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| Autoscale the DNS Service in a Cluster |
{% capture overview %} This page shows how to enable and configure autoscaling of the DNS service in a Kubernetes cluster. {% endcapture %}
{% capture prerequisites %}
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{% include task-tutorial-prereqs.md %}
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Make sure the DNS feature itself is enabled.
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Kubernetes version 1.4.0 or later is recommended.
{% endcapture %}
{% capture steps %}
Determining whether DNS horizontal autoscaling is already enabled
List the Deployments in your cluster in the kube-system namespace:
kubectl get deployment --namespace=kube-system
The output is similar to this:
NAME DESIRED CURRENT 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.
Getting the name of your DNS Deployment or ReplicationController
List the Deployments in your cluster in the kube-system namespace:
kubectl get deployment --namespace=kube-system
The output is similar to this:
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
...
kube-dns 1 1 1 1 ...
...
In Kubernetes versions earlier than 1.5 DNS is implemented using a ReplicationController instead of a Deployment. So if you don't see kube-dns, or a similar name, in the preceding output, list the ReplicationControllers in your cluster in the kube-system namespace:
kubectl get rc --namespace=kube-system
The output is similar to this:
NAME DESIRED CURRENT READY AGE
...
kube-dns-v20 1 1 1 ...
...
Determining your scale target
If you have a DNS Deployment, your scale target is:
Deployment/<your-deployment-name>
where is the name of your DNS Deployment. For example, if your DNS Deployment name is kube-dns, your scale target is Deployment/kube-dns.
If you have a DNS ReplicationController, your scale target is:
ReplicationController/<your-rc-name>
where is the name of your DNS ReplicationController. For example, if your DNS ReplicationController name is kube-dns-v20, your scale target is ReplicationController/kube-dns-v20.
Enabling DNS horizontal autoscaling
In this section, you create a 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:
{% include code.html language="yaml" file="dns-horizontal-autoscaler.yaml" ghlink="/docs/tasks/administer-cluster/dns-horizontal-autoscaler.yaml" %}
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 create -f dns-horizontal-autoscaler.yaml
The output of a successful command is:
deployment "kube-dns-autoscaler" created
DNS horizontal autoscaling is now enabled.
Tuning autoscaling parameters
Verify 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 number 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
integers.
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 autoscaling
There are a few options for turning DNS horizontal autoscaling. Which option to use depends on different conditions.
Option 1: Scale down the kube-dns-autoscaler deployment to 0 replicas
This option works for all situations. Enter this command:
kubectl scale deployment --replicas=0 kube-dns-autoscaler --namespace=kube-system
The output is:
deployment "kube-dns-autoscaler" scaled
Verify that the replica count is zero:
kubectl get deployment --namespace=kube-system
The output displays 0 in the DESIRED and CURRENT columns:
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
...
kube-dns-autoscaler 0 0 0 0 ...
...
Option 2: Delete the kube-dns-autoscaler deployment
This 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 "kube-dns-autoscaler" deleted
Option 3: Delete the kube-dns-autoscaler manifest file from the master node
This option works if kube-dns-autoscaler is under control of the Addon Manager's control, 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.
{% endcapture %}
{% capture discussion %}
Understanding how DNS horizontal autoscaling works
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The cluster-proportional-autoscaler application is deployed separately from the DNS service.
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An autoscaler Pod runs a client that polls the Kubernetes API server for the number of nodes and cores in the cluster.
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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.
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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.
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Changes to the scaling parameters are allowed without rebuilding or restarting the autoscaler Pod.
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The autoscaler provides a controller interface to support two control patterns: linear and ladder.
Future enhancements
Control patterns, in addition to linear and ladder, that consider custom metrics are under consideration as a future development.
Scaling of DNS backends based on DNS-specific metrics is under consideration as a future development. The current implementation, which uses the number of nodes and cores in cluster, is limited.
Support for custom metrics, similar to that provided by Horizontal Pod Autoscaling, is under consideration as a future development.
{% endcapture %}
{% capture whatsnext %} Learn more about the implementation of cluster-proportional-autoscaler. {% endcapture %}
{% include templates/task.md %}