Autoscale your AKS pods by using Application Gateway metrics

As incoming traffic increases, it becomes crucial to scale up your applications based on the demand.

This article explains how you can use the AvgRequestCountPerHealthyHost metric in Azure Application Gateway to scale up Azure Kubernetes Service (AKS) pods for an application. The AvgRequestCountPerHealthyHost metric measures average requests sent to a specific combination of a backend pool and a backend HTTP setting.

Use the following two components:

• Azure Kubernetes Metrics Adapter: You use this component to expose Application Gateway metrics through the metric server. It's an open-source project under Azure, similar to the Application Gateway Ingress Controller.
• Horizontal Pod Autoscaler: You use this component to apply Application Gateway metrics and target a deployment for scaling.

Set up the Azure Kubernetes Metrics Adapter

1. Create a Microsoft Entra service principal and assign it Monitoring Reader access over the Application Gateway deployment's resource group:

   applicationGatewayGroupName="<application-gateway-group-id>"
   applicationGatewayGroupId=$(az group show -g $applicationGatewayGroupName -o tsv --query "id")
   az ad sp create-for-rbac -n "azure-k8s-metric-adapter-sp" --role "Monitoring Reader" --scopes applicationGatewayGroupId
   

2. Deploy the Azure Kubernetes Metrics Adapter by using the Microsoft Entra service principal that you created previously:

   kubectl create namespace custom-metrics
   # use values from service principal created previously to create secret
   kubectl create secret generic azure-k8s-metrics-adapter -n custom-metrics \
       --from-literal=azure-tenant-id=<tenantid> \
       --from-literal=azure-client-id=<clientid> \
       --from-literal=azure-client-secret=<secret>
   kubectl apply -f kubectl apply -f https://raw.githubusercontent.com/Azure/azure-k8s-metrics-adapter/master/deploy/adapter.yaml -n custom-metrics
   

3. Create an ExternalMetric resource with the name appgw-request-count-metric. This resource instructs the metric adapter to expose the AvgRequestCountPerHealthyHost metric for the myApplicationGateway resource in the myResourceGroup resource group. You can use the filter field to target a specific backend pool and backend HTTP setting in the Application Gateway deployment.

   apiVersion: azure.com/v1alpha2
   kind: ExternalMetric
   metadata:
   name: appgw-request-count-metric
   spec:
       type: azuremonitor
       azure:
           resourceGroup: myResourceGroup # replace with your Application Gateway deployment's resource group name
           resourceName: myApplicationGateway # replace with your Application Gateway deployment's name
           resourceProviderNamespace: Microsoft.Network
           resourceType: applicationGateways
       metric:
           metricName: AvgRequestCountPerHealthyHost
           aggregation: Average
           filter: BackendSettingsPool eq '<backend-pool-name>~<backend-http-setting-name>' # optional
   

You can now make a request to the metric server to see if the new metric is being exposed:

kubectl get --raw "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/appgw-request-count-metric"
# Sample Output
# {
#   "kind": "ExternalMetricValueList",
#   "apiVersion": "external.metrics.k8s.io/v1beta1",
#   "metadata":
#     {
#       "selfLink": "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/appgw-request-count-metric",
#     },
#   "items":
#     [
#       {
#         "metricName": "appgw-request-count-metric",
#         "metricLabels": null,
#         "timestamp": "2019-11-05T00:18:51Z",
#         "value": "30",
#       },
#     ],
# }

Use the new metric to scale up the deployment

After you expose appgw-request-count-metric through the metric server, you're ready to use the Horizontal Pod Autoscaler to scale up your target deployment.

The following example targets a sample deployment named aspnet. You scale up pods when appgw-request-count-metric is 200 per pod, up to a maximum of 10 pods.

Replace your target deployment name and apply the following autoscale configuration:

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: deployment-scaler
spec:
  scaleTargetRef:
    apiVersion: networking.k8s.io/v1
    kind: Deployment
    name: aspnet # replace with your deployment's name
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: External
    external:
      metricName: appgw-request-count-metric
      targetAverageValue: 200

Test your setup by using a load test tool like ApacheBench:

ab -n10000 http://<application-gateway-ip-address>/

Related content

• Troubleshoot Application Gateway Ingress Controller issues