agentdevsl/terraform-style-guide

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 module code:

1. Start with version constraints and provider requirements in versions.tf
2. Create data sources before dependent resources
3. Build resources in dependency order with conditional creation (count or for_each)
4. Add outputs for key resource attributes, using try() for conditional resources
5. Use variables for all configurable values with secure defaults
6. Use this as the primary resource name for single instances
7. Prefer for_each over count for resource iteration (stable addresses)
8. Provider configuration belongs in examples/, NOT the root module

File Organization

| File | Purpose | |------|---------| | versions.tf | Terraform and provider version constraints (required_version + required_providers) | | main.tf | Primary resources and data sources | | variables.tf | Input variable declarations (alphabetical) | | outputs.tf | Output value declarations (alphabetical) | | locals.tf | Local value declarations |

Example Structure

# versions.tf
terraform {
  required_version = ">= 1.7"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = ">= 5.0"
    }
  }
}

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

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

# locals.tf
locals {
  common_tags = {
    Environment = var.environment
    ManagedBy   = "Terraform"
  }
}

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

  tags = merge(local.common_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

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
  }
}

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

Apply secure defaults per tf-security-baselines skill.

Version Pinning

terraform {
  required_version = ">= 1.7"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = ">= 5.0"  # Minimum version supporting required features
    }
  }
}

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