1. Start by defining the template version and description:

      AWSTemplateFormatVersion: "2010-09-09"
      Description: Create an Auto Scaling Group

2. Add a Parameters section with the required parameters that will be used later in the template:

      Parameters:
        SubnetIds:
          Description: Subnet IDs where instances can be launched
          Type: List<AWS::EC2::Subnet::Id>

3. Still under the Parameters section, add the optional instance configuration parameters:

      AmiId: 
        Description: The application server's AMI ID 
        Type: AWS::EC2::Image::Id 
        Default: ami-9be6f38c # AWS Linux in us-east-1 
      InstanceType: 
        Description: The type of instance to launch 
        Type: String 
        Default: t2.micro

4. Still under the Parameters section, add the optional auto scaling group-configuration parameters:

      MinSize: 
        Description: Minimum number of instances in the group 
        Type: Number 
        Default: 1
      MaxSize: 
        Description: Maximum number of instances in the group 
        Type: Number 
        Default: 4 
 
      ThresholdCPUHigh: 
        Description: Launch new instances when CPU utilization 
          is over this threshold 
        Type: Number 
        Default: 60 
 
      ThresholdCPULow: 
        Description: Remove instances when CPU utilization
          is under this threshold 
        Type: Number 
        Default: 40 
 
      ThresholdMinutes: 
        Description: Launch new instances when over the CPU 
          threshold for this many minutes 
        Type: Number 
        Default: 5

5. Add a Resources section, and define the auto scaling group resource:

      Resources: 
        AutoScalingGroup: 
          Type: AWS::AutoScaling::AutoScalingGroup 
          Properties: 
            MinSize: !Ref MinSize 
            MaxSize: !Ref MaxSize 
            LaunchConfigurationName: !Ref LaunchConfiguration 
            Tags: 
              - Key: Name 
                Value: !Sub "${AWS::StackName} server" 
                PropagateAtLaunch: true 
            VPCZoneIdentifier: !Ref SubnetIds

6. Still under the Resources section, define the launch configuration used by the auto scaling group:

      LaunchConfiguration: 
        Type: AWS::AutoScaling::LaunchConfiguration 
        Properties: 
          ImageId: !Ref AmiId 
          InstanceType: !Ref InstanceType 
          UserData: 
            Fn::Base64: !Sub | 
              #!/bin/bash -xe 
              # This will be run on startup, launch your application here

7. Next, define two scaling policy resources—one to scale up and the other to scale down:

        ScaleUpPolicy: 
          Type: AWS::AutoScaling::ScalingPolicy 
          Properties: 
            AdjustmentType: ChangeInCapacity 
            AutoScalingGroupName: !Ref AutoScalingGroup 
            Cooldown: 60 
            ScalingAdjustment: 1 
 
        ScaleDownPolicy: 
          Type: AWS::AutoScaling::ScalingPolicy 
          Properties: 
            AdjustmentType: ChangeInCapacity 
            AutoScalingGroupName: !Ref AutoScalingGroup 
            Cooldown: 60 
            ScalingAdjustment: -1

8. Define an alarm that will alert when the CPU goes over the ThresholdCPUHigh parameter:

      CPUHighAlarm: 
        Type: AWS::CloudWatch::Alarm 
        Properties: 
          ActionsEnabled: true 
          AlarmActions: 
            - !Ref ScaleUpPolicy 
          AlarmDescription: Scale up on CPU load 
          ComparisonOperator: GreaterThanThreshold 
          Dimensions: 
            - Name: AutoScalingGroupName 
              Value: !Ref AutoScalingGroup 
          EvaluationPeriods: !Ref ThresholdMinutes 
          MetricName: CPUUtilization 
          Namespace: AWS/EC2 
          Period: 60 
          Statistic: Average 
          Threshold: !Ref ThresholdCPUHigh

9. Finally, define an alarm that will alert when the CPU goes under the ThresholdCPULow parameter:

      CPULowAlarm: 
        Type: AWS::CloudWatch::Alarm 
        Properties: 
          ActionsEnabled: true 
          AlarmActions: 
            - !Ref ScaleDownPolicy 
          AlarmDescription: Scale down on CPU load 
          ComparisonOperator: LessThanThreshold 
          Dimensions: 
            - Name: AutoScalingGroupName 
              Value: !Ref AutoScalingGroup 
          EvaluationPeriods: !Ref ThresholdMinutes 
          MetricName: CPUUtilization 
          Namespace: AWS/EC2 
          Period: 60 
          Statistic: Average 
          Threshold: !Ref ThresholdCPULow

10. Save the template with the filename 04-auto-scaling-an-application-server.yaml.

11. Launch the template with the following AWS CLI command, supplying your subnet IDs:

      aws cloudformation create-stack  
        --stack-name asg  
        --template-body file://04-auto-scaling-an-application-server.yaml  
        --parameters  
        ParameterKey=SubnetIds,ParameterValue='<subnet-id-1>, 
          <subnet-id-2>'