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 # CloudFormation

-Deploy AWS infrastructure with native CloudFormation templates.
+Deploy AWS infrastructure with native CloudFormation templates, change sets, nested stacks, and drift detection.
+
+## When to Use This Skill
+
+- Deploying AWS resources using AWS-native Infrastructure as Code
+- Creating repeatable, parameterized infrastructure templates
+- Managing multi-environment deployments (dev, staging, prod) with the same template
+- Implementing safe deployments with change sets and rollback protection
+- Detecting and remediating configuration drift
+- Organizing large infrastructure into nested stacks
+- Exporting/importing values between stacks
+
+## Prerequisites
+
+- AWS CLI v2 installed and configured
+- IAM permissions: `cloudformation:*`, plus permissions for all resources in the template
+- (Optional) `cfn-lint` installed for template validation (`pip install cfn-lint`)
+- S3 bucket for storing templates larger than 51,200 bytes

 ## Template Structure

 ```yaml
 AWSTemplateFormatVersion: '2010-09-09'
-Description: Web application stack
+Description: Production web application infrastructure
+
+Metadata:
+  AWS::CloudFormation::Interface:
+    ParameterGroups:
+      - Label: { default: "Environment" }
+        Parameters: [Environment, InstanceType]
+      - Label: { default: "Network" }
+        Parameters: [VpcId, SubnetIds]

 Parameters:
  Environment:
    Type: String
    AllowedValues: [dev, staging, prod]
-    
+    Default: dev
+
+  InstanceType:
+    Type: String
+    Default: t3.micro
+    AllowedValues: [t3.micro, t3.small, t3.medium, t3.large]
+
+  VpcId:
+    Type: AWS::EC2::VPC::Id
+    Description: VPC to deploy into
+
+  SubnetIds:
+    Type: List<AWS::EC2::Subnet::Id>
+    Description: Subnets for the application
+
+Conditions:
+  IsProd: !Equals [!Ref Environment, prod]
+  CreateReadReplica: !Equals [!Ref Environment, prod]
+
+Mappings:
+  RegionAMI:
+    us-east-1:
+      AL2023: ami-0abcdef1234567890
+    us-west-2:
+      AL2023: ami-0fedcba9876543210
+
 Resources:
-  WebServer:
-    Type: AWS::EC2::Instance
+  SecurityGroup:
+    Type: AWS::EC2::SecurityGroup
    Properties:
-      ImageId: !Ref AMI
-      InstanceType: t3.micro
+      GroupDescription: !Sub '${Environment}-web-sg'
+      VpcId: !Ref VpcId
+      SecurityGroupIngress:
+        - IpProtocol: tcp
+          FromPort: 443
+          ToPort: 443
+          CidrIp: 0.0.0.0/0
+      Tags:
+        - Key: Name
+          Value: !Sub '${Environment}-web-sg'
+
+  LaunchTemplate:
+    Type: AWS::EC2::LaunchTemplate
+    Properties:
+      LaunchTemplateName: !Sub '${Environment}-web'
+      LaunchTemplateData:
+        ImageId: !FindInMap [RegionAMI, !Ref 'AWS::Region', AL2023]
+        InstanceType: !If [IsProd, t3.large, !Ref InstanceType]
+        MetadataOptions:
+          HttpTokens: required
+        SecurityGroupIds:
+          - !Ref SecurityGroup
+
+  AutoScalingGroup:
+    Type: AWS::AutoScaling::AutoScalingGroup
+    Properties:
+      AutoScalingGroupName: !Sub '${Environment}-web-asg'
+      LaunchTemplate:
+        LaunchTemplateId: !Ref LaunchTemplate
+        Version: !GetAtt LaunchTemplate.LatestVersionNumber
+      MinSize: !If [IsProd, 2, 1]
+      MaxSize: !If [IsProd, 10, 3]
+      DesiredCapacity: !If [IsProd, 4, 1]
+      VPCZoneIdentifier: !Ref SubnetIds
+      TargetGroupARNs:
+        - !Ref TargetGroup
+      HealthCheckType: ELB
+      HealthCheckGracePeriod: 300
      Tags:
        - Key: Name
          Value: !Sub '${Environment}-web'
-          
+          PropagateAtLaunch: true
+    UpdatePolicy:
+      AutoScalingRollingUpdate:
+        MinInstancesInService: !If [IsProd, 2, 0]
+        MaxBatchSize: 1
+        PauseTime: PT5M
+        WaitOnResourceSignals: true
+        SuspendProcesses:
+          - HealthCheck
+          - ReplaceUnhealthy
+          - AZRebalance
+          - AlarmNotification
+          - ScheduledActions
+
+  TargetGroup:
+    Type: AWS::ElasticLoadBalancingV2::TargetGroup
+    Properties:
+      Name: !Sub '${Environment}-web-tg'
+      Port: 8080
+      Protocol: HTTP
+      VpcId: !Ref VpcId
+      TargetType: instance
+      HealthCheckPath: /health
+      HealthCheckIntervalSeconds: 30
+      HealthyThresholdCount: 2
+      UnhealthyThresholdCount: 3
+
 Outputs:
-  InstanceId:
-    Value: !Ref WebServer
+  SecurityGroupId:
+    Description: Web security group ID
+    Value: !Ref SecurityGroup
    Export:
-      Name: !Sub '${Environment}-WebServerId'
+      Name: !Sub '${Environment}-WebSecurityGroup'
+
+  AutoScalingGroupName:
+    Description: ASG name
+    Value: !Ref AutoScalingGroup
+    Export:
+      Name: !Sub '${Environment}-WebASG'
 ```

 ## Stack Operations

 ```bash
-# Create stack
+# Validate a template
+aws cloudformation validate-template --template-body file://template.yaml
+
+# Lint with cfn-lint (catches more issues)
+cfn-lint template.yaml
+
+# Create a stack
 aws cloudformation create-stack \
-  --stack-name myapp \
+  --stack-name production-web \
  --template-body file://template.yaml \
-  --parameters ParameterKey=Environment,ParameterValue=prod
+  --parameters \
+    ParameterKey=Environment,ParameterValue=prod \
+    ParameterKey=VpcId,ParameterValue=vpc-abc123 \
+    ParameterKey=SubnetIds,ParameterValue="subnet-aaa\\,subnet-bbb" \
+  --capabilities CAPABILITY_IAM CAPABILITY_NAMED_IAM \
+  --tags Key=Environment,Value=production Key=Team,Value=platform \
+  --enable-termination-protection \
+  --on-failure ROLLBACK

-# Update stack
-aws cloudformation update-stack \
-  --stack-name myapp \
-  --template-body file://template.yaml
+# Wait for stack creation
+aws cloudformation wait stack-create-complete --stack-name production-web

-# Delete stack
-aws cloudformation delete-stack --stack-name myapp
+# Describe stack status and outputs
+aws cloudformation describe-stacks \
+  --stack-name production-web \
+  --query "Stacks[0].{Status:StackStatus,Outputs:Outputs}" \
+  --output table

-# Detect drift
-aws cloudformation detect-stack-drift --stack-name myapp
+# List stack resources
+aws cloudformation list-stack-resources --stack-name production-web \
+  --query "StackResourceSummaries[].{Logical:LogicalResourceId,Physical:PhysicalResourceId,Type:ResourceType,Status:ResourceStatus}" \
+  --output table
+
+# Delete a stack
+aws cloudformation delete-stack --stack-name dev-web
+aws cloudformation wait stack-delete-complete --stack-name dev-web
 ```

-## Intrinsic Functions
+## Change Sets (Safe Updates)
+
+```bash
+# Create a change set to preview changes before applying
+aws cloudformation create-change-set \
+  --stack-name production-web \
+  --change-set-name update-instance-type \
+  --template-body file://template.yaml \
+  --parameters \
+    ParameterKey=Environment,ParameterValue=prod \
+    ParameterKey=InstanceType,ParameterValue=t3.large \
+    ParameterKey=VpcId,UsePreviousValue=true \
+    ParameterKey=SubnetIds,UsePreviousValue=true \
+  --capabilities CAPABILITY_IAM
+
+# Describe the change set to review planned changes
+aws cloudformation describe-change-set \
+  --stack-name production-web \
+  --change-set-name update-instance-type \
+  --query "Changes[].{Action:ResourceChange.Action,Resource:ResourceChange.LogicalResourceId,Type:ResourceChange.ResourceType,Replacement:ResourceChange.Replacement}" \
+  --output table
+
+# Execute the change set (apply changes)
+aws cloudformation execute-change-set \
+  --stack-name production-web \
+  --change-set-name update-instance-type
+
+# Wait for update
+aws cloudformation wait stack-update-complete --stack-name production-web
+
+# Delete a change set without applying
+aws cloudformation delete-change-set \
+  --stack-name production-web \
+  --change-set-name update-instance-type
+```
+
+## Drift Detection
+
+```bash
+# Start drift detection
+DRIFT_ID=$(aws cloudformation detect-stack-drift \
+  --stack-name production-web \
+  --query 'StackDriftDetectionId' --output text)
+
+# Check drift detection status
+aws cloudformation describe-stack-drift-detection-status \
+  --stack-drift-detection-id $DRIFT_ID
+
+# View drifted resources
+aws cloudformation describe-stack-resource-drifts \
+  --stack-name production-web \
+  --stack-resource-drift-status-filters MODIFIED DELETED \
+  --query "StackResourceDrifts[].{Resource:LogicalResourceId,Status:StackResourceDriftStatus,Differences:PropertyDifferences}" \
+  --output table
+
+# Detect drift on a specific resource
+aws cloudformation detect-stack-resource-drift \
+  --stack-name production-web \
+  --logical-resource-id SecurityGroup
+```
+
+## Nested Stacks
+
+Parent template:

 ```yaml
-# Reference
-!Ref MyResource
+AWSTemplateFormatVersion: '2010-09-09'
+Description: Parent stack - full application

-# Get attribute
-!GetAtt MyResource.Arn
+Parameters:
+  Environment:
+    Type: String
+    AllowedValues: [dev, staging, prod]

-# Substitute
-!Sub 'arn:aws:s3:::${BucketName}/*'
+Resources:
+  NetworkStack:
+    Type: AWS::CloudFormation::Stack
+    Properties:
+      TemplateURL: https://s3.amazonaws.com/my-cfn-templates/network.yaml
+      Parameters:
+        Environment: !Ref Environment
+        VpcCidr: "10.0.0.0/16"
+      Tags:
+        - Key: Environment
+          Value: !Ref Environment

-# Conditional
-!If [CreateProdResources, 't3.large', 't3.micro']
+  DatabaseStack:
+    Type: AWS::CloudFormation::Stack
+    DependsOn: NetworkStack
+    Properties:
+      TemplateURL: https://s3.amazonaws.com/my-cfn-templates/database.yaml
+      Parameters:
+        Environment: !Ref Environment
+        VpcId: !GetAtt NetworkStack.Outputs.VpcId
+        SubnetIds: !GetAtt NetworkStack.Outputs.PrivateSubnetIds

-# Join
-!Join ['-', [!Ref Environment, 'app', 'bucket']]
+  AppStack:
+    Type: AWS::CloudFormation::Stack
+    DependsOn: [NetworkStack, DatabaseStack]
+    Properties:
+      TemplateURL: https://s3.amazonaws.com/my-cfn-templates/app.yaml
+      Parameters:
+        Environment: !Ref Environment
+        VpcId: !GetAtt NetworkStack.Outputs.VpcId
+        SubnetIds: !GetAtt NetworkStack.Outputs.PrivateSubnetIds
+        DbEndpoint: !GetAtt DatabaseStack.Outputs.Endpoint
+
+Outputs:
+  VpcId:
+    Value: !GetAtt NetworkStack.Outputs.VpcId
+  AppUrl:
+    Value: !GetAtt AppStack.Outputs.LoadBalancerDNS
 ```

-## Best Practices
+```bash
+# Package nested templates (uploads local references to S3)
+aws cloudformation package \
+  --template-file parent.yaml \
+  --s3-bucket my-cfn-templates \
+  --output-template-file packaged.yaml

- Use change sets before updates
- Implement stack policies
- Use nested stacks for modularity
- Enable termination protection
- Use cfn-lint for validation
+# Deploy the packaged template
+aws cloudformation deploy \
+  --template-file packaged.yaml \
+  --stack-name production-app \
+  --parameter-overrides Environment=prod \
+  --capabilities CAPABILITY_IAM CAPABILITY_AUTO_EXPAND \
+  --tags Environment=production
+```
+
+## Intrinsic Functions Reference
+
+```yaml
+# Ref - reference a parameter or resource
+SecurityGroupId: !Ref SecurityGroup
+
+# GetAtt - get an attribute of a resource
+SecurityGroupArn: !GetAtt SecurityGroup.GroupId
+
+# Sub - string substitution
+BucketName: !Sub '${Environment}-${AWS::AccountId}-data'
+
+# Join - concatenate strings
+PolicyArn: !Join ['', ['arn:aws:iam::', !Ref 'AWS::AccountId', ':policy/MyPolicy']]
+
+# Select - pick from a list
+FirstSubnet: !Select [0, !Ref SubnetIds]
+
+# Split - split a string
+FirstPart: !Select [0, !Split ['-', !Ref 'AWS::StackName']]
+
+# If - conditional value
+InstanceSize: !If [IsProd, t3.large, t3.micro]
+
+# Equals - condition definition
+Conditions:
+  IsProd: !Equals [!Ref Environment, prod]
+
+# ImportValue - cross-stack reference
+VpcId: !ImportValue production-VpcId
+
+# Cidr - generate CIDR blocks
+Subnets: !Cidr [!GetAtt VPC.CidrBlock, 6, 8]
+
+# GetAZs - list availability zones
+AZ: !Select [0, !GetAZs '']
+```
+
+## Stack Policy (Prevent Accidental Replacements)
+
+```bash
+# Apply a stack policy that prevents replacement of the database
+aws cloudformation set-stack-policy \
+  --stack-name production-web \
+  --stack-policy-body '{
+    "Statement": [
+      {
+        "Effect": "Allow",
+        "Action": "Update:*",
+        "Principal": "*",
+        "Resource": "*"
+      },
+      {
+        "Effect": "Deny",
+        "Action": "Update:Replace",
+        "Principal": "*",
+        "Resource": "LogicalResourceId/Database"
+      },
+      {
+        "Effect": "Deny",
+        "Action": "Update:Delete",
+        "Principal": "*",
+        "Resource": "LogicalResourceId/Database"
+      }
+    ]
+  }'
+```
+
+## Stack Events and Debugging
+
+```bash
+# View stack events (most recent first)
+aws cloudformation describe-stack-events \
+  --stack-name production-web \
+  --query "StackEvents[?ResourceStatus=='CREATE_FAILED' || ResourceStatus=='UPDATE_FAILED'].{Time:Timestamp,Resource:LogicalResourceId,Status:ResourceStatus,Reason:ResourceStatusReason}" \
+  --output table
+
+# Continue a rollback that is stuck
+aws cloudformation continue-update-rollback \
+  --stack-name production-web \
+  --resources-to-skip SecurityGroup
+
+# Cancel an in-progress update
+aws cloudformation cancel-update-stack --stack-name production-web
+
+# Get template from an existing stack
+aws cloudformation get-template \
+  --stack-name production-web \
+  --template-stage Processed \
+  --query TemplateBody \
+  --output text > current-template.yaml
+```
+
+## Troubleshooting
+
+| Problem | Cause | Fix |
+|---|---|---|
+| CREATE_FAILED on IAM resource | Missing CAPABILITY_IAM | Add `--capabilities CAPABILITY_IAM CAPABILITY_NAMED_IAM` |
+| Stack stuck in UPDATE_ROLLBACK_FAILED | Resource cannot be rolled back | Use `continue-update-rollback` with `--resources-to-skip` |
+| Nested stack fails | Template URL wrong or S3 access denied | Use `aws cloudformation package` to upload; check bucket policy |
+| Circular dependency error | Two resources reference each other | Break the cycle with a third resource or use `DependsOn` |
+| Drift detected | Manual changes made outside CloudFormation | Re-apply the template or update template to match current state |
+| Change set shows no changes | Template and parameters identical | Verify the diff; check if the change is parameter-only |
+| Template validation error | YAML syntax or invalid resource property | Run `cfn-lint`; check property names against docs |
+| Export name already exists | Another stack uses the same export name | Use unique export names with `!Sub '${AWS::StackName}-Name'` |
+| Delete fails - resource in use | Dependent resource outside the stack | Remove the dependency first; check for SG references |

 ## Related Skills

- [terraform-aws](../terraform-aws/) - Alternative IaC
- [aws-iam](../aws-iam/) - IAM resources
+- [terraform-aws](../terraform-aws/) - Alternative IaC with Terraform
+- [aws-iam](../aws-iam/) - IAM resources in templates
+- [aws-vpc](../aws-vpc/) - Network infrastructure templates
+- [aws-ec2](../aws-ec2/) - Compute resources in templates
+- [aws-s3](../aws-s3/) - Storage resources in templates