3004 Terraform Code Best Practicesinstructions 23280dc6

Terraform Development Best Practices

Quick Reference

[!IMPORTANT] The 4 tags above are baseline defaults. Discovered Azure Policy constraints (04-governance-constraints.md) ALWAYS take precedence. See terraform-policy-compliance.instructions.md.

File Structure (MANDATORY)

Every root module MUST follow this file layout:

Naming Conventions

Resource Identifiers

Use azurerm_resource_group.this for singleton resources (single instance per module). Use descriptive names for multiple instances: azurerm_subnet.app, azurerm_subnet.data.

# Singleton resource pattern
resource "azurerm_resource_group" "this" {
  name     = "rg-${var.project}-${var.environment}"
  location = var.location
  tags     = local.tags
}

Azure Resource Names

Use lowercase with hyphens for Azure resource names. Follow CAF abbreviations:

[!CAUTION] Storage Account names have a 24-char limit and no hyphens. Key Vault names have a 24-char limit. Always use substr() to trim long names.

Unique Suffix Pattern (CRITICAL)

Generate ONCE in the root module, pass to ALL child modules:

# versions.tf or locals.tf
resource "random_string" "suffix" {
  length  = 4
  lower   = true
  numeric = true
  special = false
}

locals {
  suffix = random_string.suffix.result

  # Length-constrained names
  kv_name  = "kv-${substr(var.project, 0, 8)}-${substr(var.environment, 0, 3)}-${local.suffix}"
  st_name  = "st${substr(replace(var.project, "-", ""), 0, 8)}${substr(var.environment, 0, 3)}${local.suffix}"
}

Provider Configuration

# versions.tf
terraform {
  required_version = ">= 1.9"

  required_providers {
    azurerm = {
      source  = "hashicorp/azurerm"
      version = "~> 4.0"
    }
    random = {
      source  = "hashicorp/random"
      version = "~> 3.0"
    }
  }
}

# providers.tf
provider "azurerm" {
  features {}
  subscription_id = var.subscription_id
}

State Backend (MANDATORY)

Use Azure Storage Account for remote state. NEVER use HCP Terraform Cloud.

# backend.tf
terraform {
  backend "azurerm" {
    resource_group_name  = "rg-tfstate-prod"
    storage_account_name = "sttfstate{suffix}"
    container_name       = "tfstate"
    key                  = "{project}.terraform.tfstate"
  }
}

Tags (MANDATORY)

[!IMPORTANT] These 4 tags are the MINIMUM baseline. Azure Policy in your subscription may enforce additional tags. Always defer to 04-governance-constraints.md for the actual required tag list.

# locals.tf
locals {
  tags = merge(var.tags, {
    Environment = var.environment
    ManagedBy   = "Terraform"
    Project     = var.project
    Owner       = var.owner
  })
}

Pass local.tags to every resource and AVM module.

Security Defaults (MANDATORY)

[!IMPORTANT] The security settings below are baseline defaults. Discovered Azure Policy security constraints (04-governance-constraints.md) ALWAYS take precedence. See terraform-policy-compliance.instructions.md.

# Storage Account
resource "azurerm_storage_account" "this" {
  # ...
  https_traffic_only_enabled    = true
  min_tls_version               = "TLS1_2"
  allow_nested_items_to_be_public = false
  shared_access_key_enabled     = false  # Policy may require this
}

# SQL Server
resource "azurerm_mssql_server" "this" {
  # ...
  minimum_tls_version          = "1.2"
  public_network_access_enabled = false
  azuread_administrator {
    azuread_authentication_only = true
  }
}

RBAC Least Privilege (MANDATORY)

Do not grant broad built-in control-plane roles to runtime app identities.

Forbidden by Default

The following roles are not allowed for application runtime identities unless explicitly approved in a tracked exception:

• Owner
• Contributor
• User Access Administrator

Approved Role Mappings

Use the smallest role and narrowest scope that satisfies the workload.

SQL-Specific Rule

For app-to-SQL access, prefer:

1. Entra-based DB user and DB roles (db_datareader, db_datawriter) where possible
2. Otherwise SQL DB Contributor at database scope

Never assign Contributor at SQL server scope for app runtime identities.

Exception Process

If a broad role is unavoidable, all of the following are required:

1. Inline comment marker on the role assignment: RBAC_EXCEPTION_APPROVED: <ticket-or-ADR>
2. Matching justification in implementation docs (ADR or implementation reference)
3. Time-bound review date and owner in the justification

Without all three, the configuration is non-compliant.

Azure Verified Modules (AVM-TF)

MANDATORY: Use AVM-TF modules for ALL resources where an AVM module exists.

Raw azurerm_* resources are only permitted when no AVM module exists AND the user explicitly approves. Document the rationale in the implementation reference.

# ✅ Use AVM-TF for Key Vault
module "key_vault" {
  source  = "Azure/avm-res-keyvault-vault/azurerm"
  version = "~> 0.9"

  name                = local.kv_name
  resource_group_name = azurerm_resource_group.this.name
  location            = var.location
  tenant_id           = data.azurerm_client_config.current.tenant_id
  tags                = local.tags
}

# ❌ Only use raw azurerm_* if no AVM module exists
# Requires explicit user approval: "approve raw terraform"

AVM-TF Module Source Format

source  = "Azure/avm-res-{service}-{resource}/azurerm"
version = "~> {major}.{minor}"

Examples:

Use mcp_terraform_get_latest_module_version or registry.terraform.io/modules/Azure to find the latest version before generating code. Update pinned minor version (~> X.Y) to the latest available.

Variables

# variables.tf
variable "location" {
  description = "Azure region for all resources."
  type        = string
  default     = "swedencentral"

  validation {
    condition     = contains(["swedencentral", "germanywestcentral", "northeurope"], var.location)
    error_message = "Location must be an approved EU region."
  }
}

variable "environment" {
  description = "Deployment environment."
  type        = string
  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be dev, staging, or prod."
  }
}

variable "tags" {
  description = "Additional tags to merge with baseline tags."
  type        = map(string)
  default     = {}
}

Outputs

# outputs.tf — every module must output BOTH ID and name
output "resource_group_id" {
  description = "Resource group resource ID."
  value       = azurerm_resource_group.this.id
}

output "resource_group_name" {
  description = "Resource group name."
  value       = azurerm_resource_group.this.name
}

Managed Identity Pattern

Prefer SystemAssigned managed identity over access keys or connection strings:

resource "azurerm_linux_web_app" "this" {
  # ...
  identity {
    type = "SystemAssigned"
  }
}

resource "azurerm_role_assignment" "app_kv" {
  scope                = module.key_vault.resource_id
  role_definition_name = "Key Vault Secrets User"
  principal_id         = azurerm_linux_web_app.this.identity[0].principal_id
}

Lifecycle Rules

# Avoid phantom diffs on externally-managed tags
lifecycle {
  ignore_changes = [tags["DateCreated"]]
}

# Prevent accidental deletion of stateful resources
lifecycle {
  prevent_destroy = true
}

Resource Renaming Without Destroy

Use moved blocks instead of destroying and re-creating:

moved {
  from = azurerm_key_vault.main
  to   = azurerm_key_vault.this
}

Patterns to Avoid

Validation Commands

terraform fmt -recursive
terraform validate
terraform plan -out=plan.tfplan
terraform show -json plan.tfplan | python scripts/analyze_plan.py  # optional: set-diff analysis

Always run terraform fmt before committing. Always run terraform validate before planning.

Cross-References

• Policy compliance: .github/instructions/terraform-policy-compliance.instructions.md
• Governance discovery: .github/instructions/governance-discovery.instructions.md
• Terraform patterns: .github/skills/terraform-patterns/SKILL.md
• Azure defaults: .github/skills/azure-defaults/SKILL.md