Terraform Style Guide

Terraform Style Guide

Ref: HashiCorp Terraform Style Guide

Code Generation Order

1. Provider configuration and version constraints
2. Data sources before dependent resources
3. Resources in dependency order
4. Outputs for key resource attributes
5. Variables for all configurable values

File Organization

Code Formatting

• Two spaces per nesting level (no tabs)
• Align equals signs for consecutive arguments
• Arguments precede blocks; 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

• Lowercase with underscores for all names
• Descriptive nouns excluding the resource type
• Singular resource names
• Use main when only one instance exists and a specific name adds no clarity

# ✗
resource "aws_instance" "webAPI-aws-instance" {}

# ✓
resource "aws_instance" "web_api" {}
resource "aws_vpc" "main" {}
variable "application_name" {}

Variables

Every variable requires 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 requires description. Mark sensitive values:

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 — stable references by name instead of index:

resource "aws_instance" "web" {
  for_each = toset(["web-1", "web-2", "web-3"])
  tags     = { Name = each.key }
}

Use count only for conditional creation:

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

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

Security

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

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
}

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"
}