Unexpected re-creation of resources when modifying variable with for_each

Introduction

Problem

When changing the values of a variable that is used by a for_each the resource is re-created.

Prerequisites

Have the following code:

locals {
  names = {
    "my_index" = {
      name = "my_name"
    }
  }
}

resource "terraform_data" "example" {
  for_each = local.names
  input = each.key
}

Observe that the input value for the terraform_data resource is using each.key.  This is referring to the index in the names local.

If you now want to change the value of the input, you will need to change the key my_index in the local names.

If we change the index my_index to my_index_changed and you run a terraform plan it will result in the following plan output:

terraform_data.example["my_index"]: Refreshing state... [id=9012385b-25d6-5fc0-403e-7cbb4291fe2d]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create
  - destroy

Terraform will perform the following actions:

  # terraform_data.example["my_index"] will be destroyed
  # (because key ["my_index"] is not in for_each map)
  - resource "terraform_data" "example" {
    - id = "9012385b-25d6-5fc0-403e-7cbb4291fe2d" -> null
    - input = "my_index" -> null
    - output = "my_index" -> null
  }

  # terraform_data.example["my_index_changed"] will be created
  + resource "terraform_data" "example" {
    + id = (known after apply)
    + input = "my_index_changed"
    + output = (known after apply)
  }

Plan: 1 to add, 0 to change, 1 to destroy.

Cause

Terraform is using the index of a for_each as a reference in the state file to keep track of the resource. By changing the index from my_index to my_index_changed, terraform determines that the resource with index my_index is in the state file, but not in the code anymore. Therefore it will want to destroy this resource.

Likewise terraform will want to create the resource with index my_index_changed, as it is not in the state file, but is present in the code.

It is the same effect as changing the resource name itself. If you would change the name of the terraform_data resource example to example_changed, terraform will also want to re-create the resource because the name changed.

Solutions:

The solution is not to use the index of a variable as input for a resource, but use value instead.

Have the following code:

locals {
  names = {
    "my_index" = {
      name = "my_name"
    }
  }
}

resource "terraform_data" "example" {
  for_each = local.names
  input = each.value.name
}

Observe that input in the terraform_data resource now has the value: each.value.name.

If we now change me_name to my_name_changed and run a plan, terraform will want to update the resource in place.

terraform_data.example["my_index"]: Refreshing state... [id=9012385b-25d6-5fc0-403e-7cbb4291fe2d]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # terraform_data.example["my_index"] will be updated in-place
  ~ resource "terraform_data" "example" {
    id = "9012385b-25d6-5fc0-403e-7cbb4291fe2d"
    ~ input = "my_name" -> "my_name_changed"
    ~ output = "my_name" -> (known after apply)
  }

Plan: 0 to add, 1 to change, 0 to destroy.

By leaving the index value in the local names the same, terraform will not want to recreate the resource.

Additional Information

• The for_each Meta-Argument