What are Terraform Modules?

Question

Accepted Answer

**Modules** — reusable, composable Terraform configurations.

```hcl
# modules/vpc/main.tf
resource "aws_vpc" "main" {
  cidr_block = var.cidr_block
  tags = {
    Name = "${var.project}-vpc"
  }
}

resource "aws_subnet" "public" {
  for_each          = toset(var.public_subnets)
  vpc_id            = aws_vpc.main.id
  cidr_block        = each.value
  availability_zone = each.key
  map_public_ip_on_launch = true
}

# modules/vpc/variables.tf
variable "cidr_block" {
  type        = string
  description = "VPC CIDR block"
}

variable "public_subnets" {
  type        = list(string)
  description = "Public subnet CIDRs"
}

# modules/vpc/outputs.tf
output "vpc_id" {
  value = aws_vpc.main.id
}

# modules/vpc/versions.tf
version: terraform >= 1.0
```

```hcl
# Root module usage
module "vpc" {
  source = "./modules/vpc"

cidr_block     = "10.0.0.0/16"
  public_subnets = ["10.0.1.0/24", "10.0.2.0/24"]
}

# From registry
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"
  # ...
}

# From Git
module "vpc" {
  source = "git::https://github.com/org/terraform-vpc.git?ref=v1.0"
}
```

**Best Practices:**
- One resource per module.
- Input validation with variables.
- Version pinning.
- Publish to Terraform Registry.

DevOps: What are Terraform Modules?