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Meta-arguments in Terraform allow you to modify the behavior of resource blocks, enabling advanced configurations such as creating multiple instances of a resource and managing dependencies. In previous tutorials, we demonstrated how to create single resources, for example, a local file or a random pet resource. In this guide, we will explain meta-arguments in detail and illustrate how they can be used to enhance your Terraform configurations.

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Creating a Single Resource

Consider the following Terraform configuration that creates a single resource—a local file: 
resource "local_file" "pet" {
  filename = var.filename
  content  = var.content
}
The accompanying variable definitions are: 
variable "filename" {
  default = "/root/pets.txt"
}

variable "content" {
  default = "I love pets!"
}

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Traditional Loop Approach

In traditional scripting, such as with bash, you might use a loop to create multiple files. The example below creates three empty files (pet1, pet2, and pet3) in the /root directory: 
#!/bin/bash
for i in {1..3}
do
    touch /root/pet${i}
done
After running the above script, listing the directory contents may produce: 
$ ls -ltr /root/
-rw-r--r-- 1 root root 0 Sep  9 02:04 pet2
-rw-r--r-- 1 root root 0 Sep  9 02:04 pet1
-rw-r--r-- 1 root root 0 Sep  9 02:04 pet3
While Terraform does not directly support loop constructs within a resource block like traditional shell scripts, its meta-arguments provide mechanisms to achieve equivalent outcomes.

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Utilizing Meta-Arguments

Terraform’s meta-arguments can be applied to any resource block to modify its behavior. Two important meta-arguments include:
  1. depends_on: Defines explicit dependencies between resources to control the order of resource creation.
  2. lifecycle: Provides rules that control how resources are created, updated, and destroyed.

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Example: Enforcing Dependencies

In the following configuration, the depends_on meta-argument is used to ensure that the local file resource is created only after the random pet resource: 
resource "local_file" "pet" {
  filename   = var.filename
  content    = var.content
  depends_on = [
    random_pet.my_pet
  ]
}

resource "random_pet" "my_pet" {
  prefix    = var.prefix
  separator = var.separator
  length    = var.length
}

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Example: Using Lifecycle Rules

The lifecycle meta-argument can be used to manage resource replacement. For instance, you can ensure that a new resource is created before the old one is destroyed by setting create_before_destroy to true: 
resource "local_file" "pet" {
  filename        = "/root/pets.txt"
  content         = "We love pets!"
  file_permission = "0700"
  
  lifecycle {
    create_before_destroy = true
  }
}

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Next Steps

In the upcoming sections, we will explore additional meta-arguments in Terraform, including those related to looping and iterating over resources, to further enhance your infrastructure automation workflows.

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Additional Resources

By understanding and utilizing meta-arguments, you can significantly enhance your Terraform configurations, making them both robust and flexible. Happy coding!

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