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.
metadata:
  lifecycle-status: active
---

# Terraform Style Guide

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

**Reference:** [HashiCorp Terraform Style Guide](https://developer.hashicorp.com/terraform/language/style)

## 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 |

### Example Structure

```hcl
# terraform.tf
terraform {
  required_version = ">= 1.14"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 6.0"
    }
  }
}

# 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

### Indentation and Alignment

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

```hcl
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:

```hcl
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
- Resource names must be singular, not plural
- Default to `main` for resources where a specific descriptive name is redundant or unavailable, provided only one instance exists

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

# Good
resource "aws_instance" "web_api" {}
resource "aws_vpc" "main" {}
variable "application_name" {}
```

## Variables

Every variable must include `type` and `description`:

```hcl
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`:

```hcl
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

```hcl
# 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

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

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

## Security Best Practices

Refer to SECURITY.md. It includes guidance on encrypting resources,
preventing sensitive data in state, and secure configurations.

## Version Pinning

```hcl
terraform {
  required_version = ">= 1.14"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 6.0"
    }
  }
}
```

Use the latest major version of each provider and the latest minor version of
Terraform, unless otherwise constrained by a dependency lock file or by other
modules used by the configuration.

**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

```hcl
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:

```bash
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

---

*Based on: [HashiCorp Terraform Style Guide](https://developer.hashicorp.com/terraform/language/style)*