terraform-patterns

Skill: Terraform Patterns

When to load

When writing new Terraform, reviewing IaC PRs, designing module structure, or debugging plan/apply failures.

Module Structure

terraform/
├── modules/
│   ├── vpc/
│   ├── eks-cluster/
│   ├── rds-postgres/
│   └── static-site/
└── environments/
    ├── staging/
    │   ├── main.tf
    │   ├── variables.tf
    │   └── terraform.tfvars
    └── production/
        ├── main.tf
        ├── variables.tf
        └── terraform.tfvars

Rule: Modules must be generic. Environment-specific values live in tfvars, never hardcoded in modules.

Resource Naming

resource "aws_s3_bucket" "this" {
  bucket = "${var.project}-${var.environment}-assets-${random_id.suffix.hex}"
  tags = merge(var.common_tags, { Name = "${var.project}-${var.environment}-assets" })
}

Remote State (Mandatory)

terraform {
  backend "s3" {
    bucket         = "my-company-terraform-state"
    key            = "${var.project}/${var.environment}/terraform.tfstate"
    region         = "us-east-1"
    encrypt        = true
    kms_key_id     = "arn:aws:kms:us-east-1:123456789:key/..."
    dynamodb_table = "terraform-state-lock"
  }
}

IAM Least Privilege

# ✅ Scoped policy
resource "aws_iam_policy" "app_s3_read" {
  policy = jsonencode({
    Statement = [{
      Effect   = "Allow"
      Action   = ["s3:GetObject", "s3:ListBucket"]
      Resource = [aws_s3_bucket.assets.arn, "${aws_s3_bucket.assets.arn}/*"]
    }]
  })
}
# ❌ Never: Action = ["s3:*"], Resource = ["*"]

Anti-Patterns

Anti-pattern	Fix
count for module variants	Use for_each with meaningful keys
Hardcoded AMI IDs	Use data "aws_ami" with filters
terraform_remote_state across all envs	Use SSM Parameter Store for cross-stack values