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Contract: Extension Resource

Gardener defines common procedures which must be passed to create a functioning shoot cluster. Well known steps are represented by special resources like Infrastructure, OperatingSystemConfig or DNS. These resources are typically reconciled by dedicated controllers setting up the infrastructure on the hyperscaler or managing DNS entries, etc.

But, some requirements don't match with those special resources or don't depend on being proceeded at a specific step in the creation / deletion flow of the shoot. They require a more generic hook. Therefore, Gardener offers the Extension resource.

What is required to register and support an Extension type?

Gardener creates one Extension resource per registered extension type in ControllerRegistration per shoot.

apiVersion: core.gardener.cloud/v1beta1
kind: ControllerRegistration
metadata:
  name: extension-example
spec:
  resources:
  - kind: Extension
    type: example
    globallyEnabled: true
    workerlessSupported: true

If spec.resources[].globallyEnabled is true, then the Extension resources of the given type is created for every shoot cluster. Set to false, the Extension resource is only created if configured in the Shoot manifest. In case of workerless Shoot, a globally enabled Extension resource is created only if spec.resources[].workerlessSupported is also set to true. If an extension configured in the spec of a workerless Shoot is not supported yet, the admission request will be rejected.

The Extension resources are created in the shoot namespace of the seed cluster.

---
apiVersion: extensions.gardener.cloud/v1alpha1
kind: Extension
metadata:
  name: example
  namespace: shoot--foo--bar
spec:
  type: example
  providerConfig: {}

Your controller needs to reconcile extensions.extensions.gardener.cloud. Since there can exist multiple Extension resources per shoot, each one holds a spec.type field to let controllers check their responsibility (similar to all other extension resources of Gardener).

ProviderConfig

It is possible to provide data in the Shoot resource which is copied to spec.providerConfig of the Extension resource.

---
apiVersion: core.gardener.cloud/v1beta1
kind: Shoot
metadata:
  name: bar
  namespace: garden-foo
spec:
  extensions:
  - type: example
    providerConfig:
      foo: bar
...

results in

---
apiVersion: extensions.gardener.cloud/v1alpha1
kind: Extension
metadata:
  name: example
  namespace: shoot--foo--bar
spec:
  type: example
  providerConfig:
    foo: bar

Shoot Reconciliation Flow and Extension Status

Gardener creates Extension resources as part of the Shoot reconciliation. Moreover, it is guaranteed that the Cluster resource exists before the Extension resource is created. Extensions can be reconciled at different stages during Shoot reconciliation depending on the defined extension lifecycle strategy in the respective ControllerRegistration resource. Please consult the Extension Lifecycle section for more information.

For an Extension controller it is crucial to maintain the Extension's status correctly. At the end Gardener checks the status of each Extension and only reports a successful shoot reconciliation if the state of the last operation is Succeeded.

apiVersion: extensions.gardener.cloud/v1alpha1
kind: Extension
metadata:
  generation: 1
  name: example
  namespace: shoot--foo--bar
spec:
  type: example
status:
  lastOperation:
    state: Succeeded
  observedGeneration: 1