Explaining the Difference between Input and Local Variables in Terraform

The aim of this page📝 is to explain how to use variables in Terraform based on the particular example of assigning values to input variables from outside of the configuration files.

• Terraform has three types of variables: input variables, local values (locals), and outputs
• Let’s focus on the difference between the first two, inputs versus locals
• Input variables are parameters of a Terraform module.
• You can set them directly in the configuration file or pass them as command-line arguments when running Terraform.
• Local values are named expressions that you can use to avoid repeating the same values or expressions multiple times in a configuration.
• Unlike input variables, you cannot directly assign values to locals from outside the configuration.
• NOTE: this does not mean that locals have to be set in the very same file, the files are collected when a plan is executed, i.e. they can be located in e.g. locals.tf file
• Instead, locals are defined within the configuration, and their values are computed based on other values or expressions within the same configuration.
• Hashicorp docs say

Unlike input variables, locals are not set directly by users of your configuration.

— https://developer.hashicorp.com/terraform/tutorials/configuration-language/locals

• “Users of your configuration” refers to anyone who is using your Terraform module or configuration.
• I am for example a member of the Infra support team and I am setting input variables by manipulating values in the Consul KV Store
• I and other users can provide input variables when they use modules or configuration, but we cannot directly assign values to local variables unless we manipulate config files directly
• We can therefore assign values to input variables in several ways:
• Command Line Flags
• From a File
• Environment Variables
• Variable Defaults
• Module Blocks
• As said, we can also use Consul and other databases to store and retrieve variable values in Terraform.

CODE

Here is a particular example of a terraform file

variable "image_id" {
  description = "The id of the machine image (AMI) to use for the server."
  type        = string
}

locals {
  instance_type = "t2.micro"
}

resource "aws_instance" "example" {
  ami           = var.image_id
  instance_type = local.instance_type

  tags = {
    Name = "example-instance"
  }
}

In this example, image_id is an input variable and instance_type is a local value. The aws_instance resource uses these variables.

LINKS

• https://learn.hashicorp.com/tutorials/terraform/variables
• https://www.consul.io/
• https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/dynamodb_table