Resource Dependencies

In this lesson, we explore various types of resource dependencies in Terraform and how they affect resource creation and deletion. Terraform uses both implicit and explicit dependencies to manage the order in which resources are provisioned.

Terraform automatically detects implicit dependencies through reference expressions. For example, when you pass the output of one resource (like a random pet) to another resource (such as a local file), Terraform understands that the random pet must be created before the local file. Similarly, during deletion, Terraform removes the resources in reverse order to maintain consistency.

Below is an example of an implicit dependency:

resource "local_file" "pet" {
  filename = var.filename
  content  = "My favorite pet is ${random_pet.my-pet.id}"
}

resource "random_pet" "my-pet" {
  prefix    = var.prefix
  separator = var.separator
  length    = var.length
}

In some scenarios, a resource might indirectly rely on another resource without any direct reference. In these cases, you can explicitly specify the dependency using the depends_on argument. This method ensures that Terraform provisions and destroys resources in the intended order.

The following configuration illustrates an explicit dependency where the local file resource explicitly depends on the random pet resource:

resource "local_file" "pet" {
  filename   = var.filename
  content    = "My favorite pet is Mr.Cat"
  depends_on = [
    random_pet.my-pet
  ]
}

resource "random_pet" "my-pet" {
  prefix    = var.prefix
  separator = var.separator
  length    = var.length
}

In the upcoming sections, we will delve into real-world use cases and best practices for managing resource dependencies in Terraform. Up next, we'll move into hands-on labs where you can practice working with both implicit and explicit dependencies to deepen your understanding of Terraform resource management.