cloudformation

## When to use this skill

Use this skill whenever the user wants to:
- Write or debug CloudFormation templates (YAML/JSON)
- Create and manage AWS resources (EC2, S3, RDS, Lambda, etc.)
- Configure CloudFormation stacks, nested stacks, or stack sets
- Implement infrastructure automation with CloudFormation best practices
- Use cross-stack references with Export/ImportValue

## How to use this skill

### Workflow

1. **Define template** — write YAML with Parameters, Resources, and Outputs
2. **Create change set** — preview changes before applying
3. **Deploy stack** — create or update the stack
4. **Validate** — check stack events and outputs

### Quick Start Example

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Description: Simple S3 bucket with versioning

Parameters:
  Environment:
    Type: String
    AllowedValues: [dev, staging, prod]
    Default: dev

Resources:
  AppBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub 'myapp-${Environment}-assets'
      VersioningConfiguration:
        Status: Enabled

Outputs:
  BucketArn:
    Value: !GetAtt AppBucket.Arn
    Export:
      Name: !Sub '${Environment}-AppBucketArn'
```

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

# Create stack with change set preview
aws cloudformation deploy \
  --template-file template.yaml \
  --stack-name myapp-dev \
  --parameter-overrides Environment=dev
```

### Cross-Stack Reference Example

```yaml
# In consuming stack — import the exported bucket ARN
Resources:
  LambdaFunction:
    Type: AWS::Lambda::Function
    Properties:
      Environment:
        Variables:
          BUCKET_ARN: !ImportValue dev-AppBucketArn
```

## Best Practices

- Use YAML over JSON for readability and version control
- Store sensitive values with `Parameter` + `NoEcho` or AWS Secrets Manager — never hardcode
- Use Mappings and Conditions for environment-specific configuration
- Prefix stack names with environment (e.g., `dev-`, `prod-`) to avoid conflicts
- Encapsulate reusable resources as nested stacks for composability

## Troubleshooting

- **Rollback on create**: Check stack events in CloudFormation console for the specific resource failure
- **Circular dependency**: Refactor with `DependsOn` or split resources across stacks
- **Drift detection**: Run `aws cloudformation detect-stack-drift` to find out-of-band changes

## Keywords

cloudformation, aws, infrastructure as code, cloudformation template, aws iac, nested stacks, cross-stack references