Node Declared Features

Kubernetes nodes use declared features to report the availability of specific features that are new or feature-gated. Control plane components utilize this information to make better decisions. The kube-scheduler, via the NodeDeclaredFeatures plugin, ensures pods are only placed on nodes that explicitly support the features the pod requires. Additionally, the NodeDeclaredFeatureValidator admission controller validates pod updates against a node's declared features.

This mechanism helps manage version skew and improve cluster stability, especially during cluster upgrades or in mixed-version environments where nodes might not all have the same features enabled. This is intended for Kubernetes feature developers introducing new node-level features and works in the background; application developers deploying Pods do not need to interact with this framework directly.

How it Works

1. Kubelet Feature Reporting: At startup, the kubelet on each node detects which managed Kubernetes features are currently enabled and reports them in the .status.declaredFeatures field of the Node. Only features under active development are included in this field.
2. Scheduler Filtering: The default kube-scheduler uses the NodeDeclaredFeatures plugin. This plugin:
   • In the PreFilter stage, checks the PodSpec to infer the set of node features required by the pod.
   • In the Filter stage, checks if the features listed in the node's .status.declaredFeatures satisfy the requirements inferred for the Pod. Pods will not be scheduled on nodes lacking the required features. Custom schedulers can also utilize the .status.declaredFeatures field to enforce similar constraints.
3. Admission Control: The nodedeclaredfeaturevalidator admission controller can reject Pods that require features not declared by the node they are bound to, preventing issues during pod updates.

Enabling node declared features

To use Node Declared Features, the NodeDeclaredFeatures feature gate must be enabled on the kube-apiserver, kube-scheduler, and kubelet components.

What's next

• Read the KEP for more details: KEP-5328: Node Declared Features
• Read about the NodeDeclaredFeatureValidator admission controller.