for each

In this lesson, we explore the powerful for_each meta-argument in Terraform. By using for_each, you can overcome some limitations of the count meta-argument and manage resources more reliably.

Using count with a List

Traditionally, resources are created using the count meta-argument. For example:

resource "local_file" "pet" {
  filename = var.filename[count.index]
  count    = length(var.filename)
}

variable "filename" {
  default = [
    "/root/pets.txt",
    "/root/dogs.txt",
    "/root/cats.txt"
  ]
}

This configuration creates resources as a list. However, updating resources based on index positions can lead to unexpected behavior when the list order changes.

Transitioning to for_each

Switching to for_each can help manage resources more effectively by assigning a unique key to each resource using each.value. An initial attempt might be:

resource "local_file" "pet" {
  filename = var.filename[count.index]
  for_each = var.filename
}

variable "filename" {
  default = [
    "/root/pets.txt",
    "/root/dogs.txt",
    "/root/cats.txt"
  ]
}

Running terraform plan with this configuration results in an error. The for_each argument only supports a map or a set of strings – not a list of strings. The error message appears similar to:

$ terraform plan
Error: Invalid for_each argument

  on main.tf line 2, in resource "local_file" "pet":
   2:   for_each = var.filename

The given "for_each" argument value is unsuitable: the "for_each"
argument must be a map, or set of strings, and you have provided a value
of type list of string.

Ensure that the data type passed to for_each complies with Terraform’s requirements—a map or a set of strings.

Correcting the Configuration

There are two approaches to resolve this issue:

Change the variable type from a list to a set (sets do not allow duplicate elements).
Convert the list into a set in the resource block using Terraform’s built-in toset function.

Below is an updated configuration that uses the toset function:

resource "local_file" "pet" {
  filename = each.value
  for_each = toset(var.filename)
}

variable "filename" {
  type    = list(string)
  default = [
    "/root/pets.txt",
    "/root/dogs.txt",
    "/root/cats.txt"
  ]
}

When you run terraform plan now, Terraform will indicate that three resources will be created:

$ terraform plan
Terraform will perform the following actions:
  # local_file.pet["/root/cats.txt"] will be created
  + resource "local_file" "pet" {
      + directory_permission = "0777"
      + file_permission      = "0777"
      + filename             = "/root/cats.txt"
    }

...<output trimmed>...
Plan: 3 to add, 0 to change, 0 to destroy.

Updating Resources by Removing an Element

Let’s simulate updating the configuration by removing an element. For example, removing /root/pets.txt from the list changes the configuration to:

resource "local_file" "pet" {
  filename = each.value
  for_each = toset(var.filename)
}

variable "filename" {
  type    = list(string)
  default = [
    "/root/dogs.txt",
    "/root/cats.txt"
  ]
}

Running terraform plan with this updated variable shows that only the resource associated with /root/pets.txt will be destroyed:

$ terraform plan
Terraform will perform the following actions:
  # local_file.pet["/root/pets.txt"] will be destroyed
  + resource "local_file" "pet" {
      + directory_permission = "0777"
      + file_permission      = "0777"
      + filename             = "/root/pets.txt"
    }

... <output trimmed> ...
Plan: 0 to add, 0 to change, 1 to destroy.

The remaining resources persist without change.

Outputting Resource Details

To visualize how Terraform manages resources using for_each, you can create an output variable that displays the resource details. Resources managed with for_each are stored as a map, keyed by their unique identifier—in this case, the filename.

resource "local_file" "pet" {
  filename = each.value
  for_each = toset(var.filename)
}

output "pets" {
  value = local_file.pet
}

Running the output command displays the resources keyed by their filenames:

$ terraform output
pets = {
  "/root/cats.txt" = {
    "directory_permission" = "0777"
    "file_permission"      = "0777"
    "filename"             = "/root/cats.txt"
    "id"                   = "da39a3ee5e6b4b0d3255bfe95601890afd80709"
  }
  "/root/dogs.txt" = {
    "directory_permission" = "0777"
    "file_permission"      = "0777"
    "filename"             = "/root/dogs.txt"
    "id"                   = "da39a3ee5e6b4b0d3255bfe95601890afd80709"
  }
}

Using for_each allows the resources to be identified by a unique key (here, the filename), reducing the risk of accidental shifts in resource management compared to using count.

Conclusion

This lesson demonstrated how to implement the for_each meta-argument in Terraform to manage resources more reliably. By converting a list of strings to a set (either via variable type modification or the toset function), you can efficiently track and update resources using a map keyed by unique identifiers. This approach minimizes errors during deletion or updates compared to using the count meta-argument. Now that you’re familiar with for_each, try practicing these concepts in your Terraform projects to streamline your infrastructure management. Happy coding!

Introduction

Introduction to Infrastructure as Code

Getting Started with Terraform

Terraform Basics

Terraform State

Working with Terraform

Terraform with AWS

Remote State

Terraform Provisioners

Terraform Import, Tainting Resources and Debugging

Terraform Modules

Terraform Functions and Conditional Expressions

Using count with a List

Transitioning to for_each

Correcting the Configuration

Updating Resources by Removing an Element

Outputting Resource Details

Conclusion

Watch Video

Practice Lab

​Using count with a List

​Transitioning to for_each

​Correcting the Configuration

​Updating Resources by Removing an Element

​Outputting Resource Details

​Conclusion

Watch Video

Practice Lab

Using count with a List

Transitioning to for_each

Correcting the Configuration

Updating Resources by Removing an Element

Outputting Resource Details

Conclusion