Meta-parameter for enabling or disabling a resource

[mattsafraid]

Terraform Version

Terraform v1.3.9
on darwin_amd64

Use Cases

The proposal here is to create a new meta-parameter that would allow to control the state of a single resource.

This is useful for cases when a resource must be destroyed and we want to keep the code, or specially when we want to enable/disable a resource without turning it into an array of resources - which means the references to the resource are kept unchanged.

A similar behaviour can be found in other configuration languages. For example in Puppet, we can use ensure => absent and in Ansible we can use state: absent.

Attempted Solutions

The current behaviour today can be achieved with count and a condition, for example:

From https://www.hashicorp.com/blog/terraform-feature-toggles-blue-green-deployments-canary-test

variable "enable_green_application" {
  default = false
  type    = bool
}

resource "aws_instance" "application_green" {
  count                  = var.enable_green_application ? 1 : 0
  instance_type          = "t2.micro"
  [...]

However in this case we don't have a single resource, but an array of resources. So, any further reference to this object must be adapted - Although we know there's only one resource because of ? 1 : 0.

resource "aws_elb" "application_green" {
  count   = var.enable_green_application ? 1 : 0
  name    = "${var.prefix}-green-elb"
  [...]
  instances = aws_instance.application_green.*.id # references an array 
  [...]

resource "aws_route53_record" "application_green" {
  count   = var.enable_green_application ? 1 : 0
  [...]
  alias {
    name                   = aws_elb.application_green.0.dns_name # references first item 
    zone_id                = aws_elb.application_green.0.zone_id
    evaluate_target_health = true
  }
  [...]

Proposal

With this proposal, not only the code would be cleaner because we don't need a condition block neither array references, but we know there is either zero or one resource.

Applying the concept to the previous snippets, and assuming here a meta-parameter called enabled which accepts a boolean value, we'd have as follows:

variable "enable_green_application" {
  default = false
  type    = bool
}

resource "aws_instance" "application_green" {
  enabled                = var.enable_green_application # new meta parameter in action 
  instance_type          = "t2.micro"
  [...]

resource "aws_elb" "application_green" {
  enabled = var.enable_green_application
  name    = "${var.prefix}-green-elb"
  [...]
  instances = aws_instance.application_green.id # no more array references
  [...]

resource "aws_route53_record" "application_green" {
  enabled = var.enable_green_application
  [...]
  alias {
    name                   = aws_elb.application_green.dns_name # no more array references
    zone_id                = aws_elb.application_green.zone_id
    evaluate_target_health = true
  }
  [...]

References

No response

[crw]

Thanks for this request!

[apparentlymart]

Indeed thanks for sharing this proposal, @mattsafraid!

I think this proposal is essentially the same as #21953. Would you agree? If so, I'd like to close this just to consolidate the discussion in one place.

[mattsafraid]

Hi @apparentlymart, thanks for looking into it!

The proposal is indeed similar to #21953, however the proposal still references an array as in

network_acl_id = aws_network_acl.public[0].id

But from the comments I see that the discussion has advanced to the same idea. This issue can be closed. Thanks!

[github-actions]

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