(ApplicationLoadBalancedFargateService): Auto scaling won't scale-in after update service with force new deployment option

The ECS(Fargate) auto scaling out is working as expect, but if I update the ECS service with force new deployment option in the AWS web UI, after the deployment process done, the ECS service CPU/MEN usage decreased and the CloudWatch Alarm which named with ‘AlarmLow’ triggered but the Desired count not decreasing.

Reproduction Steps

1. Deploy a ECS service with ApplicationLoadBalancedFargateService as following.

https://docs.aws.amazon.com/cdk/api/latest/docs/aws-ecs-patterns-readme.html#add-metric-based-auto-scaling-to-an-applicationloadbalancedfargateservice

1. Trigger auto-scale out with some test traffic
2. Click update in ECS service panel in AWS UI
3. Check ‘force new deployment’ box, don’t touch anything else and update the ECS service
4. waiting for new deployment done and the CPU/MEN usage should below the auto scale alert count.
5. Check the CloudWatch and the alarm named with ‘AlarmLow’ should in alarm status but ECS Desired Count not decrease

What did you expect to happen?

the CPU/MEN usage is below the alert setting and the ‘AlarmLow’ in alarm status, the desired count should be decreased.

What actually happened?

‘AlarmLow’ in alarm status but the ECS desired count not decreased.

Environment

• CDK CLI Version : @aws-cdk/core@1.88.0, @aws-cdk/aws-ecs-patterns@1.88.0
• Framework Version:
• Node.js Version:
• OS :
• Language (Version):

Other

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This is 🐛 Bug Report

[TrentonAdams]

Oh, I don’t have permission to close. Who can?

[TrentonAdams]

I’m not sure if this is related, but I’m having a problem with deleting a stack with an ASG for ECS.

There’s a message in the ASG configuration (activity tab) saying Could not scale to desired capacity because all remaining instances are protected from scale-in.. I go disable scale-in protection on the instance, and it continues deleting the stack almost immediately.

I’m guessing the following should default newInstancesProtectedFromScaleIn to false in the properties???

        const autoScalingGroup = new autoscaling.AutoScalingGroup(this, 'ASG', {
            vpc: props.smw_vpc,
            instanceType: new ec2.InstanceType('t2.micro'),
            machineImage: ecs.EcsOptimizedImage.amazonLinux2(),
            minCapacity: 0,
            desiredCapacity: 1,
            maxCapacity: 2,
        });

e.g.

        const autoScalingGroup = new autoscaling.AutoScalingGroup(this, 'ASG', {
            vpc: props.smw_vpc,
            instanceType: new ec2.InstanceType('t2.micro'),
            newInstancesProtectedFromScaleIn: false,
            machineImage: ecs.EcsOptimizedImage.amazonLinux2(),
            minCapacity: 0,
            desiredCapacity: 1,
            maxCapacity: 2,
        });

Should I file a separate issue, or is this related?

To be clear, I think it’s really bizarre for AWS to be putting anything in a “protected” mode without it being explicitly requested.