A module is a reusable, parameterized bundle of resources. Any folder with .tf files is a module; the top level is the root module, and it calls child modules. Modules take input variables and expose outputs, so the same module produces dev, staging, and prod with different inputs.

Source modules from a local path, a git URL, or the Terraform Registry — and pin a version for stability. Good modules are small, focused (a “vpc” module, a “database” module), and have clear inputs/outputs.

# Call a registry module with pinned version
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "~> 5.0"

  name = "prod-vpc"
  cidr = "10.0.0.0/16"
  azs  = ["eu-west-1a", "eu-west-1b"]
}

# Use the module's output
resource "aws_instance" "app" {
  subnet_id = module.vpc.private_subnets[0]
  # ...
}

1. Explain the difference between the root module and a child module.
2. Call a module with input variables and consume one of its outputs.
3. Pin a registry module to a major version and explain why.
4. Sketch the inputs/outputs of a reusable “database” module.

• ↗ Terraform — Modules