Preview - Automatically scale a cluster to meet application demands on Azure Kubernetes Service (AKS)

To keep up with application demands in Azure Kubernetes Service (AKS), you may need to adjust the number of nodes that run your workloads. The cluster autoscaler component can watch for pods in your cluster that can't be scheduled because of resource constraints. When issues are detected, the number of nodes in a node pool is increased to meet the application demand. Nodes are also regularly checked for a lack of running pods, with the number of nodes then decreased as needed. This ability to automatically scale up or down the number of nodes in your AKS cluster lets you run an efficient, cost-effective cluster.

This article shows you how to enable and manage the cluster autoscaler in an AKS cluster. The cluster autoscaler should only be tested in preview on AKS clusters.

Important

AKS preview features are self-service opt-in. Previews are provided "as-is" and "as available" and are excluded from the service level agreements and limited warranty. AKS Previews are partially covered by customer support on best effort basis. As such, these features are not meant for production use. For additional infromation, please see the following support articles:

Before you begin

This article requires that you are running the Azure CLI version 2.0.65 or later. Run az --version to find the version. If you need to install or upgrade, see Install Azure CLI.

Install aks-preview CLI extension

To use the cluster autoscaler, you need the aks-preview CLI extension version 0.4.12 or higher. Install the aks-preview Azure CLI extension using the az extension add command, then check for any available updates using the az extension update command:

# Install the aks-preview extension
az extension add --name aks-preview

# Update the extension to make sure you have the latest version installed
az extension update --name aks-preview

Limitations

The following limitations apply when you create and manage AKS clusters that use the cluster autoscaler:

  • The HTTP application routing add-on can't be used.

About the cluster autoscaler

To adjust to changing application demands, such as between the workday and evening or on a weekend, clusters often need a way to automatically scale. AKS clusters can scale in one of two ways:

  • The cluster autoscaler watches for pods that can't be scheduled on nodes because of resource constraints. The cluster then automatically increases the number of nodes.
  • The horizontal pod autoscaler uses the Metrics Server in a Kubernetes cluster to monitor the resource demand of pods. If an application needs more resources, the number of pods is automatically increased to meet the demand.

The cluster autoscaler and horizontal pod autoscaler often work together to support the required application demands

Both the horizontal pod autoscaler and cluster autoscaler can also decrease the number of pods and nodes as needed. The cluster autoscaler decreases the number of nodes when there has been unused capacity for a period of time. Pods on a node to be removed by the cluster autoscaler are safely scheduled elsewhere in the cluster. The cluster autoscaler may be unable to scale down if pods can't move, such as in the following situations:

  • A pod directly created and isn't backed by a controller object, such a deployment or replica set.
  • A pod disruption budget (PDB) is too restrictive and doesn't allow the number of pods to be fall below a certain threshold.
  • A pod uses node selectors or anti-affinity that can't be honored if scheduled on a different node.

For more information about how the cluster autoscaler may be unable to scale down, see What types of pods can prevent the cluster autoscaler from removing a node?

The cluster autoscaler uses startup parameters for things like time intervals between scale events and resource thresholds. These parameters are defined by the Azure platform, and aren't currently exposed for you to adjust. For more information on what parameters the cluster autoscaler uses, see What are the cluster autoscaler parameters?.

The cluster and horizontal pod autoscalers can work together, and are often both deployed in a cluster. When combined, the horizontal pod autoscaler is focused on running the number of pods required to meet application demand. The cluster autoscaler is focused on running the number of nodes required to support the scheduled pods.

Note

Manual scaling is disabled when you use the cluster autoscaler. Let the cluster autoscaler determine the required number of nodes. If you want to manually scale your cluster, disable the cluster autoscaler.

Create an AKS cluster and enable the cluster autoscaler

If you need to create an AKS cluster, use the az aks create command. To enable and configure the cluster autoscaler on the node pool for the cluster, use the --enable-cluster-autoscaler parameter, and specify a node --min-count and --max-count.

Important

The cluster autoscaler is a Kubernetes component. Although the AKS cluster uses a virtual machine scale set for the nodes, don't manually enable or edit settings for scale set autoscale in the Azure portal or using the Azure CLI. Let the Kubernetes cluster autoscaler manage the required scale settings. For more information, see Can I modify the AKS resources in the node resource group?

The following example creates an AKS cluster with single node pool backed by a virtual machine scale set. It also enables the cluster autoscaler on the node pool for the cluster and sets a minimum of 1 and maximum of 3 nodes:

# First create a resource group
az group create --name myResourceGroup --location eastus

# Now create the AKS cluster and enable the cluster autoscaler
az aks create \
  --resource-group myResourceGroup \
  --name myAKSCluster \
  --node-count 1 \
  --vm-set-type VirtualMachineScaleSets \
  --enable-cluster-autoscaler \
  --min-count 1 \
  --max-count 3

Note

If you specify a --kubernetes-version when running az aks create, that version must meet or exceed the minimum version number required as outlined in the preceding Before you begin section.

It takes a few minutes to create the cluster and configure the cluster autoscaler settings.

Change the cluster autoscaler settings

Important

If you have the multiple agent pools feature enabled on your subscription, skip to the autoscale with multiple agent pools section. Clusters with multiple agent pools enabled require use of the az aks nodepool command set to change node pool specific properties instead of az aks. The below instructions assume you have not enabled multiple node pools. To check if you have it enabled, run az feature list -o table and look for Microsoft.ContainerService/multiagentpoolpreview.

In the previous step to create an AKS cluster or update an existing node pool, the cluster autoscaler minimum node count was set to 1, and the maximum node count was set to 3. As your application demands change, you may need to adjust the cluster autoscaler node count.

To change the node count, use the az aks update command.

az aks update \
  --resource-group myResourceGroup \
  --name myAKSCluster \
  --update-cluster-autoscaler \
  --min-count 1 \
  --max-count 5

The above example updates cluster autoscaler on the single node pool in myAKSCluster to a minimum of 1 and maximum of 5 nodes.

Note

During preview, you can't set a higher minimum node count than is currently set for the node pool. For example, if you currently have min count set to 1, you can't update the min count to 3.

Monitor the performance of your applications and services, and adjust the cluster autoscaler node counts to match the required performance.

Disable the cluster autoscaler

If you no longer wish to use the cluster autoscaler, you can disable it using the az aks update command, specifying the --disable-cluster-autoscaler parameter. Nodes aren't removed when the cluster autoscaler is disabled.

az aks update \
  --resource-group myResourceGroup \
  --name myAKSCluster \
  --disable-cluster-autoscaler

You can manually scale your cluster after disabling the cluster autoscaler by using the az aks scale command. If you use the horizontal pod autoscaler, that feature continues to run with the cluster autoscaler disabled, but pods may end up unable to be scheduled if all node resources are in use.

Re-enable a disabled cluster autoscaler

If you wish to re-enable the cluster autoscaler on an existing cluster, you can re-enable it using the az aks update command, specifying the --enable-cluster-autoscaler parameter.

Use the cluster autoscaler with multiple node pools enabled

The cluster autoscaler can be used together with the multiple node pools preview feature enabled. Follow that document to learn how to enable multiple node pools and add additional node pools to an existing cluster. When using both features together, you enable the cluster autoscaler on each individual node pool in the cluster and can pass unique autoscaling rules to each.

The below command assumes you followed the initial instructions earlier in this document and you want to update an existing node pool's max-count from 3 to 5. Use the az aks nodepool update command to update an existing node pool's settings.

az aks nodepool update \
  --resource-group myResourceGroup \
  --cluster-name multipoolcluster \
  --name mynodepool \
  --enable-cluster-autoscaler \
  --min-count 1 \
  --max-count 5

The cluster autoscaler can be disabled with az aks nodepool update and passing the --disable-cluster-autoscaler parameter.

az aks nodepool update \
  --resource-group myResourceGroup \
  --cluster-name multipoolcluster \
  --name mynodepool \
  --disable-cluster-autoscaler

Next steps

This article showed you how to automatically scale the number of AKS nodes. You can also use the horizontal pod autoscaler to automatically adjust the number of pods that run your application. For steps on using the horizontal pod autoscaler, see Scale applications in AKS.