Autoscale your AKS pods using Application Gateway Metrics (Beta)

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

In the following tutorial, we explain how you can use Application Gateway's AvgRequestCountPerHealthyHost metric to scale up your application. AvgRequestCountPerHealthyHost measures average requests sent to a specific backend pool and backend HTTP setting combination.

We are going to use following two components:

  • Azure Kubernetes Metric Adapter - We will use the metric adapter to expose Application Gateway metrics through the metric server. The Azure Kubernetes Metric Adapter is an open source project under Azure, similar to the Application Gateway Ingress Controller.
  • Horizontal Pod Autoscaler - We will use HPA to use Application Gateway metrics and target a deployment for scaling.

Setting up Azure Kubernetes Metric Adapter

  1. We will first create an Azure AAD service principal and assign it Monitoring Reader access over Application Gateway'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. Now, We will deploy the Azure Kubernetes Metric Adapter using the AAD service principal created above.

    kubectl create namespace custom-metrics
    # use values from service principal created above 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. We will create an ExternalMetric resource with name appgw-request-count-metric. This resource will instruct the metric adapter to expose AvgRequestCountPerHealthyHost metric for myApplicationGateway resource in myResourceGroup resource group. You can use the filter field to target a specific backend pool and backend HTTP setting in the Application Gateway.

    apiVersion: azure.com/v1alpha2
    kind: ExternalMetric
    metadata:
    name: appgw-request-count-metric
    spec:
        type: azuremonitor
        azure:
            resourceGroup: myResourceGroup # replace with your application gateway's resource group name
            resourceName: myApplicationGateway # replace with your application gateway'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 our new metric is getting 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",
#       },
#     ],
# }

Using the new metric to scale up the deployment

Once we are able to expose appgw-request-count-metric through the metric server, we are ready to use Horizontal Pod Autoscaler to scale up our target deployment.

In following example, we will target a sample deployment aspnet. We will scale up Pods when appgw-request-count-metric > 200 per Pod up to a max of 10 Pods.

Replace your target deployment name and apply the following auto scale configuration:

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: deployment-scaler
spec:
  scaleTargetRef:
    apiVersion: extensions/v1beta1
    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 apache bench:

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

Next steps