writing-infrastructure-code

Infrastructure as Code

Provision and manage cloud infrastructure using code-based automation tools. This skill covers tool selection, state management, module design, and operational patterns across Terraform/OpenTofu, Pulumi, and AWS CDK.

When to Use

Use this skill when:

• Provisioning cloud infrastructure (compute, networking, databases, storage)
• Migrating from manual infrastructure to code-based workflows
• Designing reusable infrastructure modules
• Implementing multi-cloud or hybrid-cloud deployments
• Establishing state management and drift detection patterns
• Integrating infrastructure provisioning into CI/CD pipelines
• Evaluating IaC tools (Terraform vs Pulumi vs CDK)

Common requests:

• "Create a Terraform module for VPC provisioning"
• "Set up remote state with locking for team collaboration"
• "Compare Pulumi vs Terraform for our use case"
• "Design composable infrastructure modules"
• "Implement drift detection for existing infrastructure"

Core Concepts

Infrastructure as Code Fundamentals

Key Principles:

1. Declarative vs Imperative - Describe desired state (Terraform) or program infrastructure (Pulumi)
2. Idempotency - Same input produces same output, safe to re-run
3. Version Control - Infrastructure changes tracked in Git
4. State Management - Track actual infrastructure state
5. Module Composition - Reusable, versioned infrastructure components

Benefits:

• Reproducibility (same code = same infrastructure)
• Auditability (Git history shows all changes)
• Collaboration (code reviews for infrastructure changes)
• Automation (CI/CD deploys infrastructure)
• Disaster recovery (rebuild from code)

Tool Selection Framework

Choose IaC tools based on team composition and cloud strategy:

Terraform/OpenTofu - Declarative, HCL-based

• Multi-cloud and hybrid-cloud deployments
• Operations/SRE teams prefer declarative approach
• Largest provider ecosystem (AWS, GCP, Azure, 3000+ providers)
• Mature module registry and community

Pulumi - Imperative, programming language-based

• Developer-centric teams familiar with TypeScript/Python/Go
• Complex logic requires programming constructs (loops, conditionals, functions)
• Native unit testing using familiar test frameworks
• Strong typing and IDE support

AWS CDK - AWS-native, programming language-based

• AWS-only infrastructure
• Tight integration with AWS services
• L1/L2/L3 construct abstractions
• CloudFormation under the hood

Decision Tree:

Multi-cloud required?
├─ YES → Team composition?
│  ├─ Ops/SRE focused → Terraform/OpenTofu
│  └─ Developer focused → Pulumi
└─ NO → AWS only?
   ├─ YES → Language preference?
   │  ├─ HCL/declarative → Terraform
   │  ├─ TypeScript/Python → AWS CDK
   │  └─ YAML/simple → CloudFormation
   └─ NO → GCP/Azure only?
      └─ Terraform or Pulumi

State Management Architecture

Remote state with locking enables team collaboration:

Backend Selection:

| Cloud Provider | Recommended Backend | Locking Mechanism | |----------------|---------------------|-------------------| | AWS | S3 + DynamoDB | DynamoDB table | | GCP | Google Cloud Storage | Native | | Azure | Azure Blob Storage | Lease-based | | Multi-cloud | Terraform Cloud/Enterprise | Built-in | | Pulumi | Pulumi Service | Built-in |

State Isolation Strategies:

1. Directory Separation (recommended for most teams)
2. Separate directories per environment (prod/, staging/, dev/)
3. Complete state file isolation
4. No risk of cross-environment contamination

1. Workspaces
2. Single codebase, multiple environments
3. Shared state backend, environment namespacing
4. Risk: accidental cross-environment operations

1. Layered Architecture
2. Separate state files for networking, compute, data layers
3. Blast radius reduction
4. Cross-layer references via remote state data sources

Critical State Management Rules:

• Always use remote state for team environments
• Enable state file encryption at rest
• Enable versioning on state storage
• Use state locking to prevent concurrent modifications
• Never commit state files to Git
• Mark sensitive outputs as sensitive = true

Module Design Patterns

Composable Module Structure:

modules/
├── vpc/              # Network foundation
├── security-group/   # Reusable security group patterns
├── rds/              # Database with backups, encryption
├── ecs-cluster/      # Container orchestration base
├── ecs-service/      # Individual microservice
└── alb/              # Application load balancer

Module Versioning:

• Pin module versions in production (version = "5.1.0")
• Use semantic versioning for internal modules
• Test module updates in non-prod first
• Maintain CHANGELOG for module releases

Module Design Principles:

• Clear input contract (required vs optional variables)
• Documented outputs (what consumers can reference)
• Sane defaults where possible
• Validation rules for inputs
• Examples directory showing usage

When to Create a Module:

• Resource group is reused 3+ times
• Clear boundaries and responsibilities
• Stable interface contract
• Team has module maintenance capacity

When to Keep Monolithic:

• One-off infrastructure
• Rapid prototyping phase
• High coupling between resources
• Small team, simple infrastructure

Quick Reference

Terraform/OpenTofu Commands

# Initialize providers and backend
terraform init

# Plan changes (preview)
terraform plan

# Apply changes
terraform apply

# Destroy infrastructure
terraform destroy

# Format HCL files
terraform fmt

# Validate syntax
terraform validate

# Show state
terraform state list
terraform state show <resource>

# Import existing resources
terraform import <resource.name> <id>

# Workspace management
terraform workspace list
terraform workspace new staging
terraform workspace select prod

Pulumi Commands

# Initialize new project
pulumi new aws-typescript

# Preview changes
pulumi preview

# Apply changes
pulumi up

# Destroy infrastructure
pulumi destroy

# Show stack outputs
pulumi stack output

# Manage stacks
pulumi stack ls
pulumi stack select prod

# Import existing resources
pulumi import <type> <name> <id>

# Export/import state
pulumi stack export > state.json
pulumi stack import < state.json

AWS CDK Commands

# Initialize new app
cdk init app --language typescript

# Synthesize CloudFormation
cdk synth

# Preview changes
cdk diff

# Deploy stack
cdk deploy

# Destroy stack
cdk destroy

# Bootstrap account/region
cdk bootstrap

# List stacks
cdk list

Common Patterns Checklist

Infrastructure Provisioning:

• [ ] Remote state configured with locking
• [ ] State file encryption enabled
• [ ] Provider versions pinned
• [ ] Module versions pinned (production)
• [ ] Variables have descriptions and types
• [ ] Sensitive outputs marked as sensitive
• [ ] Tagging strategy implemented
• [ ] Cost allocation tags applied

Module Development:

• [ ] Clear README with usage examples
• [ ] Required vs optional variables documented
• [ ] Outputs documented with descriptions
• [ ] Validation rules for critical inputs
• [ ] Examples directory with working code
• [ ] Tests for module behavior (Terratest/CDK assertions)
• [ ] CHANGELOG for version tracking
• [ ] Semantic versioning followed

Operational Readiness:

• [ ] Drift detection scheduled
• [ ] CI/CD pipeline for plan/apply
• [ ] State backup strategy
• [ ] Disaster recovery documented
• [ ] Team access controls configured (IAM/RBAC)
• [ ] Cost estimation integrated (Infracost)
• [ ] Security scanning integrated (Checkov/tfsec)
• [ ] Documentation kept current

Detailed Documentation

For comprehensive patterns and implementation details:

Tool-Specific Patterns:

• references/terraform-patterns.md - Terraform/OpenTofu best practices, HCL patterns
• references/pulumi-patterns.md - Pulumi across TypeScript/Python/Go

**Architec