Using Local Variables for Custom Functions in Terraform

In this tutorial, we will cover how to use local variables to create custom functions in Terraform. Local variables help you define and reuse expressions or calculations within your Terraform configuration.

Defining Local Variables

Local variables are defined using the locals block. You can use local variables to store intermediate values, build complex expressions, and make your configurations more readable and maintainable.

Create a file named locals.tf and define some local variables:

locals {
  instance_name_prefix = "web-server"
  environment          = "production"
  instance_count       = 3
  instance_name_list   = [for i in range(local.instance_count) : "${local.instance_name_prefix}-${i + 1}"]
}

In this configuration:

• instance_name_prefix: A prefix for the instance names.
• environment: The environment in which the instances are deployed.
• instance_count: The number of instances to create.
• instance_name_list: A list of instance names generated using a for-loop expression.

Using Local Variables in Resources

You can reference local variables in your resources by using the local keyword. Update your main.tf file to use these local variables:

provider "aws" {
  region = "us-west-2"
}

resource "aws_instance" "example" {
  count         = local.instance_count
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name        = element(local.instance_name_list, count.index)
    Environment = local.environment
  }
}

Custom Functions with Local Variables

Local variables can also be used to create custom functions. For example, you can define a function to format resource names consistently across your configuration. Update your locals.tf file:

locals {
  format_resource_name = lambda(name, index : "${name}-${index + 1}")

  instance_name_prefix = "web-server"
  environment          = "production"
  instance_count       = 3
  instance_name_list   = [for i in range(local.instance_count) : local.format_resource_name(local.instance_name_prefix, i)]
}

In this configuration:

• format_resource_name: A custom function defined using the lambda syntax. It takes two arguments, name and index, and returns a formatted string.
• instance_name_list: A list of instance names generated using the custom function.

Applying the Configuration

To apply your configuration, follow these steps:

Step 1: Initialize Terraform

Initialize your Terraform configuration:

terraform init

Step 2: Validate the Configuration

Validate your configuration to ensure there are no syntax errors:

terraform validate

Step 3: Apply the Configuration

Apply your configuration to create the resources:

terraform apply

Review the plan and confirm the apply action.

Conclusion

By using local variables, you can create custom functions and intermediate values to make your Terraform configurations more flexible, readable, and maintainable. This tutorial covered defining local variables, using them in resources, and creating custom functions with local variables.