Interpolation in resource names: possible?

[Robert Kulagowski]

terraform 0.6.12

I'm trying to create a set of generic scripts, where a variable in variables.tf will create the proper SG, etc.

This is the relevant part; "application" is "foobar". If I can get this to work then I might use "deploymentType" as "dev", "prod", "uat", etc. First things first.

resource "aws_security_group" "SG_USE1_${upper(${var.application})}DEV_PRIVATE_ELB_V01" {

# The next line causes a syntax error

#    name = "${upper(SG_USE1_${var.application}${var.deploymentType}_PRIVATE_ELB_V${var.version})}"

# So we hard code it, but we don't want that.

    name = "SG_USE1_FOOBARDEV_PRIVATE_ELB_V01"

    description = "SG to allow access from the ELB"

    vpc_id = "${var.aws_vpc_id}"

and

resource "aws_launch_configuration" "lan-lc" {

    name_prefix = "lan-${var.deploymentType}-"

    image_id = "${lookup(var.awsAMIs, var.awsRegion)}"

    instance_type = "${lookup(var.instanceType, var.deploymentType)}"

    iam_instance_profile = "S3-SSI-LAN-RW"

    lifecycle {

        create_before_destroy = true

    }

    security_groups = ["${aws_security_group.SG_USE1_LANDEV_REM_ACC_V01.id}", "${aws_security_group.SG_USE1_FOOBARDEV_PRIVATE_ELB_V01.id}"]

Error validating: 1 error(s) occurred:

* resource 'aws_launch_configuration.lan-lc' config: unknown resource 'aws_security_group.SG_USE1_FOOBARDEV_PRIVATE_ELB_V01' referenced in variable aws_security_group.SG_USE1_FOOBARDEV_PRIVATE_ELB_V01.id

It also fails in the launch config if I replace the second security group with the same resource name structure that I use in the security group:

 security_groups = ["${aws_security_group.SG_USE1_LANDEV_REM_ACC_V01.id}", "${aws_security_group.SG_USE1_${upper(${var.application})}DEV_PRIVATE_ELB_V01.id}"]

"parse error: syntax error"

[Paul Hinze]

Hi Robert,

Thanks for the question! Terraform does not currently allow for interpolation in resource names. Dynamic resource names would make it difficult for us to maintain the predictability guarantees baked in to the `terraform plan` / `terraform apply` contract. If both names of and references to resources supported interpolation, the shape of the dependency graph could change drastically depending on the result of an interpolation. It might be a feature we explore down the line, but it's not currently on the roadmap.

In the meantime, the best way to accomplish the sort of reuse you're looking for is to use Modules, for example:

# main.tf

module "foo-prod" {

  source           = "./application"

  application      = "foo"

  deployment_type  = "prod"

  ami              = "${lookup(var.awsAMIs, var.awsRegion)}"

}

module "foo-dev" {

  source           = "./application"

  application      = "foo"

  deployment_type  = "dev"

  ami              = "${lookup(var.awsAMIs, var.awsRegion)}"

}

# ./application/main.tf

variable "application" { }

variable "deployment_type" { }

resource "aws_security_group" "private-elb" {

  # I'd avoid name and description, since these cannot be changed without

  # replacing the SG, so I'm putting these in tags here, but it's up to you! :)

  tags {

    Name = "sg-use1-${var.application}-${var.deployment_type}-private-elb"
  }
}

resource "aws_launch_configuration" "lan-lc" {

  name_prefix = "lan-${var.deploymentType}-"

  image_id    = "${var.ami}"

  # ... etc ...

}

I hope this helps! Let me know if you have any further questions,

Paul

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