Contributors to this page
Last update:

Registering Extension Controllers

Extensions are registered in the garden cluster via ControllerRegistration resources. Gardener is evaluating the registrations and creates ControllerInstallation resources which describe the request “please install this controller X to this seed Y”.

Similar to how CloudProfile or Seed resources get into the system, the Gardener administrator must deploy the ControllerRegistration resources (this does not happen automatically in any way - the administrator decides which extensions shall be enabled).

The specification mainly describes which of Gardener’s extension CRDs are managed, for example:

kind: ControllerRegistration
  name: os-coreos
  - kind: OperatingSystemConfig
    type: coreos
    primary: true

This information tells Gardener that there is an extension controller that can handle OperatingSystemConfig resources of type coreos.

Also, it specifies that this controller is the primary one responsible for the lifecycle of the OperatingSystemConfig resource. Setting primary to false would allow to register additional, secondary controllers that may also watch/react on the OperatingSystemConfig/coreos resources, however, only the primary controller may change/update the main status of the extension object (that are used to “communicate” with the Gardenlet). Particularly, only the primary controller may set .status.lastOperation, .status.lastError, .status.observedGeneration, and .status.state. Secondary controllers may contribute to the .status.conditions[] if they like, of course.

Secondary controllers might be helpful in scenarios where additional tasks need to be completed which are not part of the reconciliation logic of the primary controller but separated out into a dedicated extension.

⚠️ There must be exactly one primary controller for every registered kind/type combination. Also, please note that the primary field cannot be changed after creation of the ControllerRegistration.

Deploying Extension Controllers

Submitting above ControllerRegistration will create a ControllerInstallation resource:

kind: ControllerInstallation
  name: os-coreos
    kind: ControllerRegistration
    name: os-coreos
    kind: Seed
    name: aws-eu1

This resource expresses that Gardener requires the os-coreos extension controller to run on the aws-eu1 seed cluster.

The Gardener Controller Manager does automatically determine which extension is required on which seed cluster and will only create ControllerInstallation objects for those. Also, it will automatically delete ControllerInstallations referencing extension controllers that are no longer required on a seed (e.g., because all shoots on it have been deleted). There are additional configuration options, please see this section.

How do extension controllers get deployed to seeds?

After Gardener has written the ControllerInstallation resource some component must satisfy this request and start deploying the extension controller to the seed. Depending on the complexity of the controllers lifecycle management, configuration, etc. there are two possible scenarios:

Scenario 1: Deployed by Gardener

In many cases the extension controllers are easy to deploy and configure. It is sufficient to simply create a Helm chart (standardized way of packaging software in the Kubernetes context) and deploy it together with some static configuration values. Gardener supports this scenario and allows to provide arbitrary deployment information in the ControllerRegistration resource’s .spec section:

    type: helm
      chart: H4sIFAAAAAAA/yk...
        foo: bar

If .spec.deployment.type=helm then Gardener itself will take over the responsibility the deployment. It base64-decodes the provided Helm chart (.spec.deployment.providerConfig.chart) and deploys it with the provided static configuration (.spec.deployment.providerConfig.values). The chart and the values can be updated at any time - Gardener will recognize and re-trigger the deployment process.

In order to allow extensions to get information about the garden and the seed cluster Gardener does mix-in certain properties into the values (root level) of every deployed Helm chart:

    identifier: <uuid-of-gardener-installation>
    identifier: <seed-name>
    region: europe
    spec: <complete-seed-spec>

Extensions can use this information in their Helm chart in case they require knowledge about the garden and the seed environment. The list might be extended in the future.

:information_source: Gardener uses the UUID of the garden Namespace object in the property.

Scenario 2: Deployed by a (non-human) Kubernetes operator

Some extension controllers might be more complex and require additional domain-specific knowledge wrt. lifecycle or configuration. In this case, we encourage to follow the Kubernetes operator pattern and deploy a dedicated operator for this extension into the garden cluster. The ControllerResource's .spec.deployment.type field would then not be helm, and no Helm chart or values need to be provided there. Instead, the operator itself knows how to deploy the extension into the seed. It must watch ControllerInstallation resources and act one those referencing a ControllerRegistration the operator is responsible for.

In order to let Gardener know that the extension controller is ready and running in the seed the ControllerInstallation's .status field supports two conditions: RegistrationValid and InstallationSuccessful - both must be provided by the responsible operator:

  - lastTransitionTime: "2019-01-22T11:51:11Z"
    lastUpdateTime: "2019-01-22T11:51:11Z"
    message: Chart could be rendered successfully.
    reason: RegistrationValid
    status: "True"
    type: Valid
  - lastTransitionTime: "2019-01-22T11:51:12Z"
    lastUpdateTime: "2019-01-22T11:51:12Z"
    message: Installation of new resources succeeded.
    reason: InstallationSuccessful
    status: "True"
    type: Installed

Additionally, the .status field has a providerStatus section into which the operator can (optionally) put any arbitrary data associated with this installation.

Extensions in the garden cluster itself

The Shoot resource itself will contain some provider-specific data blobs. As a result, some extensions might also want to run in the garden cluster, e.g., to provide ValidatingWebhookConfigurations for validating the correctness of their provider-specific blobs:

kind: Shoot
  name: johndoe-aws
  namespace: garden-dev
    type: aws
    region: eu-west-1
      kind: InfrastructureConfig
        vpc: # specify either 'id' or 'cidr'
        # id: vpc-123456
      - eu-west-1a

In the above example, Gardener itself does not understand the AWS-specific provider configuration for the infrastructure. However, if this part of the Shoot resource should be validated then you should run an AWS-specific component in the garden cluster that registers a webhook. You can do it similarly if you want to default some fields of a resource (by using a MutatingWebhookConfiguration).

Again, similar to how Gardener is deployed to the garden cluster, these components must be deployed and managed by the Gardener administrator.

Extension resource configurations

The Extension resource allows to inject arbitrary steps into the shoot reconciliation flow that are unknown to Gardener. Hence, it is slightly special and allows further configuration when registering it:

kind: ControllerRegistration
  name: extension-foo
  - kind: Extension
    type: foo
    primary: true
    globallyEnabled: true
    reconcileTimeout: 30s

The globallyEnabled=true option specifies that the Extension/foo object shall be created by default for all shoots (unless they opted out by setting .spec.extensions[].enabled=false in the Shoot spec).

The reconcileTimeout tells Gardener how long it should wait during its shoot reconciliation flow for the Extension/foo's reconciliation to finish.

Deployment configuration options

The .spec.deployment resource allows to configure a deployment policy. There are the following policies:

  • OnDemand (default): Gardener will demand the deployment and deletion of the extension controller to/from seed clusters dynamically. It will automatically determine (based on other resources like Shoots) whether it is required and decide accordingly.
  • Always: Gardener will demand the deployment of the extension controller to seed clusters independent of whether it is actually required or not. This might be helpful if you want to add a new component/controller to all seed clusters by default. Another use-case is to minimize the durations until extension controllers are deployed and ready in case you have highly fluctuating seed clusters.

Also, the .spec.deployment.seedSelector allows to specify a label selector for seed clusters. Only if it matches the labels of a seed then it will be deployed to it. Please note that a seed selector can only be specified for secondary controllers (primary=false for all .spec.resources[]).