What Node Autoscaling Does

1. What Node Autoscaling Does

When your workloads exceed the cluster’s current capacity, some pods will enter a Pending state. The Cluster Autoscaler or Karpenter watches for this and increases the node count automatically.
Similarly, when resources are underused for a certain time (default 10 minutes), the scaler removes unneeded nodes to save cost.

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2. Using the Cluster Autoscaler (CA)

The Cluster Autoscaler scales EKS Managed Node Groups (Auto Scaling Groups behind the scenes). It reads Kubernetes’ scheduler information, identifies pending pods, and resizes node groups accordingly.

Step 1: Prerequisites

Make sure you have:

• A running EKS cluster
• Installed CLI tools: aws, kubectl, and eksctl
• Correct IAM permissions to create policies and service accounts
• AWS OIDC provider configured for EKS (for IRSA authentication)

Step 2: Tag Node Groups

Cluster Autoscaler discovers node groups using tags. For each Auto Scaling Group (ASG), add:

k8s.io/cluster-autoscaler/enabled
k8s.io/cluster-autoscaler/<cluster-name>

You can add these using the AWS Console or CLI:

aws autoscaling create-or-update-tags --tags Key=k8s.io/cluster-autoscaler/enabled,Value=true,PropagateAtLaunch=true
aws autoscaling create-or-update-tags --tags Key=k8s.io/cluster-autoscaler/my-eks-cluster,Value=true,PropagateAtLaunch=true

These tags tell AWS which node groups the autoscaler can modify.

Step 3: Deploy the Cluster Autoscaler

Create a deployment using the official Cluster Autoscaler image:

kubectl apply -f https://github.com/kubernetes/autoscaler/releases/download/cluster-autoscaler-chart/cluster-autoscaler-autodiscover.yaml

You can also create a manifest manually:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: cluster-autoscaler
  namespace: kube-system
spec:
  replicas: 1
  selector:
    matchLabels:
      app: cluster-autoscaler
  template:
    metadata:
      labels:
        app: cluster-autoscaler
    spec:
      serviceAccountName: cluster-autoscaler
      containers:
        - name: cluster-autoscaler
          image: registry.k8s.io/autoscaling/cluster-autoscaler:v1.28.0
          command:
            - ./cluster-autoscaler
            - --cloud-provider=aws
            - --node-group-auto-discovery=asg:tag=k8s.io/cluster-autoscaler/enabled,k8s.io/cluster-autoscaler/my-eks-cluster
            - --balance-similar-node-groups=true
            - --skip-nodes-with-system-pods=false
          env:
            - name: AWS_REGION
              value: us-east-1
          resources:
            limits:
              cpu: 100m
              memory: 300Mi

This deployment watches your cluster size and adds or removes nodes through your ASGs transparently.

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3. Using Karpenter (Next‑Gen Autoscaler)

Karpenter is AWS’s newer and faster autoscaler designed for EKS. It launches EC2 instances directly, without needing ASGs, and right-sizes nodes based on pod demand. It is more cost-efficient than Cluster Autoscaler.

Step 1: Install Karpenter

helm repo add karpenter https://charts.karpenter.sh/
helm repo update

helm install karpenter karpenter/karpenter \
  --namespace karpenter \
  --create-namespace \
  --set clusterName=my-eks-cluster \
  --set serviceAccount.create=true \
  --set aws.defaultInstanceProfile=KarpenterNodeInstanceProfile-my-eks-cluster \
  --set aws.clusterEndpoint="$(aws eks describe-cluster --name my-eks-cluster --query "cluster.endpoint" --output text)"

Step 2: Create a Karpenter Provisioner

Karpenter replaces static ASGs with flexible provisioning. Example:

apiVersion: karpenter.sh/v1alpha5
kind: Provisioner
metadata:
  name: default
spec:
  limits:
    resources:
      cpu: 1000
  providerRef:
    name: default
  requirements:
    - key: "node.kubernetes.io/instance-type"
      operator: In
      values: ["t3.medium", "t3.large"]
  ttlSecondsAfterEmpty: 300

This allows Karpenter to launch nodes matching pod resource requirements dynamically and terminate them when idle.

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4. Choosing Between Cluster Autoscaler and Karpenter

• Scaling Target

  • Cluster Autoscaler: AutoScaling Groups
  • Karpenter: Direct EC2 instances

• Speed

  • Cluster Autoscaler: Moderate
  • Karpenter: Very Fast

• Cost Efficiency

  • Cluster Autoscaler: Good
  • Karpenter: Excellent

• Setup Complexity

  • Cluster Autoscaler: Simple
  • Karpenter: Moderate

• Best For

  • Cluster Autoscaler: Existing ASGs or managed node groups
  • Karpenter: Dynamic, cost‑optimized workloads

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5. Additional Tips

• Combine Cluster Autoscaler (node scaling) with Horizontal Pod Autoscaler (HPA) for complete autoscaling (pods + nodes).
• Set your node group minSize and maxSize ranges carefully in EKS so the autoscaler knows its limits.
• Monitor autoscaling events via CloudWatch logs or kubectl describe deployment cluster-autoscaler.
• For spot instances, combine autoscaler with Mixed Instance Policies or use Karpenter, which handles spot interruptions gracefully.

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In short:
On EKS, enable node autoscaling either by deploying the Cluster Autoscaler (works with EKS node groups) or installing Karpenter (for dynamic provisioning). Both automatically add nodes when workloads increase and remove idle ones to save cost.

References

• https://devopscube.com/cluster-autoscaler/)
• https://www.stormit.cloud/blog/aws-eks-autoscaling/)