CloudFormation Advanced Features - Architecture Insights

Change Sets

Change sets preview proposed changes before executing a stack update, showing what will be Added, Modified, or Removed before any changes are applied.

Change Sets preview proposed changes before executing a stack update.

Creating Change Sets

# Create change set
aws cloudformation create-change-set \
  --stack-name my-stack \
  --change-set-name my-updates \
  --template-body file://template-updated.yaml \
  --parameters ParameterKey=InstanceType,ParameterValue=t2.medium

Reviewing Change Sets

# Describe change set
aws cloudformation describe-change-set \
  --stack-name my-stack \
  --change-set-name my-updates

Change types:

Add - New resource will be created
Modify - Resource properties will be updated
Remove - Resource will be deleted
Dynamic - Change determined at execution time

Replacement:

True - Resource will be replaced
False - Resource will be updated in place
Conditional - Depends on other changes

Executing Change Sets

# Execute change set
aws cloudformation execute-change-set \
  --stack-name my-stack \
  --change-set-name my-updates

Best Practices

Always use change sets for production
Review changes thoroughly
Get approval before execution
Document changes in change set name

Nested Stacks

Nested stacks are stacks created as part of other stacks for organizing and reusing templates.

Why Use Nested Stacks

Overcome template size limits (51,200 bytes)
Reuse common templates
Organize complex infrastructure
Separate concerns (network, compute, storage)
Update components independently

Creating Nested Stacks

Parent template:

Resources:
  NetworkStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/my-bucket/network.yaml
      Parameters:
        VpcCIDR: 10.0.0.0/16
      TimeoutInMinutes: 30

  ComputeStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/my-bucket/compute.yaml
      Parameters:
        VpcId: !GetAtt NetworkStack.Outputs.VPCId
        SubnetIds: !GetAtt NetworkStack.Outputs.PublicSubnets
      TimeoutInMinutes: 30

Child template (network.yaml):

Parameters:
  VpcCIDR:
    Type: String

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: !Ref VpcCIDR

  PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: !Select [0, !Cidr [!Ref VpcCIDR, 6, 8]]

Outputs:
  VPCId:
    Value: !Ref VPC
  PublicSubnets:
    Value: !Join [',', [!Ref PublicSubnet1, !Ref PublicSubnet2]]

Organization Pattern

templates/
├── main.yaml              # Parent template
├── network/
│   └── vpc.yaml          # Network nested stack
├── compute/
│   ├── asg.yaml          # Compute nested stack
│   └── launch-template.yaml
└── storage/
    └── s3.yaml           # Storage nested stack

StackSets

StackSets create, update, or delete stacks across multiple AWS accounts and regions.

Use Cases

Deploy baseline security across all accounts
Create standard networking in multiple regions
Enforce compliance policies organization-wide
Manage multi-region applications

Creating StackSets

# Create StackSet
aws cloudformation create-stack-set \
  --stack-set-name baseline-security \
  --template-body file://security-baseline.yaml \
  --capabilities CAPABILITY_NAMED_IAM

# Create stack instances
aws cloudformation create-stack-instances \
  --stack-set-name baseline-security \
  --accounts 111122223333 444455556666 \
  --regions us-east-1 us-west-2 \
  --operation-preferences \
    MaxConcurrentCount=2,FailureToleranceCount=1

Deployment Options

OperationPreferences:
  RegionConcurrencyType: PARALLEL  # Or SEQUENTIAL
  RegionOrder:
    - us-east-1
    - us-west-2
    - eu-west-1
  FailureToleranceCount: 1
  MaxConcurrentCount: 3

Organizational StackSets

# Create StackSet for entire organization
aws cloudformation create-stack-set \
  --stack-set-name org-wide-logging \
  --template-body file://logging.yaml \
  --permission-model SERVICE_MANAGED \
  --auto-deployment Enabled=true,RetainStacksOnAccountRemoval=false

# Deploy to organization
aws cloudformation create-stack-instances \
  --stack-set-name org-wide-logging \
  --deployment-targets OrganizationalUnitIds=ou-xxxx-yyyyyyyy \
  --regions us-east-1

Custom Resources

Custom resources enable custom provisioning logic for resources not supported by CloudFormation.

Lambda-Backed Custom Resource

Resources:
  CustomResourceFunction:
    Type: AWS::Lambda::Function
    Properties:
      Runtime: python3.9
      Handler: index.handler
      Code:
        ZipFile: |
          import cfnresponse
          import boto3

          def handler(event, context):
              try:
                  if event['RequestType'] == 'Create':
                      # Custom create logic
                      response_data = {'Result': 'Created'}
                      cfnresponse.send(event, context, cfnresponse.SUCCESS, response_data)
                  elif event['RequestType'] == 'Delete':
                      # Custom delete logic
                      cfnresponse.send(event, context, cfnresponse.SUCCESS, {})
                  else:
                      cfnresponse.send(event, context, cfnresponse.SUCCESS, {})
              except Exception as e:
                  cfnresponse.send(event, context, cfnresponse.FAILED, {'Error': str(e)})
      Role: !GetAtt LambdaExecutionRole.Arn

  CustomResource:
    Type: Custom::MyCustomResource
    Properties:
      ServiceToken: !GetAtt CustomResourceFunction.Arn
      CustomProperty: CustomValue

Use Cases

Provision resources not supported by CloudFormation
Call external APIs
Perform custom validation
Clean up or transform data

Drift Detection

Drift occurs when resources are modified outside CloudFormation.

Detect Drift

# Start drift detection
aws cloudformation detect-stack-drift \
  --stack-name my-stack

# Check status
aws cloudformation describe-stack-drift-detection-status \
  --stack-drift-detection-id <id>

# View drift results
aws cloudformation describe-stack-resource-drifts \
  --stack-name my-stack

Drift Statuses

IN_SYNC - Resource matches template
MODIFIED - Resource differs from template
DELETED - Resource deleted outside CloudFormation
NOT_CHECKED - Resource type doesn’t support drift detection

Handling Drift

Update template to match current state
Revert manual changes
Import changed resources

Best Practices

Template Organization

Consistent Structure:

infrastructure/
├── README.md
├── templates/
│   ├── network.yaml
│   ├── compute.yaml
│   └── storage.yaml
├── parameters/
│   ├── dev.json
│   ├── staging.json
│   └── prod.json
└── policies/
    └── stack-policy.json

Naming Conventions:

# Stack names: <env>-<app>-<component>
# Example: prod-webapp-network

# Resource names: <env>-<component>-<resource>
resource "aws_s3_bucket" "data" {
  bucket = "myapp-prod-s3-data"
}

Security

Least Privilege IAM:

Resources:
  EC2Role:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
          - Effect: Allow
            Principal:
              Service: ec2.amazonaws.com
            Action: sts:AssumeRole
      Policies:
        - PolicyName: S3Access
          PolicyDocument:
            Version: '2012-10-17'
            Statement:
              - Effect: Allow
                Action:
                  - s3:GetObject
                  - s3:PutObject
                Resource: !Sub ${DataBucket.Arn}/*

Encryption:

Resources:
  DataBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketEncryption:
        ServerSideEncryptionConfiguration:
          - ServerSideEncryptionByDefault:
              SSEAlgorithm: AES256

  Database:
    Type: AWS::RDS::DBInstance
    Properties:
      StorageEncrypted: true
      KmsKeyId: !Ref DatabaseKey

Secrets:

# Use Secrets Manager
Resources:
  DBSecret:
    Type: AWS::SecretsManager::Secret
    Properties:
      GenerateSecretString:
        SecretStringTemplate: '{"username": "admin"}'
        GenerateStringKey: password
        PasswordLength: 32

  Database:
    Type: AWS::RDS::DBInstance
    Properties:
      MasterUsername: !Sub '{{resolve:secretsmanager:${DBSecret}:SecretString:username}}'
      MasterUserPassword: !Sub '{{resolve:secretsmanager:${DBSecret}:SecretString:password}}'

Tagging

Resources:
  WebServer:
    Type: AWS::EC2::Instance
    Properties:
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-web-server
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: Application
          Value: !Ref ApplicationName
        - Key: Owner
          Value: !Ref OwnerEmail
        - Key: CostCenter
          Value: !Ref CostCenter
        - Key: ManagedBy
          Value: CloudFormation

Version Control

.gitignore:

# Sensitive files
*-secrets.yaml
*-secrets.json
*.key
*.pem

# IDE files
.idea/
.vscode/

# Temp files
*.swp
*.tmp

Testing

Validation Pipeline:

# GitHub Actions
name: CloudFormation CI/CD
on: [push]
jobs:
  validate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      - name: Validate template
        run: |
          aws cloudformation validate-template \
            --template-body file://template.yaml

      - name: Lint
        run: |
          pip install cfn-lint
          cfn-lint template.yaml

      - name: Security scan
        run: |
          gem install cfn-nag
          cfn_nag_scan --input-path template.yaml

  deploy-test:
    needs: validate
    runs-on: ubuntu-latest
    steps:
      - name: Deploy to test
        run: |
          aws cloudformation deploy \
            --template-file template.yaml \
            --stack-name test-stack \
            --parameter-overrides EnvironmentName=test

Troubleshooting

Common Errors

Resource creation failed:

CREATE_FAILED: The security group 'sg-xxxxx' does not exist

Solution: Check dependencies, ensure resources created in correct order.

Circular dependency:

Circular dependency between resources

Solution: Review Ref and GetAtt relationships, break circular references.

Insufficient permissions:

User is not authorized to perform: ec2:CreateVpc

Solution: Grant required IAM permissions to CloudFormation execution role.

Stack Rollback

Preventing rollback for debugging:

aws cloudformation create-stack \
  --stack-name my-stack \
  --template-body file://template.yaml \
  --on-failure DO_NOTHING  # Keep resources for debugging

Manual rollback:

# Cancel update and roll back
aws cloudformation cancel-update-stack \
  --stack-name my-stack

# Continue rollback if stuck
aws cloudformation continue-update-rollback \
  --stack-name my-stack

Viewing Logs

# View stack events
aws cloudformation describe-stack-events \
  --stack-name my-stack

# Watch events in real-time
aws cloudformation describe-stack-events \
  --stack-name my-stack \
  --query 'StackEvents[*].[Timestamp,ResourceStatus,ResourceType,LogicalResourceId,ResourceStatusReason]' \
  --output table

Debugging Tips

Use Change Sets: Preview changes before applying
Start Small: Test with minimal template first
Check Dependencies: Review implicit dependencies
Validate Templates: Use validate-template and cfn-lint
Enable Termination Protection: Prevent accidental deletion

# Enable termination protection
aws cloudformation update-termination-protection \
  --enable-termination-protection \
  --stack-name production-stack