---
name: aws-cloudformation-auto-scaling
description: Provides AWS CloudFormation patterns for Auto Scaling including EC2, ECS, and Lambda. Use when creating Auto Scaling groups, launch configurations, launch templates, scaling policies, lifecycle hooks, and predictive scaling. Covers template structure with Parameters, Outputs, Mappings, Conditions, cross-stack references, and best practices for high availability and cost optimization.
allowed-tools: Read, Write, Bash
---

# AWS CloudFormation Auto Scaling

## Overview

Create production-ready Auto Scaling infrastructure using AWS CloudFormation templates. This skill covers Auto Scaling Groups for EC2, ECS, and Lambda, launch configurations, launch templates, scaling policies, lifecycle hooks, and best practices for high availability and cost optimization.

## When to Use

Use this skill when:
- Creating Auto Scaling Groups for EC2 instances
- Configuring Launch Configurations or Launch Templates
- Implementing scaling policies (step, target tracking, simple)
- Adding lifecycle hooks for lifecycle management
- Creating scaling for ECS services
- Implementing Lambda provisioned concurrency scaling
- Organizing templates with Parameters, Outputs, Mappings, Conditions
- Implementing cross-stack references with export/import
- Using mixed instances policies for diversity

## Instructions

Follow these steps to create Auto Scaling infrastructure with CloudFormation:

### 1. Define Parameters

Specify capacity and instance settings with AWS-specific parameter types:

```yaml
Parameters:
  MinSize:
    Type: Number
    Default: 2
    Description: Minimum number of instances

  MaxSize:
    Type: Number
    Default: 10
    Description: Maximum number of instances

  DesiredCapacity:
    Type: Number
    Default: 2
    Description: Desired number of instances

  InstanceType:
    Type: AWS::EC2::Instance::Type
    Default: t3.micro
    Description: EC2 instance type

  AmiId:
    Type: AWS::EC2::Image::Id
    Description: AMI ID for instances

  SubnetIds:
    Type: List<AWS::EC2::Subnet::Id>
    Description: Subnets for Auto Scaling group
```

### 2. Create Launch Configuration

Define instance launch settings:

```yaml
Resources:
  MyLaunchConfiguration:
    Type: AWS::AutoScaling::LaunchConfiguration
    Properties:
      LaunchConfigurationName: !Sub "${AWS::StackName}-lc"
      ImageId: !Ref AmiId
      InstanceType: !Ref InstanceType
      KeyName: !Ref KeyName
      SecurityGroups:
        - !Ref InstanceSecurityGroup
      InstanceMonitoring: Enabled
      UserData:
        Fn::Base64: |
          #!/bin/bash
          yum update -y
          yum install -y httpd
          systemctl start httpd
```

### 3. Create Auto Scaling Group

Specify min/max/desired capacity and networking:

```yaml
Resources:
  MyAutoScalingGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      AutoScalingGroupName: !Sub "${AWS::StackName}-asg"
      MinSize: !Ref MinSize
      MaxSize: !Ref MaxSize
      DesiredCapacity: !Ref DesiredCapacity
      VPCZoneIdentifier: !Ref SubnetIds
      LaunchConfigurationName: !Ref MyLaunchConfiguration
      TargetGroupARNs:
        - !Ref MyTargetGroup
      HealthCheckType: ELB
      HealthCheckGracePeriod: 300
      Tags:
        - Key: Environment
          Value: !Ref Environment
          PropagateAtLaunch: true
```

### 4. Configure Load Balancer Integration

Set up ALB for traffic distribution:

```yaml
Resources:
  MyTargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      Name: !Sub "${AWS::StackName}-tg"
      Port: 80
      Protocol: HTTP
      VpcId: !Ref VPCId
      HealthCheckPath: /
      TargetType: instance

  MyLoadBalancer:
    Type: AWS::ElasticLoadBalancingV2::LoadBalancer
    Properties:
      Name: !Sub "${AWS::StackName}-alb"
      Scheme: internet-facing
      Type: application
      Subnets:
        - !Ref PublicSubnet1
        - !Ref PublicSubnet2
```

### 5. Add Scaling Policies

Implement target tracking scaling:

```yaml
Resources:
  TargetTrackingPolicy:
    Type: AWS::AutoScaling::ScalingPolicy
    Properties:
      PolicyName: !Sub "${AWS::StackName}-target-tracking"
      PolicyType: TargetTrackingScaling
      AutoScalingGroupName: !Ref MyAutoScalingGroup
      TargetTrackingConfiguration:
        PredefinedMetricSpecification:
          PredefinedMetricType: ASGAverageCPUUtilization
        TargetValue: 70
        DisableScaleIn: false
```

### 6. Configure Lifecycle Hooks

Implement hooks for graceful instance management:

```yaml
Resources:
  LifecycleHookTermination:
    Type: AWS::AutoScaling::LifecycleHook
    Properties:
      LifecycleHookName: !Sub "${AWS::StackName}-termination-hook"
      AutoScalingGroupName: !Ref MyAutoScalingGroup
      LifecycleTransition: autoscaling:EC2_INSTANCE_TERMINATING
      HeartbeatTimeout: 300
      NotificationTargetARN: !Ref SNSTopic
      RoleARN: !Ref LifecycleHookRole
```

### 7. Set Up Monitoring

Configure CloudWatch alarms for scaling triggers:

```yaml
Resources:
  HighCpuAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmName: !Sub "${AWS::StackName}-high-cpu"
      MetricName: CPUUtilization
      Namespace: AWS/EC2
      Dimensions:
        - Name: AutoScalingGroupName
          Value: !Ref MyAutoScalingGroup
      Statistic: Average
      Period: 60
      EvaluationPeriods: 3
      Threshold: 70
      ComparisonOperator: GreaterThanThreshold
```

### 8. Configure Outputs and Cross-Stack References

Export ASG configuration for other stacks:

```yaml
Outputs:
  AutoScalingGroupName:
    Description: Name of the Auto Scaling Group
    Value: !Ref MyAutoScalingGroup
    Export:
      Name: !Sub "${AWS::StackName}-AutoScalingGroupName"

  AutoScalingGroupArn:
    Description: ARN of the Auto Scaling Group
    Value: !GetAtt MyAutoScalingGroup.AutoScalingGroupArn
    Export:
      Name: !Sub "${AWS::StackName}-AutoScalingGroupArn"
```

## Examples

### Complete Auto Scaling Template

Full end-to-end template with VPC, ASG, ALB, and scaling policies:

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Description: Auto Scaling Group with ALB integration

Parameters:
  Environment:
    Type: String
    Default: production
  InstanceType:
    Type: AWS::EC2::Instance::Type
    Default: t3.micro
  AmiId:
    Type: AWS::EC2::Image::Id
  VpcId:
    Type: AWS::EC2::VPC::Id
  SubnetIds:
    Type: List<AWS::EC2::Subnet::Id>

Resources:
  InstanceSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for ASG instances
      VpcId: !Ref VpcId
      SecurityGroupEgress:
        - CidrIp: 0.0.0.0/0
          IpProtocol: "-1"

  TargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      Name: !Sub "${AWS::StackName}-tg"
      Port: 80
      Protocol: HTTP
      VpcId: !Ref VpcId
      HealthCheckPath: /health
      TargetType: instance

  LaunchTemplate:
    Type: AWS::EC2::LaunchTemplate
    Properties:
      LaunchTemplateName: !Sub "${AWS::StackName}-lt"
      ImageId: !Ref AmiId
      InstanceType: !Ref InstanceType
      SecurityGroupIds:
        - !Ref InstanceSecurityGroup

  AutoScalingGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      AutoScalingGroupName: !Sub "${AWS::StackName}-asg"
      MinSize: 2
      MaxSize: 10
      DesiredCapacity: 4
      LaunchTemplate:
        LaunchTemplateId: !Ref LaunchTemplate
        Version: !GetAtt LaunchTemplate.LatestVersionNumber
      VPCZoneIdentifier: !Ref SubnetIds
      TargetGroupARNs:
        - !Ref TargetGroup
      HealthCheckType: ELB
      HealthCheckGracePeriod: 300
      Tags:
        - Key: Name
          Value: !Sub "${AWS::StackName}-instance"
          PropagateAtLaunch: true

  ScalingPolicy:
    Type: AWS::AutoScaling::ScalingPolicy
    Properties:
      PolicyName: !Sub "${AWS::StackName}-cpu-policy"
      PolicyType: TargetTrackingScaling
      AutoScalingGroupName: !Ref AutoScalingGroup
      TargetTrackingConfiguration:
        PredefinedMetricSpecification:
          PredefinedMetricType: ASGAverageCPUUtilization
        TargetValue: 70

Outputs:
  AutoScalingGroupName:
    Value: !Ref AutoScalingGroup
    Export:
      Name: !Sub "${AWS::StackName}-ASG-Name"
```

### Validation Commands

Validate template and test changes before deployment:

```bash
# Validate CloudFormation template
aws cloudformation validate-template --template-body file://template.yaml

# Create change set to preview changes
aws cloudformation create-change-set \
  --stack-name my-asg-stack \
  --template-body file://template.yaml \
  --change-set-type CREATE

# Describe change set
aws cloudformation describe-change-set \
  --stack-name my-asg-stack \
  --change-set-name <change-set-id>

# Execute change set
aws cloudformation execute-change-set \
  --stack-name my-asg-stack \
  --change-set-name <change-set-id>
```

### Verification Commands

Verify ASG health and configuration after deployment:

```bash
# Describe Auto Scaling Groups
aws autoscaling describe-auto-scaling-groups \
  --auto-scaling-group-names my-asg-stack-asg

# Check instance health
aws autoscaling describe-auto-scaling-instances \
  --instance-ids <instance-id>

# Verify scaling policies
aws autoscaling describe-policies \
  --auto-scaling-group-name my-asg-stack-asg

# Review scaling activities
aws autoscaling describe-scaling-activities \
  --auto-scaling-group-name my-asg-stack-asg
```

## Constraints and Warnings

### Resource Limits

- ASG max size: 10000 instances (request limit increase for more)
- Max 20 launch configurations per account (use Launch Templates instead)
- Lifecycle hooks: 50 per ASG
- Scaling policies: 20 per ASG (Target Tracking limit)
- Cooldown period: 300 seconds default (decrease cautiously)

### Operational Warnings

- Do not update both Launch Configuration and Launch Template on the same ASG in one update
- Always use `UpdatePolicy` for graceful updates and instance refresh
- Set `HealthCheckGracePeriod` >= expected instance initialization time
- Lifecycle hook timeout must accommodate graceful shutdown procedures
- Spot Instances: set `InstanceMaintenancePolicy` to avoid unexpected terminations

### Security Warnings

- Never hardcode credentials; use IAM instance profiles
- Use Service-Linked Roles for Auto Scaling (`aws-autoscaling.amazonaws.com`)
- Restrict ASG describe permissions to prevent information disclosure
- KMS keys for EBS encryption must be created in same region as ASG
- Cross-stack references export names must be unique per region

## Best Practices

### High Availability

- Distribute instances across multiple AZs
- Use ALB with health checks for automatic routing
- Implement lifecycle hooks for graceful shutdown
- Configure appropriate termination policies
- Use mixed instances policies for diversity
- Set appropriate health check grace periods

### Cost Optimization

- Use Spot Instances for fault-tolerant workloads
- Implement right-sizing of instances
- Configure aggressive scale-in policies
- Use scheduled scaling for predictable patterns
- Monitor and optimize regularly
- Set appropriate minimum capacity to avoid over-provisioning

### Monitoring

- Create CloudWatch Alarms for key metrics
- Implement scaling policies based on metrics
- Use lifecycle hooks for logging and analytics
- Configure SNS notifications for scaling events
- Implement detailed monitoring for troubleshooting
- Use multiple scaling policies for optimal response

### Security

- Use IAM roles with least privilege permissions
- Encrypt EBS volumes with KMS
- Configure restrictive security groups
- Use VPC with appropriate subnets
- Implement Parameter Store for sensitive configuration
- Avoid broad managed policies (use specific permissions)
- Configure appropriate instance profiles

## References

For detailed implementation guidance, see:

- **[constraints.md](references/constraints.md)** - Resource limits (ASG limits, scaling policy limits, lifecycle hook limits), scaling constraints (cooldown periods, health check grace periods, min/max capacity), operational constraints (mixed instances policy, predictive scaling, instance refresh), security constraints (IAM roles, service-linked roles, KMS permissions), and cost considerations (Spot instance termination, over-provisioning, scaling frequency)

## Related Resources

- [Auto Scaling Documentation](https://docs.aws.amazon.com/autoscaling/)
- [AWS CloudFormation User Guide](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/)
- [Auto Scaling Best Practices](https://docs.aws.amazon.com/autoscaling/plans/userguide/auto-scaling-best-practices.html)
- [Application Auto Scaling](https://docs.aws.amazon.com/autoscaling/application/userguide/what-is-application-auto-scaling.html)