Modules

Everything so far has lived in a single, flat set of files. Real infrastructure repeats the same shapes — a web server, a VPC, a database tier — over and over, across environments and projects. Modules package a shape once and reuse it.

What a Module Is

Any directory of .tf files is a module. Whichever one terraform commands are run from is the root module — every configuration in this course so far has technically already been one, just never explicitly calling any others.

Child Modules

A module block calls another module from the root (or from another module), passing inputs as arguments matching that module's own declared variables, and reading its declared outputs back.

Module Structure & Composition

A module's own directory follows the same variables.tf/main.tf/outputs.tf convention as any configuration — inputs come in through its variable blocks, results go out through its output blocks. Everything else inside stays private to the module; the caller never sees its internal resource names.

The Public Terraform Registry

registry.terraform.io hosts community and vendor-maintained modules — a well-tested VPC module, for instance, covers subnet layout, route tables, NAT gateways, and dozens of edge cases a first attempt at writing one from scratch would likely miss.

Module Versioning

Registry modules use the same ~> version-constraint syntax as providers (Chapter 2) — pinning a module version protects against an upstream update introducing a breaking change without warning, exactly the same reproducibility guarantee the provider lock file already provides.

DRY Infrastructure

Module blocks accept for_each and count too — calling the same module multiple times with different inputs, rather than copy-pasting a resource shape once per environment.

Hands-On Exercises