terraform-style-guide

name	terraform-style-guide
description	Generate Terraform HCL code following HashiCorp's official style conventions and best practices. Use when writing, reviewing, or generating Terraform configurations.

Terraform Style Guide

Generate and maintain Terraform code following HashiCorp's official style conventions and best practices.

Reference: HashiCorp Terraform Style Guide

Code Generation Strategy

When generating Terraform code:

1. Start with provider configuration and version constraints
2. Create data sources before dependent resources
3. Build resources in dependency order
4. Add outputs for key resource attributes
5. Use variables for all configurable values

File Organization

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

1. Use modules for larger projects to group related resources.
2. Save modules in infra/modules/ and reference them from root configuration.
3. Root modules are in infra/ and infra/

Example Structure

# terraform.tf
terraform {
  required_version = ">= 1.15"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 6.43"
    }
  }
}

provider "aws" {
  region = var.aws_region
  default_tags {
    tags = local.common_tags
  }
}

# variables.tf
variable "environment" {
  description = "Target deployment environment"
  type        = string

  validation {
    condition     = contains(["dev", "uat", "prod"], var.environment)
    error_message = "Environment must be dev, uat, or prod."
  }
}

# locals.tf
locals {
  common_tags = {
    Environment = var.environment
    CreatedBy   = "Terraform"
    Project     = var.project_name
    Owner       = "DOS68kSlack"
    Source      = "https://github.com/pagopa/DOS68kSlack"
    CostCenter  = "PNRR 1.4.3 Cloud Gaap"
  }
}

# main.tf
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_hostnames = true

  tags = {
    Name = "${var.project_name}-${var.environment}-vpc"
  }
}

# outputs.tf
output "vpc_id" {
  description = "ID of the created VPC"
  value       = aws_vpc.main.id
}

Code Formatting

Indentation and Alignment

• Use two spaces per nesting level (no tabs)
• Align equals signs for consecutive arguments

resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  subnet_id     = "subnet-12345678"

  tags = {
    Name        = "web-server"
    Environment = "production"
  }
}

Block Organization

Arguments precede blocks, with meta-arguments first:

resource "aws_instance" "example" {
  # Meta-arguments
  count = 3

  # Arguments
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  # Blocks
  root_block_device {
    volume_size = 20
  }

  # Lifecycle last
  lifecycle {
    create_before_destroy = true
  }
}

Naming Conventions

• Use lowercase with underscores for all names
• Use descriptive nouns excluding the resource type
• Be specific and meaningful

# Bad
resource "aws_instance" "webAPI-aws-instance" {}
variable "name" {}

# Good
resource "aws_instance" "web_api" {}
variable "application_name" {}

Variables

Every variable must include type and description:

variable "instance_type" {
  description = "EC2 instance type for the web server"
  type        = string
  default     = "t2.micro"

  validation {
    condition     = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type)
    error_message = "Instance type must be t2.micro, t2.small, or t2.medium."
  }
}

variable "database_password" {
  description = "Password for the database admin user"
  type        = string
  sensitive   = true
}

Outputs

Every output must include description:

output "instance_id" {
  description = "ID of the EC2 instance"
  value       = aws_instance.web.id
}

output "database_password" {
  description = "Database administrator password"
  value       = aws_db_instance.main.password
  sensitive   = true
}

Dynamic Resource Creation

Prefer for_each over count

# Bad - count for multiple resources
resource "aws_instance" "web" {
  count = var.instance_count
  tags  = { Name = "web-${count.index}" }
}

# Good - for_each with named instances
variable "instance_names" {
  type    = set(string)
  default = ["web-1", "web-2", "web-3"]
}

resource "aws_instance" "web" {
  for_each = var.instance_names
  tags     = { Name = each.key }
}

count for Conditional Creation

resource "aws_cloudwatch_metric_alarm" "cpu" {
  count = var.enable_monitoring ? 1 : 0

  alarm_name = "high-cpu-usage"
  threshold  = 80
}

Security Best Practices

When generating code, apply security hardening:

• Enable encryption at rest by default
• Configure private networking where applicable
• Apply principle of least privilege for security groups
• Enable logging and monitoring
• Never hardcode credentials or secrets
• Mark sensitive outputs with sensitive = true

Example: Secure S3 Bucket

resource "aws_s3_bucket" "data" {
  bucket = "${var.project}-${var.environment}-data"
  tags   = local.common_tags
}

resource "aws_s3_bucket_versioning" "data" {
  bucket = aws_s3_bucket.data.id

  versioning_configuration {
    status = "Enabled"
  }
}

resource "aws_s3_bucket_server_side_encryption_configuration" "data" {
  bucket = aws_s3_bucket.data.id

  rule {
    apply_server_side_encryption_by_default {
      sse_algorithm     = "aws:kms"
      kms_master_key_id = aws_kms_key.s3.arn
    }
  }
}

resource "aws_s3_bucket_public_access_block" "data" {
  bucket = aws_s3_bucket.data.id

  block_public_acls       = true
  block_public_policy     = true
  ignore_public_acls      = true
  restrict_public_buckets = true
}

Version Pinning

terraform {
  required_version = ">= 1.7"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"  # Allow minor updates
    }
  }
}

Version constraint operators:

• = 1.0.0 - Exact version
• >= 1.0.0 - Greater than or equal
• ~> 1.0 - Allow rightmost component to increment
• >= 1.0, < 2.0 - Version range

Provider Configuration

provider "aws" {
  region = "us-west-2"

  default_tags {
    tags = {
      ManagedBy = "Terraform"
      Project   = var.project_name
    }
  }
}

# Aliased provider for multi-region
provider "aws" {
  alias  = "east"
  region = "us-east-1"
}

Version Control

Never commit:

• terraform.tfstate, terraform.tfstate.backup
• .terraform/ directory
• *.tfplan
• .tfvars files with sensitive data

Always commit:

• All .tf configuration files
• .terraform.lock.hcl (dependency lock file)

Validation Tools

Run before committing:

terraform fmt -recursive
terraform validate

Additional tools:

• tflint - Linting and best practices
• checkov / tfsec - Security scanning

Code Review Checklist

• Code formatted with terraform fmt
• Configuration validated with terraform validate
• Files organized according to standard structure
• All variables have type and description
• All outputs have descriptions
• Resource names use descriptive nouns with underscores
• Version constraints pinned explicitly
• Sensitive values marked with sensitive = true
• No hardcoded credentials or secrets
• Security best practices applied

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Based on: HashiCorp Terraform Style Guide