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Kubernetes autoscaler charm

The Kubernetes autoscaler charm is an operator charm designed to be run on top of a Charmed Kubernetes cluster. The charm automatically adjusts the size of the Kubernetes cluster when one of the following conditions is true:

  • there are pods that failed to be scheduled in the cluster due to insufficient resources
  • there are nodes in the cluster that have been underutilized for an extended period of time and their pods can be placed on other existing nodes

This work is based on the upstream autoscaler code - see the FAQ for more details.

Deploying the autoscaler

The autoscaler charm must be deployed in a Kubernetes model on your cluster, not a machine model (i.e., it runs on Kubernetes). These instructions assume you have read and followed the instructions for adding your Charmed Kubernetes cluster to your Juju controller. These instructions assume you have a cluster registered with Juju and named ck8s. Please adjust the commands given appropriately if you have used a different name.

It is also recommended to schedule the autoscaler charm pods on control-plane nodes, not worker nodes. In order to schedule pods on control-plane nodes, ensure the control-plane nodes do not have the taint juju.is/kubernetes-control-plane=true:NoSchedule applied:

kubectl get nodes -o custom-columns=NAME:.metadata.name,TAINTS:.spec.taints --no-headers

In order to remove the taint, for each tainted control-plane node run:

kubectl taint node $NODE juju.is/kubernetes-control-plane=true:NoSchedule-

Assuming you have a Charmed-Kubernetes deployment running and have copied the kube config file from your control-plane:

If necessary, add a new model to deploy the autoscaler charm into. Adding a model creates a Kubernetes namespace behind the scenes:

juju add-model autoscaler-model ck8s

Next, deploy the autoscaler charm. Note that it must be deployed with trust:

juju deploy kubernetes-autoscaler --trust --constraints "tags=node.juju-application=kubernetes-control-plane"

The --trust switch means that the autoscaler will have access to the Juju credentials required to add and remove nodes.

Also note that the node constraints do not currently work as intended (see LP1970991). The following workaround can be used to patch the autoscaler pods onto control-plane nodes:

kubectl patch statefulset kubernetes-autoscaler -p '{"spec": {"template": {"spec": {"nodeSelector": {"juju-application": "kubernetes-control-plane"}}}}}' -n autoscaler-model

Configuration

Once deployed, the charm must be configured in order to communicate with the Juju controller. You will need to retrieve some information about your Juju controller first. Note that the controller of interest is the Juju controller which contains the Charmed Kubernetes model:

KUBE_CONTROLLER=ck8s
API_ENDPOINTS=$(juju show-controller $KUBE_CONTROLLER --format json | jq -rc '.[].details["api-endpoints"] | join(",")' )
CA_CERT=$(juju show-controller $KUBE_CONTROLLER --format json | jq -rc '.[].details["ca-cert"]' | base64 -w0)
USERNAME=$(juju show-controller $KUBE_CONTROLLER --format json | jq -rc '.[].account.user')
PASSWORD=$(juju show-controller $KUBE_CONTROLLER --show-password --format json | jq -rc '.[].account.password')

Next configure the charm:

juju config kubernetes-autoscaler \
    juju_api_endpoints="${API_ENDPOINTS}" \
    juju_ca_cert="${CA_CERT}"\
    juju_username="${USERNAME}"\
    juju_password="${PASSWORD}"

The charm also needs to know the UUID of your Charmed Kubernetes model. You can obtain a list of the available models using the command below:

juju models -c $KUBE_CONTROLLER --format json | jq -cr '.models[]|{name,"model-uuid"}'

Obtain the UUID of the Charmed Kubernetes model from the list, then configure the default model UUID (replace <CHARMED_K8s_MODEL_UUID> with the UUID of your Charmed Kubernetes model):

juju config kubernetes-autoscaler juju_default_model_uuid="<CHARMED_K8s_MODEL_UUID>"

Finally, configure the scaling. You will need to provide a minimum number of nodes, maximum number of nodes, and also the name of your kubernetes-worker application (typically this is kubernetes-worker). Note that currently the autoscaler charm does not support scaling to/from 0 nodes, so the minimum must be greater than 0:

juju config kubernetes-autoscaler juju_scale="- {min: 1, max: 3, application: kubernetes-worker}"

Additional configuration of autoscaler behaviour can be made via the autoscaler_extra_args configuration option, which allows additional parameters. An example of this configuration is shown below:

juju config kubernetes-autoscaler autoscaler_extra_args="{v: 5, scale-down-delay-after-add: 1m0s, scale-down-unneeded-time: 1m0s, unremovable-node-recheck-timeout: 1m0s}"

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