# Autoscale the DNS Service in a Cluster

Autoscale the DNS Service in a ClusterThis page shows how to enable and configure autoscaling of the DNS service in your Kubernetes cluster.Before you beginYou need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:iximiuz LabsKillercodaKodeKloudPlay with KubernetesTo check the version, enter kubectl version.This guide assumes your nodes use the AMD64 or Intel 64 CPU architecture.Make sure Kubernetes DNS is enabled.Determine whether DNS horizontal autoscaling is already enabledList the Deployments in your cluster in the kube-system namespace:kubectl get deployment --namespace=kube-system The output is similar to this:NAME READY UP-TO-DATE AVAILABLE AGE ... kube-dns-autoscaler 1/1 1 1 ... ... If you see "kube-dns-autoscaler" in the output, DNS horizontal autoscaling is already enabled, and you can skip to Tuning autoscaling parameters.Get the name of your DNS DeploymentList the DNS deployments in your cluster in the kube-system namespace:kubectl get deployment -l k8s-app=kube-dns --namespace=kube-system The output is similar to this:NAME READY UP-TO-DATE AVAILABLE AGE ... coredns 2/2 2 2 ... ... If you don't see a Deployment for DNS services, you can also look for it by name:kubectl get deployment --namespace=kube-system and look for a deployment named coredns or kube-dns.Your scale target isDeployment/<your-deployment-name> where <your-deployment-name> is the name of your DNS Deployment. For example, if the name of your Deployment for DNS is coredns, your scale target is Deployment/coredns.Note:CoreDNS is the default DNS service for Kubernetes. CoreDNS sets the label k8s-app=kube-dns so that it can work in clusters that originally used kube-dns.Enable DNS horizontal autoscalingIn this section, you create a new Deployment. The Pods in the Deployment run a container based on the cluster-proportional-autoscaler-amd64 image.Create a file named dns-horizontal-autoscaler.yaml with this content:admin/dns/dns-horizontal-autoscaler.yaml kind: ServiceAccount apiVersion: v1 metadata: name: kube-dns-autoscaler namespace: kube-system --- kind: ClusterRole apiVersion: rbac.authorization.k8s.io/v1 metadata: name: system:kube-dns-autoscaler rules: - apiGroups: [""] resources: ["nodes"] verbs: ["list", "watch"] - apiGroups: [""] resources: ["replicationcontrollers/scale"] verbs: ["get", "update"] - apiGroups: ["apps"] resources: ["deployments/scale", "replicasets/scale"] verbs: ["get", "update"] # Remove the configmaps rule once below issue is fixed: # kubernetes-incubator/cluster-proportional-autoscaler#16 - apiGroups: [""] resources: ["configmaps"] verbs: ["get", "create"] --- kind: ClusterRoleBinding apiVersion: rbac.authorization.k8s.io/v1 metadata: name: system:kube-dns-autoscaler subjects: - kind: ServiceAccount name: kube-dns-autoscaler namespace: kube-system roleRef: kind: ClusterRole name: system:kube-dns-autoscaler apiGroup: rbac.authorization.k8s.io --- apiVersion: apps/v1 kind: Deployment metadata: name: kube-dns-autoscaler namespace: kube-system labels: k8s-app: kube-dns-autoscaler kubernetes.io/cluster-service: "true" spec: selector: matchLabels: k8s-app: kube-dns-autoscaler template: metadata: labels: k8s-app: kube-dns-autoscaler spec: priorityClassName: system-cluster-critical securityContext: seccompProfile: type: RuntimeDefault supplementalGroups: [ 65534 ] fsGroup: 65534 nodeSelector: kubernetes.io/os: linux containers: - name: autoscaler image: registry.k8s.io/cpa/cluster-proportional-autoscaler:1.8.4 resources: requests: cpu: "20m" memory: "10Mi" command: - /cluster-proportional-autoscaler - --namespace=kube-system - --configmap=kube-dns-autoscaler # Should keep target in sync with cluster/addons/dns/kube-dns.yaml.base - --target=<SCALE_TARGET> # When cluster is using large nodes(with more cores), "coresPerReplica" should dominate. # If using small nodes, "nodesPerReplica" should dominate. - --default-params={"linear":{"coresPerReplica":256,"nodesPerReplica":16,"preventSinglePointFailure":true,"includeUnschedulableNodes":true}} - --logtostderr=true - --v=2 tolerations: - key: "CriticalAddonsOnly" operator: "Exists" serviceAccountName: kube-dns-autoscaler In the file, replace <SCALE_TARGET> with your scale target.Go to the directory that contains your configuration file, and enter this command to create the Deployment:kubectl apply -f dns-horizontal-autoscaler.yaml The output of a successful command is:deployment.apps/kube-dns-autoscaler created DNS horizontal autoscaling is now enabled.Tune DNS autoscaling parametersVerify that the kube-dns-autoscaler ConfigMap exists:kubectl get configmap --namespace=kube-system The output is similar to this:NAME DATA AGE ... kube-dns-autoscaler 1 ... ... Modify the data in the ConfigMap:kubectl edit configmap kube-dns-autoscaler --namespace=kube-system Look for this line:linear: '{"coresPerReplica":256,"min":1,"nodesPerReplica":16}' Modify the fields according to your needs. The "min" field indicates the minimal number of DNS backends. The actual number of backends is calculated using this equation:replicas = max( ceil( cores × 1/coresPerReplica ) , ceil( nodes × 1/nodesPerReplica ) ) Note that the values of both coresPerReplica and nodesPerReplica are floats.The idea is that when a cluster is using nodes that have many cores, coresPerReplica dominates. When a cluster is using nodes that have fewer cores, nodesPerReplica dominates.There are other supported scaling patterns. For details, see cluster-proportional-autoscaler.Disable DNS horizontal autoscalingThere are a few options for tuning DNS horizontal autoscaling. Which option to use depends on different conditions.Option 1: Scale down the kube-dns-autoscaler deployment to 0 replicasThis option works for all situations. Enter this command:kubectl scale deployment --replicas=0 kube-dns-autoscaler --namespace=kube-system The output is:deployment.apps/kube-dns-autoscaler scaled Verify that the replica count is zero:kubectl get rs --namespace=kube-system The output displays 0 in the DESIRED and CURRENT columns:NAME DESIRED CURRENT READY AGE ... kube-dns-autoscaler-6b59789fc8 0 0 0 ... ... Option 2: Delete the kube-dns-autoscaler deploymentThis option works if kube-dns-autoscaler is under your own control, which means no one will re-create it:kubectl delete deployment kube-dns-autoscaler --namespace=kube-system The output is:deployment.apps "kube-dns-autoscaler" deleted Option 3: Delete the kube-dns-autoscaler manifest file from the master nodeThis option works if kube-dns-autoscaler is under control of the (deprecated) Addon Manager, and you have write access to the master node.Sign in to the master node and delete the corresponding manifest file. The common path for this kube-dns-autoscaler is:/etc/kubernetes/addons/dns-horizontal-autoscaler/dns-horizontal-autoscaler.yaml After the manifest file is deleted, the Addon Manager will delete the kube-dns-autoscaler Deployment.Understanding how DNS horizontal autoscaling worksThe cluster-proportional-autoscaler application is deployed separately from the DNS service.An autoscaler Pod runs a client that polls the Kubernetes API server for the number of nodes and cores in the cluster.A desired replica count is calculated and applied to the DNS backends based on the current schedulable nodes and cores and the given scaling parameters.The scaling parameters and data points are provided via a ConfigMap to the autoscaler, and it refreshes its parameters table every poll interval to be up to date with the latest desired scaling parameters.Changes to the scaling parameters are allowed without rebuilding or restarting the autoscaler Pod.The autoscaler provides a controller interface to support two control patterns: linear and ladder.What's nextRead about Guaranteed Scheduling For Critical Add-On Pods.Learn more about the implementation of cluster-proportional-autoscaler.