Additional HCL Language Features

In this lesson, we will explore some advanced features of the HashiCorp Configuration Language (HCL) used in Terraform. These features can help make your Terraform code more expressive and modular.

Expressions, Operators, and Functions

Terraform provides various expressions, operators, and functions to build dynamic strings, perform arithmetic operations, and use built-in functions. You can refer to the Terraform documentation for a complete list and usage examples.

Some examples include:

• Template strings: Similar to JavaScript, you can use template strings with curly braces to reference variables within a string.
• Operators: Arithmetic and logical operators like multiplication, division, and checking equality are available.
• Conditionals: Ternary syntax can be used for conditional expressions.
• For loops: for loops can be used to loop over a list of configurations.
• Splat expressions: This expands values in a list.
• Functions: Math functions, date and time functions, and hash/crypto functions can be used in your code.

Meta Arguments

Terraform provides various meta arguments to control the behavior of resources, such as depends_on, count, for_each, and lifecycle.

You can find more information in the Terraform documentation.

depends_on

This meta argument is used when one resource implicitly depends on another, but there's no direct connection within the config. For example, if an instance needs access to an S3 bucket, you need to provision the role policy first.

resource "aws_instance" "example" {

  # ...

  depends_on = [aws_iam_role_policy.example]
}

count

Use count to create multiple copies of a resource. This is useful when you have nearly identical resources.

resource "aws_instance" "example" {

  # ...

  count = 4
  tags = {
    Name = "server-${count.index + 1}"
  }
}

### for_each

`for_each` is similar to count, but it provides more control over each resource. It allows you to loop through an iterable and create resources based on its values.

```json
locals {
  subnets = ["subnet-abc123", "subnet-def456"]
}

resource "aws_instance" "example" {

  # ...

  for_each = toset(local.subnets)
  subnet_id = each.value
}

lifecycle

The lifecycle meta argument is used to specify the order in which Terraform takes actions, like creating resources before destroying them, or ignoring changes.

resource "aws_instance" "example" {

  # ...

  lifecycle {
    create_before_destroy = true
    ignore_changes = [tags]
    prevent_destroy = true
  }
}

Provisioners

Provisioners allow you to perform actions locally or on remote machines. You can use file, local-exec, or remote-exec provisioners, or use vendor-specific provisioners like the Ansible provisioner.

For more information, refer to the Terraform documentation on provisioners.

Throughout the course, we will continue to use some of these advanced language features as we develop our web application infrastructure. To get the most up-to-date and detailed reference, make sure to consult the Terraform documentation.