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ControlPlane Customization Webhooks

Gardener creates the Shoot controlplane in several steps of the Shoot flow. At different point of this flow, it:

• Deploys standard controlplane components such as kube-apiserver, kube-controller-manager, and kube-scheduler by creating the corresponding deployments, services, and other resources in the Shoot namespace.
• Initiates the deployment of custom controlplane components by ControlPlane controllers by creating a ControlPlane resource in the Shoot namespace.

In order to apply any provider-specific changes to the configuration provided by Gardener for the standard controlplane components, cloud extension providers can install mutating admission webhooks for the resources created by Gardener in the Shoot namespace.

What needs to be implemented to support a new cloud provider?

In order to support a new cloud provider, you should install “controlplane” mutating webhooks for any of the following resources:

• Deployment with name kube-apiserver, kube-controller-manager, or kube-scheduler
• Service with name kube-apiserver
• OperatingSystemConfig with any name, and purpose reconcile

See Contract Specification for more details on the contract that Gardener and webhooks should adhere to regarding the content of the above resources.

You can install 2 different kinds of controlplane webhooks:

• Shoot, or controlplane webhooks apply changes needed by the Shoot cloud provider, for example the --cloud-provider command line flag of kube-apiserver and kube-controller-manager. Such webhooks should only operate on Shoot namespaces labeled with shoot.gardener.cloud/provider=<provider>.
• Seed, or seedprovider webhooks apply changes needed by the Seed cloud provider, for example adapting the storage class and capacity on Etcd objects. Such webhooks should only operate on Shoot namespaces labeled with seed.gardener.cloud/provider=<provider>.

The labels shoot.gardener.cloud/provider and seed.gardener.cloud/provider are added by Gardener when it creates the Shoot namespace.

The resources mutated by the “controlplane” mutating webhooks are labeled with provider.extensions.gardener.cloud/mutated-by-controlplane-webhook: true by gardenlet. The provider extensions can add an object selector to their “controlplane” mutating webhooks to not intercept requests for unrelated objects.

Contract Specification

This section specifies the contract that Gardener and webhooks should adhere to in order to ensure smooth interoperability. Note that this contract can’t be specified formally and is therefore easy to violate, especially by Gardener. The Gardener team will nevertheless do its best to adhere to this contract in the future and to ensure via additional measures (tests, validations) that it’s not unintentionally broken. If it needs to be changed intentionally, this can only happen after proper communication has taken place to ensure that the affected provider webhooks could be adapted to work with the new version of the contract.

Note: The contract described below may not necessarily be what Gardener does currently (as of May 2019). Rather, it reflects the target state after changes for Gardener extensibility have been introduced.

kube-apiserver

To deploy kube-apiserver, Gardener shall create a deployment and a service both named kube-apiserver in the Shoot namespace. They can be mutated by webhooks to apply any provider-specific changes to the standard configuration provided by Gardener.

The pod template of the kube-apiserver deployment shall contain a container named kube-apiserver.

The command field of the kube-apiserver container shall contain the kube-apiserver command line. It shall contain a number of provider-independent flags that should be ignored by webhooks, such as:

• admission plugins (--enable-admission-plugins, --disable-admission-plugins)
• secure communications (--etcd-cafile, --etcd-certfile, --etcd-keyfile, …)
• audit log (--audit-log-*)
• ports (--secure-port)

The kube-apiserver command line shall not contain any provider-specific flags, such as:

• --cloud-provider
• --cloud-config

These flags can be added by webhooks if needed.

The kube-apiserver command line may contain a number of additional provider-independent flags. In general, webhooks should ignore these unless they are known to interfere with the desired kube-apiserver behavior for the specific provider. Among the flags to be considered are:

• --endpoint-reconciler-type
• --advertise-address
• --feature-gates

Gardener uses SNI to expose the apiserver. In this case, Gardener expects that the --endpoint-reconciler-type and --advertise-address flags of the kube-apiserver’s Deployment are not modified.

The --enable-admission-plugins flag may contain admission plugins that are not compatible with CSI plugins such as PersistentVolumeLabel. Webhooks should therefore ensure that such admission plugins are either explicitly enabled (if CSI plugins are not used) or disabled (otherwise).

The env field of the kube-apiserver container shall not contain any provider-specific environment variables (so it will be empty). If any provider-specific environment variables are needed, they should be added by webhooks.

The volumes field of the pod template of the kube-apiserver deployment, and respectively the volumeMounts field of the kube-apiserver container shall not contain any provider-specific Secret or ConfigMap resources. If such resources should be mounted as volumes, this should be done by webhooks.

The kube-apiserver Service will be of type ClusterIP. In this case, Gardener expects that for this Service no mutations happen.

kube-controller-manager

To deploy kube-controller-manager, Gardener shall create a deployment named kube-controller-manager in the Shoot namespace. It can be mutated by webhooks to apply any provider-specific changes to the standard configuration provided by Gardener.

The pod template of the kube-controller-manager deployment shall contain a container named kube-controller-manager.

The command field of the kube-controller-manager container shall contain the kube-controller-manager command line. It shall contain a number of provider-independent flags that should be ignored by webhooks, such as:

• --kubeconfig, --authentication-kubeconfig, --authorization-kubeconfig
• --leader-elect
• secure communications (--tls-cert-file, --tls-private-key-file, …)
• cluster CIDR and identity (--cluster-cidr, --cluster-name)
• sync settings (--concurrent-deployment-syncs, --concurrent-replicaset-syncs)
• horizontal pod autoscaler (--horizontal-pod-autoscaler-*)
• ports (--port, --secure-port)

The kube-controller-manager command line shall not contain any provider-specific flags, such as:

• --cloud-provider
• --cloud-config
• --configure-cloud-routes
• --external-cloud-volume-plugin

These flags can be added by webhooks if needed.

The kube-controller-manager command line may contain a number of additional provider-independent flags. In general, webhooks should ignore these unless they are known to interfere with the desired kube-controller-manager behavior for the specific provider. Among the flags to be considered are:

• --feature-gates

The env field of the kube-controller-manager container shall not contain any provider-specific environment variables (so it will be empty). If any provider-specific environment variables are needed, they should be added by webhooks.

The volumes field of the pod template of the kube-controller-manager deployment, and respectively the volumeMounts field of the kube-controller-manager container shall not contain any provider-specific Secret or ConfigMap resources. If such resources should be mounted as volumes, this should be done by webhooks.

kube-scheduler

To deploy kube-scheduler, Gardener shall create a deployment named kube-scheduler in the Shoot namespace. It can be mutated by webhooks to apply any provider-specific changes to the standard configuration provided by Gardener.

The pod template of the kube-scheduler deployment shall contain a container named kube-scheduler.

The command field of the kube-scheduler container shall contain the kube-scheduler command line. It shall contain a number of provider-independent flags that should be ignored by webhooks, such as:

• --config
• --authentication-kubeconfig, --authorization-kubeconfig
• secure communications (--tls-cert-file, --tls-private-key-file, …)
• ports (--port, --secure-port)

The kube-scheduler command line may contain additional provider-independent flags. In general, webhooks should ignore these unless they are known to interfere with the desired kube-controller-manager behavior for the specific provider. Among the flags to be considered are:

• --feature-gates

The kube-scheduler command line can’t contain provider-specific flags, and it makes no sense to specify provider-specific environment variables or mount provider-specific Secret or ConfigMap resources as volumes.

etcd-main and etcd-events

To deploy etcd, Gardener shall create 2 Etcd named etcd-main and etcd-events in the Shoot namespace. They can be mutated by webhooks to apply any provider-specific changes to the standard configuration provided by Gardener.

Gardener shall configure the Etcd resource completely to set up an etcd cluster which uses the default storage class of the seed cluster.

cloud-controller-manager

Gardener shall not deploy a cloud-controller-manager. If it is needed, it should be added by a ControlPlane controller

CSI Controllers

Gardener shall not deploy a CSI controller. If it is needed, it should be added by a ControlPlane controller

kubelet

To specify the kubelet configuration, Gardener shall create a OperatingSystemConfig resource with any name and purpose reconcile in the Shoot namespace. It can therefore also be mutated by webhooks to apply any provider-specific changes to the standard configuration provided by Gardener. Gardener may write multiple such resources with different type to the same Shoot namespaces if multiple OSs are used.

The OSC resource shall contain a unit named kubelet.service, containing the corresponding systemd unit configuration file. The [Service] section of this file shall contain a single ExecStart option having the kubelet command line as its value.

The OSC resource shall contain a file with path /var/lib/kubelet/config/kubelet, which contains a KubeletConfiguration resource in YAML format. Most of the flags that can be specified in the kubelet command line can alternatively be specified as options in this configuration as well.

The kubelet command line shall contain a number of provider-independent flags that should be ignored by webhooks, such as:

• --config
• --bootstrap-kubeconfig, --kubeconfig
• --network-plugin (and, if it equals cni, also --cni-bin-dir and --cni-conf-dir)
• --node-labels

The kubelet command line shall not contain any provider-specific flags, such as:

• --cloud-provider
• --cloud-config
• --provider-id

These flags can be added by webhooks if needed.

The kubelet command line / configuration may contain a number of additional provider-independent flags / options. In general, webhooks should ignore these unless they are known to interfere with the desired kubelet behavior for the specific provider. Among the flags / options to be considered are:

• --enable-controller-attach-detach (enableControllerAttachDetach) - should be set to true if CSI plugins are used, but in general can also be ignored since its default value is also true, and this should work both with and without CSI plugins.
• --feature-gates (featureGates) - should contain a list of specific feature gates if CSI plugins are used. If CSI plugins are not used, the corresponding feature gates can be ignored since enabling them should not harm in any way.