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 Component | Kubernetes Component |
|---|---|
gardener-apiserver |
kube-apiserver |
gardener-controller-manager |
kube-controller-manager |
gardener-scheduler |
kube-scheduler |
gardenlet |
kubelet |
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:
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.
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:
-
The gardenlet starts up with a bootstrap
kubeconfighaving a bootstrap token that allows to createCertificateSigningRequest(CSR) resources. -
After the CSR is signed, the gardenlet downloads the created client certificate, creates a new
kubeconfigwith it, and stores it inside aSecretin the seed cluster. -
The gardenlet deletes the bootstrap
kubeconfigsecret, and starts up with its newkubeconfig. -
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.
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.
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.
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.
The usage of the gardenlet is flexible:
| Usage | Description |
|---|---|
seedConfig |
Run one gardenlet per seed cluster inside the seed cluster itself. |
seedSelector |
Use 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
seedConfigoption, because it makes scaling easier and leads to a better distribution of responsibilities.
-
Provide a
seedConfigthat 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 itsSeedobject itself. -
Provide a
seedSelectorthat 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 theSeedobjects together with akubeconfigpointing to the cluster yourself.
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.
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.
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/healthzreports500 Internal Server Errorif 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 reports200 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.
Gardener users can use shoot clusters as seed clusters, so-called "shooted seeds",
by using shoot cluster annotation shoot.gardener.cloud/use-as-seed.
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 shooted seed cluster.
If you want to prevent the automatic gardenlet deployment,
use the no-gardenlet value in the shoot.gardener.cloud/use-as-seed annotation.
In this case, you have to deploy the gardenlet on your own into the seed cluster.
For example, if you annotate the shoot cluster with
shoot.gardener.cloud/use-as-seed="true,no-gardenlet,invisible"the shooted seed is created without gardenlet, and thegarden-schedulerignores it (it’s invisible).
More information: Create Shooted Seed Cluster
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 shooted, as mentioned earlier).
More information: Deploy a Gardenlet for all instructions.