I want to upgrade my AKS cluster using terraform without or with minimal downtime. 1. What happens to the workloads during the cluster upgrade. 2. Can i do the AKS cluster upgrade and node upgrade same time. 3. Azure provides the Scheduled AKS cluster maintenance (preview feature) , is it Azure does the cluster upgrade?
AKS Cluster uses concept of buffer node when upgrade is performed. It brings a buffer node, move the workload to buffer node and upgrades the actual node. Time taken to upgrade the cluster depends on number of nodes in the cluster.
https://docs.microsoft.com/en-us/azure/aks/upgrade-cluster#upgrade-an-aks-cluster
You can upgrade Control Plane as well as Hosted Plane using Azure CLI.
#az aks upgrade --resource-group <ResourceGroup> --name <ClusterName> -k <KubernetesVersion>