promotion looks like moving it out of the
feature gate into a dedicated multiCluster section on EndpointPickerConfig, once it
matures or needs structured configuration (peer discovery, cluster identity). The
Runtime consumes a single WithMultiCluster option today, so that promotion is a
config-surface change, not a behavior change.

Signed-off-by: Sam Batschelet <sbatsche@redhat.com>

Author: hexfusion

cmd/epp/runner/runner.go

@@ -685,6 +685,7 @@ func (r *Runner) parseConfigurationPhaseOne(ctx context.Context, opts *runserver
 
 	loader.RegisterFeatureGate(flowcontrol.FeatureGate, false)
 	loader.RegisterFeatureGate(runserver.HAPopulateNonLeaderDatastoreFeatureGate, true)
+	loader.RegisterFeatureGate(fwkdl.MultiClusterFeatureGate, false)
 
 	r.registerInTreePlugins()
 
@@ -784,7 +785,8 @@ func (r *Runner) configureAndStartDatalayer(ctx context.Context, cfg *datalayer.
 }
 
 func (r *Runner) setupMetricsCollection(opts *runserver.Options) datalayer.EndpointFactory {
-	r.dlRuntime = datalayer.NewRuntime(opts.RefreshMetricsInterval)
+	r.dlRuntime = datalayer.NewRuntime(opts.RefreshMetricsInterval,
+		datalayer.WithMultiCluster(r.featureGates[fwkdl.MultiClusterFeatureGate]))
 	return r.dlRuntime
 }
 

pkg/epp/datalayer/runtime.go

@@ -59,20 +59,30 @@ type Runtime struct {
 
 	collectors *collectorManager // per-endpoint poller, keyed by namespaced name
 	logger     logr.Logger       // Set in Configure; used where no context is available (e.g. ReleaseEndpoint).
+
+	multiCluster bool // marks created endpoints as peer clusters
 }
 
 const (
 	defaultRefreshInterval = 50 * time.Millisecond
 )
 
+// RuntimeOption configures a Runtime at construction.
+type RuntimeOption func(*Runtime)
+
+// WithMultiCluster marks endpoints the Runtime creates as peer clusters.
+func WithMultiCluster(v bool) RuntimeOption {
+	return func(r *Runtime) { r.multiCluster = v }
+}
+
 // NewRuntime creates a new Runtime with the given polling interval.
 // If duration is <= 0, uses the defaultRefreshInterval.
-func NewRuntime(pollingInterval time.Duration) *Runtime {
+func NewRuntime(pollingInterval time.Duration, opts ...RuntimeOption) *Runtime {
 	interval := defaultRefreshInterval
 	if pollingInterval > 0 {
 		interval = pollingInterval
 	}
-	return &Runtime{
+	r := &Runtime{
 		pollingInterval: interval,
 		dispatchers:     newPollingDispatchers(),
 		notification:    newNotificationManager(),
@@ -81,6 +91,10 @@ func NewRuntime(pollingInterval time.Duration) *Runtime {
 		collectors:      newCollectorManager(),
 		logger:          logr.Discard(),
 	}
+	for _, o := range opts {
+		o(r)
+	}
+	return r
 }
 
 // Configure is called to transform the configuration information into the Runtime's
@@ -372,6 +386,9 @@ func (r *Runtime) NewEndpoint(ctx context.Context, endpointMetadata *fwkdl.Endpo
 	logger = logger.WithValues("endpoint", endpointMetadata.GetNamespacedName())
 
 	endpoint := fwkdl.NewEndpoint(endpointMetadata, nil)
+	if r.multiCluster {
+		fwkdl.SetMultiCluster(endpoint)
+	}
 
 	dispatchers := make([]fwkdl.PollingDispatcher, 0, r.dispatchers.Count())
 	for _, d := range r.dispatchers.Dispatchers() {

pkg/epp/framework/interface/datalayer/multicluster.go

@@ -0,0 +1,45 @@
+/*
+Copyright 2026 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 datalayer
+
+// MultiClusterFeatureGate enables multi-cluster mode.
+const MultiClusterFeatureGate = "multicluster"
+
+// MultiClusterAttrKey marks an endpoint that represents a peer cluster rather than
+// a local inference server. It is an attribute, not an identity field, so only the
+// multi-cluster plugins that set and read it carry the concept. Every other
+// consumer is unaffected, and absence reads as not-multi-cluster.
+const MultiClusterAttrKey = "llm-d.ai/multicluster"
+
+type multiClusterAttr bool
+
+func (m multiClusterAttr) Clone() Cloneable { return m }
+
+// SetMultiCluster marks ep as a peer-cluster endpoint.
+func SetMultiCluster(ep Endpoint) {
+	ep.GetAttributes().Put(MultiClusterAttrKey, multiClusterAttr(true))
+}
+
+// IsMultiCluster reports whether ep represents a peer cluster. Absence is false.
+func IsMultiCluster(ep Endpoint) bool {
+	if v, ok := ep.GetAttributes().Get(MultiClusterAttrKey); ok {
+		if m, ok := v.(multiClusterAttr); ok {
+			return bool(m)
+		}
+	}
+	return false
+}

pkg/epp/framework/interface/datalayer/multicluster_test.go

@@ -0,0 +1,33 @@
+/*
+Copyright 2026 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 datalayer
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMultiClusterAttribute(t *testing.T) {
+	// Absence reads as not-multi-cluster.
+	ep := NewEndpoint(nil, nil)
+	assert.False(t, IsMultiCluster(ep), "unmarked endpoint should not be multi-cluster")
+
+	// A marked endpoint reads back true through the attribute map.
+	SetMultiCluster(ep)
+	assert.True(t, IsMultiCluster(ep), "marked endpoint should read back as multi-cluster")
+}