How can a Kubernetes cluster be upgraded without downtime?

Updating a Kubernetes cluster without stopping its operation is achieved through phased component updates with minimal downtime. The main approaches:

• Rolling update of nodes: cluster nodes are updated one by one. First, one node is taken out of service, its software is updated, then it is returned to the cluster, and the process repeats with other nodes.

• Using controllers and ReplicaSets: applications are deployed with multiple replicas. When one replica is updated, others continue to operate, ensuring availability.

• Drain and cordon nodes: before updating, a node is marked as unavailable for new pods (cordon), then all pods are moved from it to other nodes (drain).

• Updating cluster components (control plane): usually done in phases using Kubernetes' built-in mechanisms to avoid interrupting operation.

Example command for safe node update:

kubectl drain <node-name> --ignore-daemonsets
# update software on the node
kubectl uncordon <node-name>

Thus, thanks to Kubernetes architecture and proper update strategies, continuous cluster operation is achieved.