Terraform Modules Deep Dive Cheat Sheet

Detailed reference for structuring, calling, and versioning reusable Terraform modules including inputs, outputs, and remote sources.

2 PagesAdvancedFeb 8, 2026

Standard Module Layout

Conventional file layout for a reusable Terraform module.

bash

modules/vpc/├── main.tf          # Resource definitions├── variables.tf     # Input variable declarations├── outputs.tf       # Output value declarations├── versions.tf       # required_providers / required_version└── README.md        # Usage docs

Variables & Outputs

Declaring typed inputs and exposing outputs from a module.

hcl

# variables.tfvariable "cidr_block" {  type        = string  description = "CIDR range for the VPC"  default     = "10.0.0.0/16"}variable "subnet_count" {  type    = number  default = 2}# outputs.tfoutput "vpc_id" {  value       = aws_vpc.this.id  description = "ID of the created VPC"}

Calling a Module

Referencing local and remote (registry/Git) module sources.

hcl

module "vpc" {  source      = "./modules/vpc"      # Local path  cidr_block  = "10.1.0.0/16"  subnet_count = 3}module "vpc_registry" {  source  = "terraform-aws-modules/vpc/aws"  # Registry  version = "~> 5.0"  cidr    = "10.2.0.0/16"}module "vpc_git" {  source = "git::https://github.com/org/tf-modules.git//vpc?ref=v1.4.0"}# Reference an output from another moduleresource "aws_instance" "app" {  subnet_id = module.vpc.public_subnet_ids[0]}

Module Composition Features

Meta-arguments and patterns for flexible, reusable modules.

count / for_each on module blocks- Instantiate a module multiple times, e.g. for_each = toset(var.environments)
depends_on (module)- Force explicit ordering when dependencies aren't inferable from references
providers = { aws = aws.west }- Pass specific provider configurations/aliases into a child module
terraform_remote_state- Data source to read outputs from another Terraform state/module
locals block- Compute derived values once and reuse them across a module for DRY expressions
moved block- Declares a resource/module was renamed or relocated, avoiding destroy/recreate on refactors

Module Versioning & Sources

Pinning module versions for reproducible infrastructure.

version = "~> 5.0"- Registry modules support version constraints, resolved via terraform init
?ref=v1.4.0- Git sources pin to a tag, branch, or commit SHA
terraform init -upgrade- Re-resolve modules/providers to the latest versions matching constraints
.terraform.lock.hcl- Lock file recording exact provider versions and checksums (not module versions)

Pro Tip

Pin remote module sources to an immutable ref (a Git tag or commit SHA, or a registry version constraint) rather than a branch like `main` — otherwise a plan can silently pick up unreviewed upstream changes.

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