How to reference datasources using a dynamic string?

Terraform
1

Hello,

I am trying to generate a string with the same name as a datasource resource and am hoping you can suggest me a way to make the string reference the datasource.

This is what I have in code.

variable "env_subdomains" {
  description = "different region specific envs for hosted zones"
  type        = list(string)
  default = [
    "dev",
    "qa",
    "uat"
  ]
}

variable "region_subdomains" {
  description = "different region values for hosted zones"
  type        = list(string)
  default = [
    "au",  #australia
    "sg",  #singapore
    "my" #malaysia
  ]
}

variable "app_subdomains" {
  description = "different apps for subdomains within region"
  type        = list(string)
  default = [
    "kamailo",
   "nginx"
  ]
}

data "aws_route53_zone" "uat_au_region_hosted_zone" {
  name = "uat.au.${var.TLD}"
}

locals {
  all_subdomains     = distinct(setproduct(var.app_subdomains, var.env_subdomains, var.region_subdomains))
}

module "nonprod_acm_certs" {
  count       = length(local.all_subdomains)
  source      = "terraform-aws-modules/acm/aws"
  version     = "~> v2.0"
  domain_name = "*.${local.all_subdomains[count.index].0}.${local.all_subdomains[count.index].1}.${local.all_subdomains[count.index].2}.${var.TLD}"
  zone_id     = "$${data.aws_route53_zone.${local.all_subdomains[count.index].1}_${local.all_subdomains[count.index].2}_region_hosted_zone.zone_id}"
  subject_alternative_names = [
    "${local.all_subdomains[count.index].0}.${local.all_subdomains[count.index].1}.${local.all_subdomains[count.index].2}.${var.TLD}",
  ]
  certificate_transparency_logging_preference = true
  wait_for_validation                         = true
  tags = merge(
    local.common_tags,
    {
      Name = "${local.all_subdomains[count.index].0}.${local.all_subdomains[count.index].1}.${local.all_subdomains[count.index].2}.${var.TLD}"
    }
  )
}

What I trying to achieve is:

  • The zoneID mentioned below in one of it’s iteration would be the same name as the datasource - uat_au_region_hosted_zone. So, I am trying to set the interpolation in such a way that the resultant string would automatically refer the datasource.
 zone_id     = "$${data.aws_route53_zone.${local.all_subdomains[count.index].1}_${local.all_subdomains[count.index].2}_region_hosted_zone.zone_id}"

But when I run TFPlan, it shows like in the image below. Not sure, if in the apply stage, it will automatically resolve to the datasource value.

image
image1302×454 27.5 KB

Can anyone please help me with this ?

2

You can’t dynamically set the name of any sort of resource/module/data source reference.

Instead you should look at using for_each or count, and then you can do things like data.datasource_name["some_value"].attribute with some_value being calculated (using things like each.value, etc.)

3

Another possibility is to gather the separate data resource results up into a mapping yourself in a local value, and then you can look up elements of that mapping dynamically by key:

locals {
  route53_zones = {
    uat_au = data.aws_route53_zone.uat_au_region_hosted_zone
    # (and any others you want to use)
  }
}

You can then use local.route53_zones["uat_au"] to access that specific element, or you can put an arbitrary expression inside those brackets to generate a lookup key dynamically based on values from elsewhere.

When you use for_each in a resource or data block Terraform will construct a similar map for you automatically, but if you need something more custom than that there’s no reason you can’t construct the map directly yourself.

1 Like
4

Thanks for the idea @apparentlymart . This solves my problem.