Adapt applications for use in mixed-OS Kubernetes clusters

Applies to: AKS on Azure Stack HCI 22H2, AKS on Windows Server

AKS enabled by Azure Arc enables you to run Kubernetes clusters with both Linux and Windows nodes, but you must make small edits to your apps for use in these mixed-OS clusters. In this how-to guide, you learn how to ensure your application gets scheduled on the right host OS using either node selectors or taints and tolerations.

This article assumes a basic understanding of Kubernetes concepts. For more information, see Kubernetes core concepts for AKS hybrid.

Node selectors

A Node Selector is a simple field in the pod specification YAML that constrains pods to only be scheduled onto healthy nodes matching the operating system. In your pod specification YAML, specify a nodeSelector: Windows or Linux, as shown in the following examples: = Windows

or, = Linux

For more information about nodeSelectors, see node selectors.

Taints and tolerations

Taints and tolerations work together to ensure that pods aren't scheduled on nodes unintentionally. A node can be "tainted" to reject pods that don't explicitly tolerate its taint through a "toleration" in the pod specification YAML.

Windows OS nodes in AKS Arc can be tainted when created with the New-AksHciNodePool or the New-AksHciCluster commands. You can also use these commands to taint Linux OS nodes. The following example taints Windows nodes.

Apply taint to new cluster

If you also create a new cluster, run the following command to create a Windows node pool with a taint. If you have an existing cluster to which you want to add a node pool with a taint, see the next example, which uses the New-AksHciNodePool command.

New-AksHciCluster -name mycluster -nodePoolName taintnp -nodeCount 1 -osType Windows -osSku Windows2022 -taints sku=Windows:NoSchedule

Add tainted node pool to existing cluster

To add a tainted node pool to an existing cluster, run the following command:

New-AksHciNodePool -clusterName <cluster-name> -nodePoolNAme taintnp -count 1 -osType Windows -osSku Windows2022 -taints sku=Windows:NoSchedule

To check that the node pool was successfully deployed with the taint, run the following command:

Get-AksHciNodePool -clusterName <cluster-name> -name taintnp

Example output:

Status       : {Phase, Details}
ClusterName  : mycluster
NodePoolName : taintnp
Version      : v1.20.7-kvapkg.1
OsType       : Windows
NodeCount    : 0
VmSize       : Standard_K8S3_v1
Phase        : Deployed
Taints       : {sku=Windows:NoSchedule}

Specify toleration for pod

You can specify a toleration for a pod in the pod specification YAML. The following toleration "matches" the taint created by the kubectl taint line shown in the previous example. The result is that a pod with the toleration can schedule onto the tainted nodes.

- key:
  operator: Equal
  value: Windows
  effect: NoSchedule

The steps in this section work well if you're in control of the pod spec that you're deploying. However, in some cases, users have a pre-existing large number of deployments for Linux containers, as well as an ecosystem of common configurations, such as community Helm charts. You won't have access to the pod spec unless you want to download and edit the chart.

If you deploy these Helm charts to a mixed cluster environment with both Linux and Windows worker nodes, your application pods fail with the error "ImagePullBackOff". For example:

kubectl get pods
NAMESPACE              NAME                                                    READY   STATUS              RESTARTS   AGE
default                nginx-deployment-558fc78868-795dp                       0/1     ImagePullBackOff    0          6m24s
default                nginx-deployment-6b474476c4-gpb77                       0/1     ImagePullBackOff    0          11m

In this instance, you can use taints to help with this. Windows Server nodes can be tainted with the key-value pair

For more information about taints and tolerations, see Taints and Tolerations.

Next steps

In this how-to guide, you learned how to add node selectors or taints and tolerations to your Kubernetes clusters using kubectl. Next, you can: