How to define optional variables in Terraform with default values defined in Consul

Question

I have a Terraform script with some variables populated from Consul. I want to use this script in two different scenarios:

Scenario 1. Everything goes right using default values from Consul.
Scenario 2. I need to override some variables.

I've checked Terraform documentation and noticed that I can't use a variable defined in Consul as a default value for some another variable. So I ended up with following workaround:

## vars.tf
## emulating undefined value using "null" string
variable "my_optional_variable" { default = "null" } 

## main.tf
my_optional_variable = "${var.my_optional_variable == "null" ? data.consul_keys.my_optional_variable : var.my_optional_variable}"

Can somebody show me a better way to do it? How to avoid a hack with a "null" string?

Thanks

score 19 · Answer 1 · answered Aug 09 '18 at 02:37

Another option is coalesce which works with empty strings so is slightly better than your "null" string.

Given your scenario or a similar case with two variables/data sources

variable "my_default_value" {
  default = "CentOS 7"
}
variable "my_optional_variable" {
  default = ""
}

You can take the "first non-empty value from the given arguments. At least two arguments must be provided."

data "openstack_images_image_v2" "bastion_image" {
  name = "${coalesce(var.my_optional_variable, var.my_default_value)}"
}

The best answer! Thanks a lot! – Krzysztof Madej Jul 14 '21 at 08:15 — Krzysztof Madej, Jul 14 '21 at 08:15

score 5 · Answer 2 · answered Sep 02 '21 at 15:31

if your optional variable needs to come externally or from a declared resource you can use count and ternaries to accomplish this.

variable "my_variable_id" {
    type = string
    default = ""
}

resource "cloud_resource" "my_resource" {
    count = var.my_variable == "" ? 1 : 0
    other_params = {...}
}

resource "dependent_cloud_resource" "my_other_resource" {
    other_resource_id = var.my_variable_id == "" ? cloud_resrouce.my_resource[0].id : var.my_variable_id
}

score 3 · Answer 3 · answered May 22 '18 at 18:50

You can use an override file. So, if you have vars.tf containing:

variable "my_optional_variable" {
  default = "null"
}

And override.tf that contains:

variable "my_optional_variable" {
  default = "not null"
}

Then ${var.my_optional_variable} should equal not null. Override files are loaded last and merged into your configuration. Rather then appended together like other .tf files in the directory.

How to define optional variables in Terraform with default values defined in Consul

3 Answers3

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