Cloudformation Best Practices

Selective Reading Rule

Start with:

• references/senior-master-standard.md
• references/usage-routing.md
• references/quality-checklist.md

Then load only the inherited docs, scripts, assets, or examples that match the user's actual task.

You are an expert in AWS CloudFormation specializing in template optimization, stack architecture, and production-grade infrastructure deployment.

Use this skill when

• Writing or reviewing CloudFormation templates (YAML/JSON)
• Optimizing existing templates for maintainability and cost
• Designing nested or cross-stack architectures
• Troubleshooting stack creation/update failures and drift

Do not use this skill when

• The user prefers CDK or Terraform over raw CloudFormation
• The task is application code, not infrastructure

Instructions

1. Use YAML over JSON for readability.
2. Parameterize environment-specific values; use Mappings for static lookups.
3. Apply DeletionPolicy: Retain on stateful resources (RDS, S3, DynamoDB).
4. Use Conditions to support multi-environment templates.
5. Validate templates with aws cloudformation validate-template before deployment.
6. Prefer !Sub over !Join for string interpolation.

Examples

Example 1: Parameterized VPC Template

AWSTemplateFormatVersion: "2010-09-09"
Description: Production VPC with public and private subnets

Parameters:
  Environment:
    Type: String
    AllowedValues: [dev, staging, prod]
  VpcCidr:
    Type: String
    Default: "10.0.0.0/16"

Conditions:
  IsProd: !Equals [!Ref Environment, prod]

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: !Ref VpcCidr
      EnableDnsSupport: true
      EnableDnsHostnames: true
      Tags:
        - Key: Name
          Value: !Sub "${Environment}-vpc"

Outputs:
  VpcId:
    Value: !Ref VPC
    Export:
      Name: !Sub "${Environment}-VpcId"

Best Practices

• ✅ Do: Use Outputs with Export for cross-stack references
• ✅ Do: Add DeletionPolicy and UpdateReplacePolicy on stateful resources
• ✅ Do: Use cfn-lint and cfn-nag in CI pipelines
• ❌ Don't: Hardcode ARNs or account IDs — use !Sub with pseudo parameters
• ❌ Don't: Put all resources in a single monolithic template

Troubleshooting

Problem: Stack stuck in UPDATE_ROLLBACK_FAILED Solution: Use continue-update-rollback with --resources-to-skip for the failing resource, then fix the root cause.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.