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In this lesson, we’ll explore how OpenTofu records resource attributes and manages dependencies during provisioning. Understanding these concepts is key to writing reliable Infrastructure as Code.

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Exported Attributes

When you define a resource in OpenTofu, several attributes are exported after creation. For example, create an AWS key pair: 
resource "aws_key_pair" "alpha" {
  key_name   = "alpha"
  public_key = "ssh-rsa AAAAB3NzaC1yc2EAAAAADAQABAAABAQD3.....alpha@a-server"
}
Here, public_key is a required argument. After running tofu apply, inspect the exported values using: 
tofu show
Example output: 
# aws_key_pair.alpha:
resource "aws_key_pair" "alpha" {
  arn         = "arn:aws:ec2:us-east-1:key-pair/alpha"
  fingerprint = "d7:ff:a6:63:18:64:9c:57:a1:ee:ca:a4:ad:c2:81:62"
  id          = "alpha"
  key_name    = "alpha"
  public_key  = "ssh-rsa AAAAB3NzaC1yc2EAAAAADAQABAAABAQD3F6ty... alpha@a-server"
  tags_all    = {}
}
AttributeDescription
arnAmazon Resource Name for the key pair
fingerprintSHA1 fingerprint of the public key
idUnique identifier (same as key_name)
key_nameName assigned to the key pair
public_keySSH public key supplied by the user
tags_allCombined map of resource and provider-level tags
You can target a specific resource by running tofu show aws_key_pair.alpha.
Learn more in the [OpenTofu CLI Docs].

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Referencing Exported Attributes

Exported attributes become inputs for other resources. For instance, associate the key pair with an EC2 instance: 
resource "aws_instance" "cerberus" {
  ami           = var.ami
  instance_type = var.instance_type
  key_name      = aws_key_pair.alpha.key_name
}
The reference aws_key_pair.alpha.key_name follows the format: 
resource_type.resource_name.attribute
This creates an implicit dependency, ensuring aws_key_pair.alpha is created before aws_instance.cerberus.

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Implicit Dependencies in Action

When you run tofu apply, OpenTofu will build the dependency graph and provision resources in the correct order: 
$ tofu apply
aws_key_pair.alpha: Creating...
aws_key_pair.alpha: Creation complete after 1s [id=alpha]
aws_instance.cerberus: Creating...
aws_instance.cerberus: Creation complete after 10s [id=i-c791dc46a6639d4a7]
Apply complete! Resources: 2 added, 0 changed, 0 destroyed
During tofu destroy, the reverse order is applied: the EC2 instance is terminated before the key pair.
Implicit dependencies eliminate race conditions and ensure resources are created or destroyed in the correct sequence.
Read more about the [AWS Provider] for detailed attribute information.

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Explicit Dependencies with depends_on

If resources lack direct attribute references but still require ordering, use the depends_on meta-argument: 
resource "aws_instance" "db" {
  ami           = var.db_ami
  instance_type = var.db_instance_type
}

resource "aws_instance" "web" {
  ami           = var.web_ami
  instance_type = var.web_instance_type

  depends_on = [
    aws_instance.db
  ]
}
Here, aws_instance.web will wait for aws_instance.db to finish creation first.
Overusing depends_on can complicate your configuration. Prefer implicit references whenever possible.
For advanced dependency control, see the [Infrastructure as Code] best practices.
That’s it for this lesson on resource attributes and dependencies in OpenTofu. In the next module, we’ll cover outputs and remote backends to share state across your team.

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