Follow these steps to create a CloudFormation template that launches a stack with an autoscaling group:

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 minimum and maximum sizes:

  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 

5. Then, add the settings for the CPU thresholds:

  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

6. Add a Resources section and define the autoscaling 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

7. Still under the Resources section, define the launch configuration that's used by the autoscaling 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

8. 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

9. Define an alarm that will alert you 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

10. Finally, define an alarm that will alert you 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

11. Save the template with the filename 04-01-AutoScaling.yml.

12. Launch the template with the following AWS CLI command, supplying your subnet IDs in place of <subnet-id-1> and <subnet-id-2>:

 aws cloudformation create-stack 
 --stack-name asg 
 --template-body file://04-01-AutoScaling.yml 
 --parameters 
 ParameterKey=SubnetIds,ParameterValue='<subnet-id-1>,<subnet-id-2>' 

13. At this point, the CFN service is provisioning all the resources in the template and will take a few minutes to complete. Once the stack has reached a CREATE_COMPLETE status, you can confirm that the autoscaling group is working correctly by checking for a new EC2 instance with the name asg-server.
14. Delete the stack to prevent future charges for the resources that were created in this recipe.