AWS::AutoScaling::AutoScalingGroup - False-positive drift if DefaultInstanceWarmup is set

[rgoltz]

Name of the resource

AWS::AutoScaling::AutoScalingGroup

Resource Name

No response

Issue Description

Once you create a stack containing a AWS::AutoScaling::AutoScalingGroup and the property DefaultInstanceWarmup is set in CloudFormation template, the CloudFormation drift detection reporting such stack as drifted - Even the DefaultInstanceWarmup is set correctly on the created ASG (details see "Observed Behavior").

Expected Behavior

Stack is in state IN_SYNC, since template and resource have the same, correct setting.

Observed Behavior

Following CloudFormation documentation the setting DefaultInstanceWarmup is supported. Furthermore CDK also added support for this property recently aws/aws-cdk#23285 + aws/aws-cdk#23180.

If you set DefaultInstanceWarmup in your CFN template (e.g. to 500), your stack / this resource is in state DRIFTED after running CFN Drift detection - Details see: "Test Case".

Test Cases

** Create new Stack using following example (please mind to adjust LaunchTemplateId + VPCZoneIdentifier fitting your account) in eu-central-1

---
AWSTemplateFormatVersion: '2010-09-09'
Description: CFN Drift detection - ASG - DefaultInstanceWarmup issue - case 11177102011

Resources:
  AsgWithDefaultInstanceWarmupSet:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      AutoScalingGroupName: test-asg-rogo-1
      MinSize: 0
      MaxSize: 0
      LaunchTemplate:
        LaunchTemplateId: lt-IdAddedHere
        Version: 20
      VPCZoneIdentifier:
      - subnet-111IdAddedHere
      - subnet-222IdAddedHere
      - subnet-333IdAddedHere
      Tags:
      - Key: ReferenceID
        Value: 11177102011
        PropagateAtLaunch: false
      DefaultInstanceWarmup: 500

** After stack is created, please check your ASG resource in Console for DefaultInstanceWarmup (EC2 -> Auto Scaling Groups -> Click on your ASG -> Go down to "Advanced configurations" -> "Default instance warmup") - Here in the example it's set to 500 Sec correctly.

** Go to CFN Console and run CFN Drift detection via: (in Console using "Stack actions" => "Detect drift". After you triggered this process, please wait for ca. 1 min and check again the Drift status: Stack went in state DRIFTED. Check the Drift-Details - You will the Drift for DefaultInstanceWarmup property.

Other Details

• It's also an follow-up of Support DefaultInstanceWarmup of AutoScalingGroup #1216
• AWS-News regarding this Property https://aws.amazon.com/about-aws/whats-new/2022/04/ec2-auto-scaling-default-instance-warm-up-replacement-actions/?nc1=h_ls

[rgoltz]

Update after checking with CFN-team: Currently they prioritizing +planning a fix. Our next checkpoint with CFN-team is middle of March 2023.