Recently I tested out Amazon Web Service's new offering of managed Kubernetes service named EKS. Amazon is great about providing detailed tutorials to get started using their services and EKS is no exception. Check out the tutorial here

During the creation of the EKS cluster one of the steps is to use CloudFormation to create the cluster's worker nodes.  Tutorial here

After creating a cluster and putting it to use I was happy to finally have an AWS managed K8s cluster and all I would need to worry about is paying for use of the EC2 instances. A few days passed and I realized the cluster was using the maximum number of worker nodes (NodeAutoScalingGroupMaxSize) specified in the CloudFormation template. After contacting support they informed me there is a setting in the template to control the desired capacity (DesiredCapacity). Below are the modifications to the template to add a field for specifying the DesiredCapacity of your EKS cluster.

First I added a field for the ClusterDesiredCapacity

ClusterDesiredCapacity:
  Type: Number
  Description: Desired size of Node Group ASG.
  Default: 2

Next and Lastly I changed the setting for DesiredCapacity: under the NodeGroup section from its default setting of !Ref NodeAutoScalingGroupMaxSize to using the new field we just added !Ref ClusterDesiredCapacity

Here is the entire NodeGroup: section for reference with the change applied.

  NodeGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      DesiredCapacity: !Ref ClusterDesiredCapacity
      LaunchConfigurationName: !Ref NodeLaunchConfig
      MinSize: !Ref NodeAutoScalingGroupMinSize
      MaxSize: !Ref NodeAutoScalingGroupMaxSize
      VPCZoneIdentifier:
        !Ref Subnets
      Tags:
      - Key: Name
        Value: !Sub "${ClusterName}-${NodeGroupName}-Node"
        PropagateAtLaunch: 'true'
      - Key: !Sub 'kubernetes.io/cluster/${ClusterName}'
        Value: 'owned'
        PropagateAtLaunch: 'true'
    UpdatePolicy:
      AutoScalingRollingUpdate:
        MinInstancesInService: '1'
        MaxBatchSize: '1'

Note: Futher below I've included the entire template as well as a link to the template hosted in S3.

Now that we have a new cloudformation template we can create a new stack.

When creating the new stack you will now have a field under Other Parameters for ClusterDesiredCapacity

After creating a new stack the role arn for the worker node group will have changed so you will need to modify the AWS auth configmap to allow these new worker nodes to join your cluster. Here is an example of that config map:

apiVersion: v1
kind: ConfigMap
metadata:
  name: aws-auth
  namespace: kube-system
data:
  mapRoles: |
    - rolearn: [PUT_THE_ROLEARN_GENERATED_BY_CLOUDFORMATION_HERE]
      username: system:node:{{EC2PrivateDNSName}}
      groups:
        - system:bootstrappers
        - system:nodes
  mapUsers: |
    - userarn: arn:aws:iam::123456789123:user/youreanawesomeuser
      username: arn:aws:iam::123456789123:user/youreanawesomeuser
      groups:
        - system:masters

The final thing to watch out for is security groups. Part of the stack is a security group for the nodes being created. If you need to allow your worker nodes to access other AWS resources now would be the time to change up those security groups.

Customized Amazon S3 template URL

https://s3-external-1.amazonaws.com/cf-templates-1emn2kp68x0yx-us-east-1/2018270oW3-AWS-EKS-AutoScalingGroupTemplate_2018-08-3019lv3d6dbwn

---
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Amazon EKS - Node Group - Released 2018-08-30'

Parameters:

  KeyName:
    Description: The EC2 Key Pair to allow SSH access to the instances
    Type: AWS::EC2::KeyPair::KeyName

  NodeImageId:
    Type: AWS::EC2::Image::Id
    Description: AMI id for the node instances.

  NodeInstanceType:
    Description: EC2 instance type for the node instances
    Type: String
    Default: t2.medium
    AllowedValues:
    - t2.small
    - t2.medium
    - t2.large
    - t2.xlarge
    - t2.2xlarge
    - m3.medium
    - m3.large
    - m3.xlarge
    - m3.2xlarge
    - m4.large
    - m4.xlarge
    - m4.2xlarge
    - m4.4xlarge
    - m4.10xlarge
    - m5.large
    - m5.xlarge
    - m5.2xlarge
    - m5.4xlarge
    - m5.12xlarge
    - m5.24xlarge
    - c4.large
    - c4.xlarge
    - c4.2xlarge
    - c4.4xlarge
    - c4.8xlarge
    - c5.large
    - c5.xlarge
    - c5.2xlarge
    - c5.4xlarge
    - c5.9xlarge
    - c5.18xlarge
    - i3.large
    - i3.xlarge
    - i3.2xlarge
    - i3.4xlarge
    - i3.8xlarge
    - i3.16xlarge
    - r3.xlarge
    - r3.2xlarge
    - r3.4xlarge
    - r3.8xlarge
    - r4.large
    - r4.xlarge
    - r4.2xlarge
    - r4.4xlarge
    - r4.8xlarge
    - r4.16xlarge
    - x1.16xlarge
    - x1.32xlarge
    - p2.xlarge
    - p2.8xlarge
    - p2.16xlarge
    - p3.2xlarge
    - p3.8xlarge
    - p3.16xlarge
    ConstraintDescription: Must be a valid EC2 instance type

  NodeAutoScalingGroupMinSize:
    Type: Number
    Description: Minimum size of Node Group ASG.
    Default: 1

  NodeAutoScalingGroupMaxSize:
    Type: Number
    Description: Maximum size of Node Group ASG.
    Default: 3

  ClusterDesiredCapacity:
    Type: Number
    Description: Desired size of Node Group ASG.
    Default: 2

  NodeVolumeSize:
    Type: Number
    Description: Node volume size
    Default: 20

  ClusterName:
    Description: The cluster name provided when the cluster was created. If it is incorrect, nodes will not be able to join the cluster.
    Type: String

  BootstrapArguments:
    Description: Arguments to pass to the bootstrap script. See files/bootstrap.sh in https://github.com/awslabs/amazon-eks-ami
    Default: ""
    Type: String

  NodeGroupName:
    Description: Unique identifier for the Node Group.
    Type: String

  ClusterControlPlaneSecurityGroup:
    Description: The security group of the cluster control plane.
    Type: AWS::EC2::SecurityGroup::Id

  VpcId:
    Description: The VPC of the worker instances
    Type: AWS::EC2::VPC::Id

  Subnets:
    Description: The subnets where workers can be created.
    Type: List<AWS::EC2::Subnet::Id>

Metadata:
  AWS::CloudFormation::Interface:
    ParameterGroups:
      -
        Label:
          default: "EKS Cluster"
        Parameters:
          - ClusterName
          - ClusterControlPlaneSecurityGroup
      -
        Label:
          default: "Worker Node Configuration"
        Parameters:
          - NodeGroupName
          - NodeAutoScalingGroupMinSize
          - NodeAutoScalingGroupMaxSize
          - NodeInstanceType
          - NodeImageId
          - NodeVolumeSize
          - KeyName
          - BootstrapArguments
      -
        Label:
          default: "Worker Network Configuration"
        Parameters:
          - VpcId
          - Subnets

Resources:

  NodeInstanceProfile:
    Type: AWS::IAM::InstanceProfile
    Properties:
      Path: "/"
      Roles:
      - !Ref NodeInstanceRole

  NodeInstanceRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service:
            - ec2.amazonaws.com
          Action:
          - sts:AssumeRole
      Path: "/"
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/AmazonEKSWorkerNodePolicy
        - arn:aws:iam::aws:policy/AmazonEKS_CNI_Policy
        - arn:aws:iam::aws:policy/AmazonEC2ContainerRegistryReadOnly

  NodeSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for all nodes in the cluster
      VpcId:
        !Ref VpcId
      Tags:
      - Key: !Sub "kubernetes.io/cluster/${ClusterName}"
        Value: 'owned'

  NodeSecurityGroupIngress:
    Type: AWS::EC2::SecurityGroupIngress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow node to communicate with each other
      GroupId: !Ref NodeSecurityGroup
      SourceSecurityGroupId: !Ref NodeSecurityGroup
      IpProtocol: '-1'
      FromPort: 0
      ToPort: 65535

  NodeSecurityGroupFromControlPlaneIngress:
    Type: AWS::EC2::SecurityGroupIngress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow worker Kubelets and pods to receive communication from the cluster control plane
      GroupId: !Ref NodeSecurityGroup
      SourceSecurityGroupId: !Ref ClusterControlPlaneSecurityGroup
      IpProtocol: tcp
      FromPort: 1025
      ToPort: 65535

  ControlPlaneEgressToNodeSecurityGroup:
    Type: AWS::EC2::SecurityGroupEgress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow the cluster control plane to communicate with worker Kubelet and pods
      GroupId: !Ref ClusterControlPlaneSecurityGroup
      DestinationSecurityGroupId: !Ref NodeSecurityGroup
      IpProtocol: tcp
      FromPort: 1025
      ToPort: 65535

  NodeSecurityGroupFromControlPlaneOn443Ingress:
    Type: AWS::EC2::SecurityGroupIngress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow pods running extension API servers on port 443 to receive communication from cluster control plane
      GroupId: !Ref NodeSecurityGroup
      SourceSecurityGroupId: !Ref ClusterControlPlaneSecurityGroup
      IpProtocol: tcp
      FromPort: 443
      ToPort: 443

  ControlPlaneEgressToNodeSecurityGroupOn443:
    Type: AWS::EC2::SecurityGroupEgress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow the cluster control plane to communicate with pods running extension API servers on port 443
      GroupId: !Ref ClusterControlPlaneSecurityGroup
      DestinationSecurityGroupId: !Ref NodeSecurityGroup
      IpProtocol: tcp
      FromPort: 443
      ToPort: 443

  ClusterControlPlaneSecurityGroupIngress:
    Type: AWS::EC2::SecurityGroupIngress
    DependsOn: NodeSecurityGroup
    Properties:
      Description: Allow pods to communicate with the cluster API Server
      GroupId: !Ref ClusterControlPlaneSecurityGroup
      SourceSecurityGroupId: !Ref NodeSecurityGroup
      IpProtocol: tcp
      ToPort: 443
      FromPort: 443

  NodeGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      DesiredCapacity: !Ref ClusterDesiredCapacity
      LaunchConfigurationName: !Ref NodeLaunchConfig
      MinSize: !Ref NodeAutoScalingGroupMinSize
      MaxSize: !Ref NodeAutoScalingGroupMaxSize
      VPCZoneIdentifier:
        !Ref Subnets
      Tags:
      - Key: Name
        Value: !Sub "${ClusterName}-${NodeGroupName}-Node"
        PropagateAtLaunch: 'true'
      - Key: !Sub 'kubernetes.io/cluster/${ClusterName}'
        Value: 'owned'
        PropagateAtLaunch: 'true'
    UpdatePolicy:
      AutoScalingRollingUpdate:
        MinInstancesInService: '1'
        MaxBatchSize: '1'

  NodeLaunchConfig:
    Type: AWS::AutoScaling::LaunchConfiguration
    Properties:
      AssociatePublicIpAddress: 'true'
      IamInstanceProfile: !Ref NodeInstanceProfile
      ImageId: !Ref NodeImageId
      InstanceType: !Ref NodeInstanceType
      KeyName: !Ref KeyName
      SecurityGroups:
      - !Ref NodeSecurityGroup
      BlockDeviceMappings:
        - DeviceName: /dev/xvda
          Ebs:
            VolumeSize: !Ref NodeVolumeSize
            VolumeType: gp2
            DeleteOnTermination: true
      UserData:
        Fn::Base64:
          !Sub |
            #!/bin/bash
            set -o xtrace
            /etc/eks/bootstrap.sh ${ClusterName} ${BootstrapArguments}
            /opt/aws/bin/cfn-signal --exit-code $? \
                     --stack  ${AWS::StackName} \
                     --resource NodeGroup  \
                     --region ${AWS::Region}

Outputs:
  NodeInstanceRole:
    Description: The node instance role
    Value: !GetAtt NodeInstanceRole.Arn