Terraform use case to create multiple almost identical copies of infrastructure

Question

I have TF templates whose purpose is to create multiple copies of the same cloud infrastructure. For example you have multiple business units inside a big organization, and you want to build out the same basic networks. Or you want an easy way for a developer to spin up the stack that he's working on. The only difference between "tf apply" invokations is the variable BUSINESS_UNIT, for example, which is passed in as an environment variable.

Is anyone else using a system like this, and if so, how do you manage the state files ?

Answer 1

You should use a Terraform Module. Creating a module is nothing special: just put any Terraform templates in a folder. What makes a module special is how you use it.

Let's say you put the Terraform code for your infrastructure in the folder /terraform/modules/common-infra. Then, in the templates that actually define your live infrastructure (e.g. /terraform/live/business-units/main.tf), you could use the module as follows:

module "business-unit-a" {
  source = "/terraform/modules/common-infra"
}

To create the infrastructure for multiple business units, you could use the same module multiple times:

module "business-unit-a" {
  source = "/terraform/modules/common-infra"
}

module "business-unit-b" {
  source = "/terraform/modules/common-infra"
}

module "business-unit-c" {
  source = "/terraform/modules/common-infra"
}

If each business unit needs to customize some parameters, then all you need to do is define an input variable in the module (e.g. under /terraform/modules/common-infra/vars.tf):

variable "business_unit_name" {
  description = "The name of the business unit"
}

Now you can set this variable to a different value each time you use the module:

module "business-unit-a" {
  source = "/terraform/modules/common-infra"
  business_unit_name = "a"
}

module "business-unit-b" {
  source = "/terraform/modules/common-infra"
  business_unit_name = "b"
}

module "business-unit-c" {
  source = "/terraform/modules/common-infra"
  business_unit_name = "c"
}

For more information, see How to create reusable infrastructure with Terraform modules and Terraform: Up & Running.