How to Configure count or for_each with Terraform Modules – HashiCorp Help Center

Introduction

According to the Terraform Style Guide, "The for_each and count meta-arguments let you create multiple resources from a single resource block depending on run-time conditions. You can use these meta-arguments to make your code flexible and reduce duplicate resource blocks. If the resources are almost identical, use count. If some of arguments need distinct values that you cannot derive from an integer, use for_each(1)."

With a resource block this is quite straightforward; an example can be found here (2). What happens when these are used with a module?

Use case

Users configure count or for_each when they want multiple instances of a resource or an entire module. Below is an extremely simple example to get started and to see the difference in resource address. It is important to discuss the different syntax when considering refactoring code to use count instead of for_each and vice versa. The resource addresses recorded in state are not the same and the user will need to rename the resources.

Procedure

Example

Filesystem

$ tree
.
├── main.tf
└── module
    └── src.tf

./main.tf

# COUNT 
module "using_count" {
  source = "./module"
  count = 2

  name   = "test-${count.index}"
}

# FOR_EACH
variable "project" {
  type = map(any)

  default = {
    a = {
      some_num = 2,
      env      = "dev"
    },
    b = {
      some_num = 1,
      env      = "stage"
    }
  }
}

module "using_foreach" {
  source   = "./module"
  for_each = var.project

  name     = "test-${each.key}-${each.value.env}"
}

./module/src.tf

variable "name" {}

resource "random_pet" "r" {
  prefix = var.name
}

Execute terraform init to initialize providers and these 2 modules.
Run terraform plan and see the output Plan: 5 to add, 0 to change, 0 to destroy.
Execute terraform apply and notice the different resource addresses. The module using count will reference the newly created resources using an index: module.using_count[0].random_pet.r while the module using for_each will reference them using the key provided: module.using_foreach["a"].random_pet.r.

Considerations

What if you have a module that used count and now you want to refactor to use for_each? This would require a considerable amount of work. In the above example, the resource addresses are different and the name argument for random_pet is generated using different information: count uses the index and for_each uses key/value.

In this case, your best course of action is to

Make backups of your configuration files and terraform.tfstate file.
Adjust your terraform configuration to use the new syntax.
Remove the incorrect/old resources from the state file using terraform state rm ADDRESS (3) or the remove block (4).
Import them into state using the correct information and either the command terraform import ADDRESS <value> (5) or import block (6).