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Readiness of Shoot Worker Nodes


When registering new Nodes, kubelet adds the node.kubernetes.io/not-ready taint to prevent scheduling workload Pods to the Node until the Ready condition gets True. However, the kubelet does not consider the readiness of node-critical Pods. Hence, the Ready condition might get True and the node.kubernetes.io/not-ready taint might get removed, for example, before the CNI daemon Pod (e.g., calico-node) has successfully placed the CNI binaries on the machine.

This problem has been discussed extensively in kubernetes, e.g., in kubernetes/kubernetes#75890. However, several proposals have been rejected because the problem can be solved by using the --register-with-taints kubelet flag and dedicated controllers (ref).

Implementation in Gardener

Gardener makes sure that workload Pods are only scheduled to Nodes where all node-critical components required for running workload Pods are ready. For this, Gardener follows the proposed solution by the Kubernetes community and registers new Node objects with the node.gardener.cloud/critical-components-not-ready taint (effect NoSchedule). gardener-resource-manager’s Node controller reacts on newly created Node objects that have this taint. The controller removes the taint once all node-critical Pods are ready (determined by checking the Pods’ Ready conditions).

The Node controller considers all DaemonSets and Pods with the label node.gardener.cloud/critical-component=true as node-critical. If there are DaemonSets that contain the node.gardener.cloud/critical-component=true label in their metadata and in their Pod template, the Node controller waits for corresponding daemon Pods to be scheduled and to get ready before removing the taint.

Additionally, the Node controller checks for the readiness of csi-driver-node components if a respective Pod indicates that it uses such a driver. This is achieved through a well-defined annotation prefix (node.gardener.cloud/wait-for-csi-node-). For example, the csi-driver-node Pod for Openstack Cinder is annotated with node.gardener.cloud/wait-for-csi-node-cinder=cinder.csi.openstack.org. A key prefix is used instead of a “regular” annotation to allow for multiple CSI drivers being registered by one csi-driver-node Pod. The annotation key’s suffix can be chosen arbitrarily (in this case cinder) and the annotation value needs to match the actual driver name as specified in the CSINode object. The Node controller will verify that the used driver is properly registered in this object before removing the node.gardener.cloud/critical-components-not-ready taint. Note that the csi-driver-node Pod still needs to be labelled and tolerate the taint as described above to be considered in this additional check.

Marking Node-Critical Components

To make use of this feature, node-critical DaemonSets and Pods need to:

  • Tolerate the node.gardener.cloud/critical-components-not-ready NoSchedule taint.
  • Be labelled with node.gardener.cloud/critical-component=true.

csi-driver-node Pods additionally need to:

  • Be annotated with node.gardener.cloud/wait-for-csi-node-<name>=<full-driver-name>. It’s required that these Pods fulfill the above criteria (label and toleration) as well.

Gardener already marks components like kube-proxy, apiserver-proxy and node-local-dns as node-critical. Provider extensions mark components like csi-driver-node as node-critical and add the wait-for-csi-node annotation. Network extensions mark components responsible for setting up CNI on worker Nodes (e.g., calico-node) as node-critical. If shoot owners manage any additional node-critical components, they can make use of this feature as well.