Terraform & OpenTofu

File Organization & Naming

File	Purpose
terraform.tf	Terraform + provider version requirements
providers.tf	Provider configurations
main.tf	Primary resources and data sources
variables.tf	Input variables (alphabetical)
outputs.tf	Output values (alphabetical)
locals.tf	Local values

• Lowercase with underscores: web_api, not webAPI or web-api
• Descriptive nouns excluding resource type: aws_instance.web_api not aws_instance.web_api_instance
• Singular, not plural
• this for singleton resources (one of that type per module)
• Contextual variable prefixes: vpc_cidr_block not cidr

Block Ordering

Resources: count/for_each (blank line after) → arguments → nested blocks → tags → depends_on → lifecycle (last)

Variables: description → type → default → validation → nullable

Every variable needs type + description. Every output needs description. Mark secrets sensitive = true.

Module Structure

Type	Scope	Example
Resource Module	Single logical group	VPC + subnets, SG + rules
Infrastructure Module	Collection of resource modules	Networking + compute for one region
Composition	Complete infrastructure	Spans regions/accounts

module-name/
├── main.tf, variables.tf, outputs.tf, versions.tf
├── examples/
│   ├── minimal/
│   └── complete/
└── tests/
    └── defaults.tftest.hcl

Keep modules small (single responsibility). examples/ double as documentation and integration test fixtures. Semantic versioning for all published modules.

count vs for_each

Scenario	Use
Boolean toggle (create or skip)	count = condition ? 1 : 0
Named/keyed items that may reorder	for_each = toset(list) or map
Fixed identical replicas	count = N

Default to for_each -- removing a middle item from a count list recreates all subsequent resources. Use count only for boolean conditionals or truly identical replicas.

Testing

Situation	Approach
Quick validation	terraform fmt -check && terraform validate
Pre-commit	+ tflint + trivy config . / checkov -d .
Logic validation (1.6+)	Native terraform test with command = plan
Cost-free unit tests (1.7+)	Native tests + mock_provider
Real infra validation	Native tests with command = apply, or Terratest (Go)

Native test essentials (.tftest.hcl in tests/):

• command = plan for fast unit tests; command = apply for integration (default)
• assert { condition = expr; error_message = "..." } -- multiple per run block
• expect_failures = [var.name] for negative testing (validate rejection of bad input)
• mock_provider "aws" { mock_resource "..." { defaults = { ... } } } -- plan-mode only, no credentials, fast CI
• variables {} at file level (all runs) or within a run block (override)
• Reference prior run outputs: run.setup.vpc_id
• parallel = true on independent runs with separate state -- creates sync point at next sequential run
• state_key = "name" required for parallel = true runs with independent state
• File naming: *_unit_test.tftest.hcl (plan mode) vs *_integration_test.tftest.hcl (apply mode)

Version Pinning

Component	Strategy	Example
Terraform	Pin minor	required_version = "~> 1.9"
Providers	Pin major	version = "~> 5.0"
Modules (prod)	Pin exact	version = "5.1.2"
Modules (dev)	Allow patch	version = "~> 5.1"

Key modern features: moved blocks (1.1+), optional() with defaults (1.3+), native testing (1.6+), mock providers (1.7+), cross-variable validation (1.9+), write-only arguments (1.11+). Stacks (HCP, preview): orchestrates multiple configs as a single deployment unit -- evaluate for multi-environment patterns.

State & Security

• Remote backend with locking: S3+DynamoDB, Azure Blob, GCS, or Terraform Cloud. Never local state for shared infrastructure.
• Encrypt state at rest. Never commit .tfstate, .terraform/, or *.tfplan. Always commit .terraform.lock.hcl.
• default_tags on provider for consistent resource tagging.
• Encryption at rest on all storage. Private networking by default -- public access is opt-in.
• Least-privilege security groups. No 0.0.0.0/0 ingress without explicit justification.
• Never hardcode credentials -- use assume_role, OIDC, or secrets managers.
• Pre-commit: terraform fmt -recursive && terraform validate && trivy config .
• moved { from = old; to = new } for refactoring resource names/modules without destroy-recreate. Remove block after apply.

Troubleshooting

• State lock stuck: terraform force-unlock <ID> -- only after confirming no other operation running
• Resource drift: terraform plan -refresh-only to detect, terraform apply -refresh-only to accept
• Replace tainted: terraform apply -replace=ADDR (not deprecated terraform taint)
• Import existing: import blocks (1.5+) for declarative import, or terraform import ADDR ID

Dependency Management

Use locals with try() to control deletion ordering without explicit depends_on:

locals {
  vpc_id = try(aws_vpc_ipv4_cidr_block_association.this[0].vpc_id, aws_vpc.this.id, "")
}

This forces Terraform to destroy subnets before CIDR associations -- prevents deletion errors.

• cidrsubnet(var.vpc_cidr, 8, count.index) for calculated subnet CIDRs -- never hardcode subnets
• Multi-region: provider "aws" { alias = "eu_west_1" } + providers = { aws = aws.eu_west_1 } in module blocks

Verify

Run before declaring done:

terraform fmt -check && terraform validate && tflint && trivy config .

All commands must pass with zero errors.