Terraform modules: structure and calling conventions for reusable infrastructure

Placing all resources in a single flat main.tf file makes infrastructure difficult to reuse, test, and maintain as it grows. Teams end up copying and pasting HCL across environments, leading to drift between dev and prod.

Extract related resources into child modules with a clear interface (variables, outputs). Publish shared modules to a private registry or reference them via relative paths or git sources.

Module directory structure:

modules/
  rds-postgres/
    main.tf       # resources
    variables.tf  # input variables
    outputs.tf    # output values
    versions.tf   # required_providers
    README.md

Calling a module:

module "database" {
  source  = "./modules/rds-postgres"
  # or: source = "git::https://github.com/org/tf-modules.git//rds-postgres?ref=v1.2.0"

  identifier     = "app-db-${var.environment}"
  instance_class = "db.t3.medium"
  vpc_id         = module.networking.vpc_id
  subnet_ids     = module.networking.private_subnet_ids
}

Modules are the primary unit of reuse in Terraform. They enforce consistent resource configuration across environments and make it easy to upgrade shared infrastructure patterns in one place.

Managing infrastructure across multiple environments or teams with shared patterns