Terraform & OpenTofu

Working rules

• Preserve state and resource addresses during refactoring; inspect the plan for unintended replacement.
• Separate plan-only checks from apply-mode tests that create real infrastructure and incur cost.

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.

Version Pinning

Component	Strategy	Example
Terraform	Pin minor	required_version = "~> 1.9.0"
Providers	Pin major	version = "~> 5.0"
Modules (prod)	Pin exact	version = "5.1.2"
Modules (dev)	Allow patch	version = "~> 5.1.0"

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 -- check current release status): orchestrates multiple configs as a single deployment unit -- evaluate for multi-environment patterns.

State & Security

• Remote backend with locking: S3 with use_lockfile = true (1.10+), Azure Blob, GCS, or Terraform Cloud. Never local state for shared infrastructure. DynamoDB-based S3 locking (dynamodb_table) is deprecated and slated for removal -- prefer use_lockfile; both may be set at once while migrating an existing table off.
• 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: auto-format first (terraform fmt -recursive -- rewrites files), then verify (terraform validate && tflint && trivy config .)
• Use moved blocks with from and to addresses for refactoring resource names/modules without destroy-recreate. Retain historical moves for downstream upgrades; remove only after every affected state has migrated, or as an explicitly breaking module release.
• lifecycle { ignore_changes = [attr] } suppresses updates only, and it substitutes the prior state value at plan time -- on the first plan after the config change, with no "first apply" exception. Two consequences reviewers get backwards: (1) on an already-provisioned resource the literal in the config is never written, and ForceNew never fires because ignore_changes erased the diff before replacement is evaluated -- so a change that replaces a committed value with a placeholder scrubs the repository and leaves the remote value live; (2) ignore_changes does not apply on create, so any later -replace, taint, state rm + re-add, or manual deletion re-seeds the placeholder over a value that was set out of band. Keep only the container resource in configuration and provision the value entirely out of band, or state the restore step in the runbook for every replace path.

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. Where plan-mode tests exist, add terraform test -filter=<unit-test-file> -- restrict this to plan-mode suites, since apply-mode tests stand up real infrastructure and do not belong in a pre-completion check.

Task-specific references

Read the relevant reference before implementing or reviewing the matching behavior:

• For native plan/apply tests, fixture ordering, or CI test selection: native-test-patterns.md.