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Terraform Associate: Terraform modules

Terraform Associate · Work through your exam, one topic at a time.

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Terraform modules

What a module is

Any directory of .tf files is a module. The directory you run Terraform in is the root module; anything invoked with a module block is a child module. Modules package reusable, versioned infrastructure behind a narrow input/output contract.

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.5.1"
  cidr    = "10.0.0.0/16"
}

output "vpc_id" { value = module.vpc.vpc_id }

Mechanics

  • source supports the public/private registry, Git URLs (git::https://host/repo.git?ref=v1.2.0), object storage buckets, and local paths (./modules/network).
  • version is honored only for registry-style sources; local paths and raw Git use ?ref= instead.
  • Inputs are the child's declared variables; outputs are read as module.<name>.<output>. A child's internal resources are never addressable from the parent.
  • terraform init installs modules into .terraform/modules; changing source or version requires init, and init -upgrade moves to newer allowed versions.
  • Modules accept count and for_each, producing addresses like module.vpc["prod"].vpc_id.
  • Pass aliased providers explicitly: providers = { aws = aws.west }.

Traps

  • Declaring a provider block inside a reusable module is discouraged; the root should configure and pass providers.
  • A module output must be explicitly declared - forgetting it is the classic "why can't the parent see this attribute" bug.
  • A version argument on a local-path module is silently meaningless, not an error worth relying on.

Rule of thumb

If you are copy-pasting the same resource block into a second environment, that block belongs in a module with variables for what differs.