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Runtime Class

This page describes the RuntimeClass resource and runtime selection mechanism.

Warning: RuntimeClass includes breaking changes in the beta upgrade in v1.14. If you were using RuntimeClass prior to v1.14, see Upgrading RuntimeClass from Alpha to Beta.

• Runtime Class

Runtime Class

RuntimeClass is a feature for selecting the container runtime configuration. The container runtime configuration is used to run a Pod’s containers.

Set Up

Ensure the RuntimeClass feature gate is enabled (it is by default). See Feature Gates for an explanation of enabling feature gates. The RuntimeClass feature gate must be enabled on apiservers and kubelets.

1. Configure the CRI implementation on nodes (runtime dependent)
2. Create the corresponding RuntimeClass resources

1. Configure the CRI implementation on nodes

The configurations available through RuntimeClass are Container Runtime Interface (CRI) implementation dependent. See the corresponding documentation (below) for your CRI implementation for how to configure.

Note: RuntimeClass currently assumes a homogeneous node configuration across the cluster (which means that all nodes are configured the same way with respect to container runtimes). Any heterogeneity (varying configurations) must be managed independently of RuntimeClass through scheduling features (see Assigning Pods to Nodes).

The configurations have a corresponding handler name, referenced by the RuntimeClass. The handler must be a valid DNS 1123 label (alpha-numeric + - characters).

2. Create the corresponding RuntimeClass resources

The configurations setup in step 1 should each have an associated handler name, which identifies the configuration. For each handler, create a corresponding RuntimeClass object.

The RuntimeClass resource currently only has 2 significant fields: the RuntimeClass name (metadata.name) and the handler (handler). The object definition looks like this:

apiVersion: node.k8s.io/v1beta1  # RuntimeClass is defined in the node.k8s.io API group
kind: RuntimeClass
metadata:
  name: myclass  # The name the RuntimeClass will be referenced by
  # RuntimeClass is a non-namespaced resource
handler: myconfiguration  # The name of the corresponding CRI configuration

Note: It is recommended that RuntimeClass write operations (create/update/patch/delete) be restricted to the cluster administrator. This is typically the default. See Authorization Overview for more details.

Usage

Once RuntimeClasses are configured for the cluster, using them is very simple. Specify a runtimeClassName in the Pod spec. For example:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  runtimeClassName: myclass
  # ...

This will instruct the Kubelet to use the named RuntimeClass to run this pod. If the named RuntimeClass does not exist, or the CRI cannot run the corresponding handler, the pod will enter the Failed terminal phase. Look for a corresponding event for an error message.

If no runtimeClassName is specified, the default RuntimeHandler will be used, which is equivalent to the behavior when the RuntimeClass feature is disabled.

CRI Configuration

For more details on setting up CRI runtimes, see CRI installation.

dockershim

Kubernetes built-in dockershim CRI does not support runtime handlers.

containerd

Runtime handlers are configured through containerd’s configuration at /etc/containerd/config.toml. Valid handlers are configured under the runtimes section:

[plugins.cri.containerd.runtimes.${HANDLER_NAME}]

See containerd’s config documentation for more details: https://github.com/containerd/cri/blob/master/docs/config.md

cri-o

Runtime handlers are configured through cri-o’s configuration at /etc/crio/crio.conf. Valid handlers are configured under the crio.runtime table:

[crio.runtime.runtimes.${HANDLER_NAME}]
  runtime_path = "${PATH_TO_BINARY}"

See cri-o’s config documentation for more details: https://github.com/kubernetes-sigs/cri-o/blob/master/cmd/crio/config.go

Upgrading RuntimeClass from Alpha to Beta

The RuntimeClass Beta feature includes the following changes:

• The node.k8s.io API group and runtimeclasses.node.k8s.io resource have been migrated to a built-in API from a CustomResourceDefinition.
• The spec has been inlined in the RuntimeClass definition (i.e. there is no more RuntimeClassSpec).
• The runtimeHandler field has been renamed handler.
• The handler field is now required in all API versions. This means the runtimeHandler field in the Alpha API is also required.
• The handler field must be a valid DNS label (RFC 1123), meaning it can no longer contain . characters (in all versions). Valid handlers match the following regular expression: ^[a-z0-9]([-a-z0-9]*[a-z0-9])?$.

Action Required: The following actions are required to upgrade from the alpha version of the RuntimeClass feature to the beta version:

• RuntimeClass resources must be recreated after upgrading to v1.14, and the runtimeclasses.node.k8s.io CRD should be manually deleted: kubectl delete customresourcedefinitions.apiextensions.k8s.io runtimeclasses.node.k8s.io
• Alpha RuntimeClasses with an unspecified or empty runtimeHandler or those using a . character in the handler are no longer valid, and must be migrated to a valid handler configuration (see above).

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