Decision toggles in Terraform

00_basic_decision.tf

variable "decision_toggle" { default="false" }
variable "val_a" { default="3" }
variable "val_b" { default="3" }

output "toggle_result_0" {
    value = "${ var.decision_toggle ? var.val_a : 0 }"
}

output "toggle_result_1" {
    value = "${ var.decision_toggle ? var.val_a : var.val_a+var.val_b }"
}

01_resource_routing.tf

variable "dedicated_etcd" { default="false" }
variable "etcd_hosts" { default="3" }
variable "app_hosts" { default="3" }

resource "aws_instance" "etcd" {
  count = "${ var.decision_toggle ? etcd_hosts : 0 }"
  ami = "ami-b6bb47d4"
  instance_type = "t2.micro"

  tags {
    is_etcd = "true"
    is_app = "false"
  }
}

resource "aws_instance" "app" {
  count = "${ var.decision_toggle ? var.app_hosts : var.app_hosts+var.etcd_hosts }"
  ami = "ami-b6bb47d4"
  instance_type = "t2.micro"

  tags {
    is_etcd = "${ var.decision_toggle ? "false" : "true" }"
    is_app = "true"
  }
}

02_component_allocation.tf

variable "dmz_subnet" { default="false" }
variable "web_hosts" { default="3" }
variable "api_hosts" { default="3" }

data "aws_subnet" "public_network_zone" {
  filter {
    name = "tag:NetworkZone"
    values = ["public"]
  }
}

data "aws_subnet" "dmz_network_zone" {
  filter {
    name = "tag:NetworkZone"
    values = ["dmz"]
  }
}

resource "aws_instance" "web" {
  count = "${ var.web_hosts }"
  ami = "ami-b6bb47d4"
  instance_type = "t2.micro"
  subnet_id = "${element(data.aws_subnet.public_network_zone.*.id, count.index)}"

  tags {
    in_dmz = "false"
  }
}

resource "aws_instance" "dmz" {
  count = "${ var.dmz_subnet }"
  ami = "ami-b6bb47d4"
  instance_type = "t2.micro"
  subnet_id = "${var.dmz_toggle ? element(data.aws_subnet.public_network_zone.*.id, count.index) : element(data.aws_subnet.public_network_zone.*.id, count.index)}"
  tags {
    in_dmz = "${ var.dmz_subbnet ? "true" : "false" }"
  }
}