# Cloudformation Best Practices

> CloudFormation template optimization, nested stacks, drift detection, and production-ready patterns. Use when writing or reviewing CF templates.

- Skill: `phoroth/cloudformation-best-practices` (Agent Skill)
- Install (CLI): `npx skillmds add phoroth/cloudformation-best-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/phoroth/cloudformation-best-practices/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: phoroth (https://skillmd.com/u/phoroth)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/phoroth/cloudformation-best-practices

---

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 enprojectnment-specific values; use `Mappings` for static lookups.
3. Apply `DeletionPolicy: Retain` on stateful resources (RDS, S3, DynamoDB).
4. Use `Conditions` to support multi-enprojectnment 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

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

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

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

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

Outputs:
  VpcId:
    Value: !Ref VPC
    Export:
      Name: !Sub "${Enprojectnment}-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 enprojectnment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.