Conditionals

expected time	requirements
30 minutes	A computer with Terraform installed, terraform knowledge.

Goal: Learn how to conditionally create resources.

Explanation

Sometimes you may want to create a resource on certain conditions. In many other programming languages this will be done using an if or when statement. In Terraform the syntax is a little weird, so some explanation may help.

Howto

A conditional is expressed using:

condition ? true_val : false_val

For example:

var.feature ? 1 : 0

That would return 1 when var.feature is true, otherwise 0 is returned.

To optionally create a resource, this conditional can be used on the count parameter. That’s quite unintuitive:

resource "x" "y" {
  count = var.feature ? 1 : 0
  name = "z"
}

You can use this count trick on resources and data objects.

You can test quite some things/conditions, here are some examples:

# Compare a string:
var.variable == "something" ? 1 : 0

# Negative compare:
var.variable != "something" ? 1 : 0

# You can also return some value on a condition. (That would not be in the `count` parameter.)
var.variable == "europe" ? "amsterdam": "other"

# Or compare a value:
var.variable > 10 ? "large" : "small"

# Or use a function:
regex('pattern', var.variable) ? "found" : "not found"

Assignment

Write (pseudo) code to:
- Define a variable my_var.
- Create a resource example.
- Make the example resource conditionally, when my_var is an odd number.

Here are some hints:

Check the variable documentation.

resource "example" "default" {
  name = "whatever"
}

Odd numbers can be tested with the ’%’ operator.

Questions:

Can you explain these conditionals:

count = var.vpc_id == "" ? 1 : 0

count = var.vpc_id == "" ? min(length(data.aws_availability_zones.default.names), var.amount) : 0

count = var.vpc_id != "" && var.bastion_host ? 1 : 0

Solution

variable "my_var" {
  type = number
}

resource "example" "default" {
  count = var.my_var % 2 != 0 ? 1 : 0
  # Or the reverse:
  # count = var.my_var % 2 == 1 ? 1 : 0
  name = "whatever"
}