terraform-skill

Activate this skill when:

• Creating new Terraform or OpenTofu configurations or modules
• Setting up testing infrastructure for IaC code
• Deciding between testing approaches (validate, plan, frameworks)
• Structuring multi-environment deployments
• Implementing CI/CD for infrastructure-as-code
• Reviewing or refactoring existing Terraform/OpenTofu projects
• Choosing between module patterns or state management approaches

Don't use this skill for:

• Basic Terraform/OpenTofu syntax questions (Claude knows this)
• Provider-specific API reference (link to docs instead)
• Cloud platform questions unrelated to Terraform/OpenTofu

1. Code Structure Philosophy

Module Hierarchy:

| Type | When to Use | Scope |

|------|-------------|-------|

| Resource Module | Single logical group of connected resources | VPC + subnets, Security group + rules |

| Infrastructure Module | Collection of resource modules for a purpose | Multiple resource modules in one region/account |

| Composition | Complete infrastructure | Spans multiple regions/accounts |

Hierarchy: Resource → Resource Module → Infrastructure Module → Composition

Directory Structure:

```

environments/ # Environment-specific configurations

├── prod/

├── staging/

└── dev/

modules/ # Reusable modules

├── networking/

├── compute/

└── data/

examples/ # Module usage examples (also serve as tests)

├── complete/

└── minimal/

```

Key principle from terraform-best-practices.com:

• Separate environments (prod, staging) from modules (reusable components)
• Use examples/ as both documentation and integration test fixtures
• Keep modules small and focused (single responsibility)

For detailed module architecture, see: [Code Patterns: Module Types & Hierarchy](references/code-patterns.md)

2. Naming Conventions

Resources:

```hcl

# Good: Descriptive, contextual

resource "aws_instance" "web_server" { }

resource "aws_s3_bucket" "application_logs" { }

# Good: "this" for singleton resources (only one of that type)

resource "aws_vpc" "this" { }

resource "aws_security_group" "this" { }

# Avoid: Generic names for non-singletons

resource "aws_instance" "main" { }

resource "aws_s3_bucket" "bucket" { }

```

Singleton Resources:

Use "this" when your module creates only one resource of that type:

✅ DO:

```hcl

resource "aws_vpc" "this" {} # Module creates one VPC

resource "aws_security_group" "this" {} # Module creates one SG

```

❌ DON'T use "this" for multiple resources:

```hcl

resource "aws_subnet" "this" {} # If creating multiple subnets

```

Use descriptive names when creating multiple resources of the same type.

Variables:

```hcl

# Prefix with context when needed

var.vpc_cidr_block # Not just "cidr"

var.database_instance_class # Not just "instance_class"

```

Files:

• main.tf - Primary resources
• variables.tf - Input variables
• outputs.tf - Output values
• versions.tf - Provider versions
• data.tf - Data sources (optional)

Decision Matrix: Which Testing Approach?

| Your Situation | Recommended Approach | Tools | Cost |

|----------------|---------------------|-------|------|

| Quick syntax check | Static analysis | terraform validate, fmt | Free |

| Pre-commit validation | Static + lint | validate, tflint, trivy, checkov | Free |

| Terraform 1.6+, simple logic | Native test framework | Built-in terraform test | Free-Low |

| Pre-1.6, or Go expertise | Integration testing | Terratest | Low-Med |

| Security/compliance focus | Policy as code | OPA, Sentinel | Free |

| Cost-sensitive workflow | Mock providers (1.7+) | Native tests + mocking | Free |

| Multi-cloud, complex | Full integration | Terratest + real infra | Med-High |

Testing Pyramid for Infrastructure

```

/\

/ \ End-to-End Tests (Expensive)

/____\ - Full environment deployment

/ \ - Production-like setup

/________\

/ \ Integration Tests (Moderate)

/____________\ - Module testing in isolation

/ \ - Real resources in test account

/________________\ Static Analysis (Cheap)

- validate, fmt, lint

- Security scanning

```

Native Test Best Practices (1.6+)

Before generating test code:

1. Validate schemas with Terraform MCP:

```

Search provider docs → Get resource schema → Identify block types

```

1. Choose correct command mode:

- command = plan - Fast, for input validation

- command = apply - Required for computed values and set-type blocks

1. Handle set-type blocks correctly:

- Cannot index with [0]

- Use for expressions to iterate

- Or use command = apply to materialize

Common patterns:

• S3 encryption rules: set (use for expressions)
• Lifecycle transitions: set (use for expressions)
• IAM policy statements: set (use for expressions)

For detailed testing guides, see:

• [Testing Frameworks Guide](references/testing-frameworks.md) - Deep dive into static analysis, native tests, and Terratest
• [Quick Reference](references/quick-reference.md#testing-approach-selection) - Decision flowchart and command cheat sheet

Resource Block Ordering

Strict ordering for consistency:

1. count or for_each FIRST (blank line after)
2. Other arguments
3. tags as last real argument
4. depends_on after tags (if needed)
5. lifecycle at the very end (if needed)

```hcl

# ✅ GOOD - Correct ordering

resource "aws_nat_gateway" "this" {

count = var.create_nat_gateway ? 1 : 0

allocation_id = aws_eip.this[0].id

subnet_id = aws_subnet.public[0].id

tags = {

Name = "${var.name}-nat"

}

depends_on = [aws_internet_gateway.this]

lifecycle {

create_before_destroy = true

}

}

```

Variable Block Ordering

1. description (ALWAYS required)
2. type
3. default
4. validation
5. nullable (when setting to false)

```hcl

variable "environment" {

description = "Environment name for resource tagging"

type = string

default = "dev"

validation {

condition = contains(["dev", "staging", "prod"], var.environment)

error_message = "Environment must be one of: dev, staging, prod."

}

nullable = false

}

```

For complete structure guidelines, see: [Code Patterns: Block Ordering & Structure](references/code-patterns.md#block-ordering--structure)

Quick Decision Guide

| Scenario | Use | Why |

|----------|-----|-----|

| Boolean condition (create or don't) | count = condition ? 1 : 0 | Simple on/off toggle |

| Simple numeric replication | count = 3 | Fixed number of identical resources |

| Items may be reordered/removed | for_each = toset(list) | Stable resource addresses |

| Reference by key | for_each = map | Named access to resources |

| Multiple named resources | for_each | Better maintainability |

Common Patterns

Boolean conditions:

```hcl

# ✅ GOOD - Boolean condition

resource "aws_nat_gateway" "this" {

count = var.create_nat_gateway ? 1 : 0

# ...

}

```

Stable addressing with for_each:

```hcl

# ✅ GOOD - Removing "us-east-1b" only affects that subnet

resource "aws_subnet" "private" {

for_each = toset(var.availability_zones)

availability_zone = each.key

# ...

}

# ❌ BAD - Removing middle AZ recreates all subsequent subnets

resource "aws_subnet" "private" {

count = length(var.availability_zones)

availability_zone = var.availability_zones[count.index]

# ...

}

```

For migration guides and detailed examples, see: [Code Patterns: Count vs For_Each](references/code-patterns.md#count-vs-for_each-deep-dive)

Use locals to ensure correct resource deletion order:

```hcl

# Problem: Subnets might be deleted after CIDR blocks, causing errors

# Solution: Use try() in locals to hint deletion order

locals {

# References secondary CIDR first, falling back to VPC

# Forces Terraform to delete subnets before CIDR association

vpc_id = try(

aws_vpc_ipv4_cidr_block_association.this[0].vpc_id,

aws_vpc.this.id,

""

)

}

resource "aws_vpc" "this" {

cidr_block = "10.0.0.0/16"

}

resource "aws_vpc_ipv4_cidr_block_association" "this" {

count = var.add_secondary_cidr ? 1 : 0

vpc_id = aws_vpc.this.id

cidr_block = "10.1.0.0/16"

}

resource "aws_subnet" "public" {

vpc_id = local.vpc_id # Uses local, not direct reference

cidr_block = "10.1.0.0/24"

}

```

Why this matters:

• Prevents deletion errors when destroying infrastructure
• Ensures correct dependency order without explicit depends_on
• Particularly useful for VPC configurations with secondary CIDR blocks

For detailed examples, see: [Code Patterns: Locals for Dependency Management](references/code-patterns.md#locals-for-dependency-management)

Standard Module Structure

```

my-module/

├── README.md # Usage documentation

├── main.tf # Primary resources

├── variables.tf # Input variables with descriptions

├── outputs.tf # Output values

├── versions.tf # Provider version constraints

├── examples/

│ ├── minimal/ # Minimal working example

│ └── complete/ # Full-featured example

└── tests/ # Test files

└── module_test.tftest.hcl # Or .go

```

Best Practices Summary

Variables:

• ✅ Always include description
• ✅ Use explicit type constraints
• ✅ Provide sensible default values where appropriate
• ✅ Add validation blocks for complex constraints
• ✅ Use sensitive = true for secrets

Outputs:

• ✅ Always include description
• ✅ Mark sensitive outputs with sensitive = true
• ✅ Consider returning objects for related values
• ✅ Document what consumers should do with each output

For detailed module patterns, see:

• [Module Patterns Guide](references/module-patterns.md) - Variable best practices, output design, ✅ DO vs ❌ DON'T patterns
• [Quick Reference](references/quick-reference.md#common-patterns) - Resource naming, variable naming, file organization

Recommended Workflow Stages

1. Validate - Format check + syntax validation + linting
2. Test - Run automated tests (native or Terratest)
3. Plan - Generate and review execution plan
4. Apply - Execute changes (with approvals for production)

Cost Optimization Strategy

1. Use mocking for PR validation (free)
2. Run integration tests only on main branch (controlled cost)
3. Implement auto-cleanup (prevent orphaned resources)
4. Tag all test resources (track spending)

For complete CI/CD templates, see:

• [CI/CD Workflows Guide](references/ci-cd-workflows.md) - GitHub Actions, GitLab CI, Atlantis integration, cost optimization
• [Quick Reference](references/quick-reference.md#troubleshooting-guide) - Common CI/CD issues and solutions

Essential Security Checks

```bash

# Static security scanning

trivy config .

checkov -d .

```

Common Issues to Avoid

❌ Don't:

• Store secrets in variables
• Use default VPC
• Skip encryption
• Open security groups to 0.0.0.0/0

✅ Do:

• Use AWS Secrets Manager / Parameter Store
• Create dedicated VPCs
• Enable encryption at rest
• Use least-privilege security groups

For detailed security guidance, see:

• [Security & Compliance Guide](references/security-compliance.md) - Trivy/Checkov integration, secrets management, state file security, compliance testing

Version Constraint Syntax

```hcl

version = "5.0.0" # Exact (avoid - inflexible)

version = "~> 5.0" # Recommended: 5.0.x only

version = ">= 5.0" # Minimum (risky - breaking changes)

```

Strategy by Component

| Component | Strategy | Example |

|-----------|----------|---------|

| Terraform | Pin minor version | required_version = "~> 1.9" |

| Providers | Pin major version | version = "~> 5.0" |

| Modules (prod) | Pin exact version | version = "5.1.2" |

| Modules (dev) | Allow patch updates | version = "~> 5.1" |

Update Workflow

```bash

# Lock versions initially

terraform init # Creates .terraform.lock.hcl

# Update to latest within constraints

terraform init -upgrade # Updates providers

# Review and test

terraform plan

```

For detailed version management, see: [Code Patterns: Version Management](references/code-patterns.md#version-management)

Feature Availability by Version

| Feature | Version | Use Case |

|---------|---------|----------|

| try() function | 0.13+ | Safe fallbacks, replaces element(concat()) |

| nullable = false | 1.1+ | Prevent null values in variables |

| moved blocks | 1.1+ | Refactor without destroy/recreate |

| optional() with defaults | 1.3+ | Optional object attributes |

| Native testing | 1.6+ | Built-in test framework |

| Mock providers | 1.7+ | Cost-free unit testing |

| Provider functions | 1.8+ | Provider-specific data transformation |

| Cross-variable validation | 1.9+ | Validate relationships between variables |

| Write-only arguments | 1.11+ | Secrets never stored in state |

Quick Examples

```hcl

# try() - Safe fallbacks (0.13+)

output "sg_id" {

value = try(aws_security_group.this[0].id, "")

}

# optional() - Optional attributes with defaults (1.3+)

variable "config" {

type = object({

name = string

timeout = optional(number, 300) # Default: 300

})

}

# Cross-variable validation (1.9+)

variable "environment" { type = string }

variable "backup_days" {

type = number

validation {

condition = var.environment == "prod" ? var.backup_days >= 7 : true

error_message = "Production requires backup_days >= 7"

}

}

```

For complete patterns and examples, see: [Code Patterns: Modern Terraform Features](references/code-patterns.md#modern-terraform-features-10)

Terraform 1.0-1.5

• Use Terratest for testing
• No native testing framework available
• Focus on static analysis and plan validation

Terraform 1.6+ / OpenTofu 1.6+

• New: Native terraform test / tofu test command
• Consider migrating from external frameworks for simple tests
• Keep Terratest only for complex integration tests

Terraform 1.7+ / OpenTofu 1.7+

• New: Mock providers for unit testing
• Reduce cost by mocking external dependencies
• Use real integration tests for final validation

Terraform vs OpenTofu

Both are fully supported by this skill. For licensing, governance, and feature comparison, see [Quick Reference: Terraform vs OpenTofu](references/quick-reference.md#terraform-vs-opentofu-comparison).

This skill uses progressive disclosure - essential information is in this main file, detailed guides are available when needed:

📚 Reference Files:

• [Testing Frameworks](references/testing-frameworks.md) - In-depth guide to static analysis, native tests, and Terratest
• [Module Patterns](references/module-patterns.md) - Module structure, variable/output best practices, ✅ DO vs ❌ DON'T patterns
• [CI/CD Workflows](references/ci-cd-workflows.md) - GitHub Actions, GitLab CI templates, cost optimization, automated cleanup
• [Security & Compliance](references/security-compliance.md) - Trivy/Checkov integration, secrets management, compliance testing
• [Quick Reference](references/quick-reference.md) - Command cheat sheets, decision flowcharts, troubleshooting guide

How to use: When you need detailed information on a topic, reference the appropriate guide. Claude will load it on demand to provide comprehensive guidance.

This skill is licensed under the Apache License 2.0. See the LICENSE file for full terms.

Copyright © 2026 Anton Babenko