• When we download a K8s release, the 5 core components shown on the left in the image above are at the same version.
• The K8s components can be at different versions. The kube-apiserver must be at the highest version compared to other components, except kubectl which can be one minor version above.
• Upgrading the version of a cluster basically means upgrading the version of various K8s components running in the cluster.
• K8s supports 3 latest minor versions. If the current version of a K8s component is unsupported, we should upgrade it one minor version at a time.
• Upgrading the cluster on a managed K8s engine like GKE is very easy and takes only a few clicks.
• k get nodes command shows the version of kubelet running on each node.

Upgrading the master node

First the master node is upgraded, during which the control plane components are unavailable. The worker nodes keep functioning and the application is up. While the master node is getting updated, all management functions are down. We cannot run kubectl commands as kube-apiserver is down. If a pod were to crash, a new one will not be spawned as the kube-controller-manager is down.

Upgrading worker nodes

Once the master node has been upgraded, we need to upgrade the worker nodes (upgrade the k8s components running on them). As the worker nodes serve traffic, there are various strategies to upgrade them.

Strategy 1: All at once

Upgrade all the worker nodes at once. This will lead to downtime as no pods will be running during the upgrade.

Strategy 2: One at a time

Move the pods of the first node to the remaining nodes and upgrade the first node. Then repeat the same for the rest of the nodes.