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Gardenlet

Gardener is implemented using the operator pattern: It uses custom controllers that act on our own custom resources, and apply Kubernetes principles to manage clusters instead of containers. Following this analogy, you can recognize components of the Gardener architecture as well-known Kubernetes components, for example, shoot clusters can be compared with pods, and seed clusters can be seen as worker nodes.

The following Gardener components play a similar role as the corresponding components in the Kubernetes architecture:

Gardener ComponentKubernetes Component
gardener-apiserverkube-apiserver
gardener-controller-managerkube-controller-manager
gardener-schedulerkube-scheduler
gardenletkubelet

Similar to how the kube-scheduler of Kubernetes finds an appropriate node for newly created pods, the gardener-scheduler of Gardener finds an appropriate seed cluster to host the control plane for newly ordered clusters. By providing multiple seed clusters for a region or provider, and distributing the workload, Gardener also reduces the blast radius of potential issues.

Kubernetes runs a primary “agent” on every node, the kubelet, which is responsible for managing pods and containers on its particular node. Decentralizing the responsibility to the kubelet has the advantage that the overall system is scalable. Gardener achieves the same for cluster management by using a gardenlet as primary “agent” on every seed cluster, and is only responsible for shoot clusters located in its particular seed cluster:

Counterparts in the Gardener Architecture and the Kubernetes Architecture

The gardener-controller-manager has control loops to manage resources of the Gardener API. However, instead of letting the gardener-controller-manager talk directly to seed clusters or shoot clusters, the responsibility isn’t only delegated to the gardenlet, but also managed using a reversed control flow: It’s up to the gardenlet to contact the Gardener API server, for example, to share a status for its managed seed clusters.

Reversing the control flow allows placing seed clusters or shoot clusters behind firewalls without the necessity of direct access via VPN tunnels anymore.

Reversed Control Flow Using a Gardenlet

TLS Bootstrapping

Kubernetes doesn’t manage worker nodes itself, and it’s also not responsible for the lifecycle of the kubelet running on the workers. Similarly, Gardener doesn’t manage seed clusters itself, so Gardener is also not responsible for the lifecycle of the gardenlet running on the seeds. As a consequence, both the gardenlet and the kubelet need to prepare a trusted connection to the Gardener API server and the Kubernetes API server correspondingly.

To prepare a trusted connection between the gardenlet and the Gardener API server, the gardenlet initializes a bootstrapping process after you deployed it into your seed clusters:

  1. The gardenlet starts up with a bootstrap kubeconfig having a bootstrap token that allows to create CertificateSigningRequest (CSR) resources.

  2. After the CSR is signed, the gardenlet downloads the created client certificate, creates a new kubeconfig with it, and stores it inside a Secret in the seed cluster.

  3. The gardenlet deletes the bootstrap kubeconfig secret, and starts up with its new kubeconfig.

  4. The gardenlet starts normal operation.

The gardener-controller-manager runs a control loop that automatically signs CSRs created by gardenlets.

The gardenlet bootstrapping process is based on the kubelet bootstrapping process. More information: Kubelet’s TLS bootstrapping.

If you don’t want to run this bootstrap process you can create a kubeconfig pointing to the garden cluster for the gardenlet yourself, and use field gardenClientConnection.kubeconfig in the gardenlet configuration to share it with the gardenlet.

Gardenlet Certificate Rotation

The certificate used to authenticate the gardenlet against the API server is valid for a year. After about 10 months, the gardenlet tries to automatically replace the current certificate with a new one (certificate rotation).

To use certificate rotation, you need to specify the secret to store the kubeconfig with the rotated certificate in field .gardenClientConnection.kubeconfigSecret of the gardenlet component configuration.

Rotate certificates using bootstrap kubeconfig

If the gardenlet created the certificate during the initial TLS Bootstrapping using the Bootstrap kubeconfig, certificates can be rotated automatically. The same control loop in the gardener-controller-manager that signs the CSRs during the initial TLS Bootstrapping also automatically signs the CSR during a certificate rotation.

Rotate Certificate Using Custom kubeconfig

When trying to rotate a custom certificate that wasn’t created by gardenlet as part of the TLS Bootstrap, the x509 certificate’s Subject field needs to conform to the following:

  • the Common Name (CN) is prefixed with gardener.cloud:system:seed:
  • the Organization (O) equals gardener.cloud:system:seeds

Otherwise, the gardener-controller-manager doesn’t automatically sign the CSR. In this case, an external component or user needs to approve the CSR manually, for example, using command kubectl certificate approve seed-csr-<...>). If that doesn’t happen within 15 minutes, the gardenlet repeats the process and creates another CSR.

Seed Config versus Seed Selector

The usage of the gardenlet is flexible:

UsageDescription
seedConfigRun one gardenlet per seed cluster inside the seed cluster itself.
seedSelectorUse one gardenlet to manage multiple seed clusters. The gardenlet can run outside of the seed cluster.

For production use it’s recommended to go for the seedConfig option, because it makes scaling easier and leads to a better distribution of responsibilities.

  • Provide a seedConfig that contains information about the seed cluster itself if you want the gardenlet in the standard way, see the example gardenlet configuration. Once bootstrapped, the gardenlet creates and updates its Seed object itself.

  • Provide a seedSelector that incorporates a label selector for the targeted seed clusters if you want the gardenlet to manage multiple seeds, see the example gardenlet configuration. In this case, you have to create the Seed objects together with a kubeconfig pointing to the cluster yourself.

Component Configuration

In the component configuration for the gardenlet, it’s possible to define:

  • settings for the Kubernetes clients interacting with the various clusters
  • settings for the control loops inside the gardenlet
  • settings for leader election and log levels, feature gates, and seed selection or seed configuration.

More information: Example Gardenlet Component Configuration.

Heartbeats

Similar to how Kubernetes uses Lease objects for node heart beats (see KEP), the gardenlet is using Lease objects for heart beats of the seed cluster. Every two seconds, the gardenlet checks that the seed cluster’s /healthz endpoint returns HTTP status code 200. If that is the case, the gardenlet renews the lease in the Garden cluster in the gardener-system-seed-lease namespace and updates the GardenletReady condition in the status.conditions field of the Seed resource(s).

Similarly to the node-lifecycle-controller inside the kube-controller-manager, the gardener-controller-manager features a seed-lifecycle-controller that sets the GardenletReady condition to Unknown in case the gardenlet fails to renew the lease. As a consequence, the gardener-scheduler doesn’t consider this seed cluster for newly created shoot clusters anymore.

/healthz Endpoint

The gardenlet includes an HTTPS server that serves a /healthz endpoint. It’s used as a liveness probe in the Deployment of the gardenlet. If the gardenlet fails to renew its lease then the endpoint returns 500 Internal Server Error, otherwise it returns 200 OK.

⚠️
In case the gardenlet is managing multiple seeds (that is, a seed selector is used) then the /healthz reports 500 Internal Server Error if there is at least one seed for which it couldn’t renew its lease. Only if it can renew the lease for all seeds it reports 200 OK.

Please note, that the /healthz only indicates whether the gardenlet could successfully probe the Seed’s API server and renew the lease with the Garden cluster. It does not show that the Gardener extension API server (with the Gardener resource groups) is available. However, the Gardenlet is designed to withstand such connection outages and retries until the connection is reestablished.

Control Loops

The gardenlet consists out of several controllers which are now described in more detail.

⚠️ This section is not necessarily complete and might be under construction.

BackupEntry Controller

The BackupEntry controller reconciles those core.gardener.cloud/v1beta1.BackupEntry resources whose .spec.seedName value is equal to the name of a Seed the respective gardenlet is responsible for. Those resources are created by the Shoot controller (only if backup is enabled for the respective Seed) and there is exactly one BackupEntry per Shoot.

The controller creates an extensions.gardener.cloud/v1alpha1.BackupEntry resource (non-namespaced) in the seed cluster and waits until the responsible extension controller reconciled it (see this for more details). The status is populated in the .status.lastOperation field.

The core.gardener.cloud/v1beta1.BackupEntry resource has an owner reference pointing to the corresponding Shoot. Hence, if the Shoot is deleted, also the BackupEntry resource gets deleted. In this case, the controller deletes the extensions.gardener.cloud/v1alpha1.BackupEntry resource in the seed cluster and waits until the responsible extension controller has deleted it. Afterwards, the finalizer of the core.gardener.cloud/v1beta1.BackupEntry resource is released so that it finally disappears from the system.

Keep Backup for Deleted Shoots

In some scenarios it might be beneficial to not immediately delete the BackupEntrys (and with them, the etcd backup) for deleted Shoots.

In this case you can configure the .controllers.backupEntry.deletionGracePeriodHours field in the component configuration of the gardenlet. For example, if you set it to 48, then the BackupEntrys for deleted Shoots will only be deleted 48 hours after the Shoot was deleted.

Additionally, you can limit the shoot purposes for which this applies by setting .controllers.backupEntry.deletionGracePeriodShootPurposes[]. For example, if you set it to [production] then only the BackupEntrys for Shoots with .spec.purpose=production will be deleted after the configured grace period. All others will be deleted immediately after the Shoot deletion.

Managed Seeds

Gardener users can use shoot clusters as seed clusters, so-called “managed seeds” (aka “shooted seeds”), by creating ManagedSeed resources. By default, the gardenlet that manages this shoot cluster then automatically creates a clone of itself with the same version and the same configuration that it currently has. Then it deploys the gardenlet clone into the managed seed cluster.

If you want to prevent the automatic gardenlet deployment, specify the seedTemplate section in the ManagedSeed resource, and don’t specify the gardenlet section. In this case, you have to deploy the gardenlet on your own into the seed cluster.

More information: Register Shoot as Seed

Migrating from Previous Gardener Versions

If your Gardener version doesn’t support gardenlets yet, no special migration is required, but the following prerequisites must be met:

  • Your Gardener version is at least 0.31 before upgrading to v1.
  • You have to make sure that your garden cluster is exposed in a way that it’s reachable from all your seed clusters.

With previous Gardener versions, you had deployed the Gardener Helm chart (incorporating the API server, controller-manager, and scheduler). With v1, this stays the same, but you now have to deploy the gardenlet Helm chart as well into all of your seeds (if they aren’t managed, as mentioned earlier).

More information: Deploy a Gardenlet for all instructions.

Gardener Architecture

Issue #356: Implement Gardener Scheduler

PR #2309: Add /healthz endpoint for Gardenlet