Demo Resource Dependencies

This lesson demonstrates how to manage resource dependencies in OpenTofu, covering both explicit and implicit approaches using depends_on and attribute references.

Table of Contents

1. Understanding Resource Dependencies
2. Generating a TLS Private Key
3. Writing the Key to a Local File
4. Cleanup
5. Explicit Dependency with depends_on
6. Links and References

Understanding Resource Dependencies

OpenTofu resources can depend on each other in two ways:

First, we set an explicit dependency using the depends_on argument when Resource A does not reference Resource B’s attributes:

Next, an implicit dependency is created by referencing one resource’s attributes inside another:

Generating a TLS Private Key

Navigate to your project’s key-generator directory and create key.tf:

resource "tls_private_key" "pvtkey" {
  algorithm = "RSA"
  rsa_bits  = 4096
}

Initialize, plan, and apply the configuration:

opentofu init
opentofu plan
opentofu apply
# Type "yes" to confirm

Inspect the generated key:

opentofu show tls_private_key.pvtkey

Writing the Key to a Local File

Update key.tf to include a local_file resource:

resource "tls_private_key" "pvtkey" {
  algorithm = "RSA"
  rsa_bits  = 4096
}

resource "local_file" "key_details" {
  filename = "/root/key.txt"
  content  = tls_private_key.pvtkey.private_key_pem
}

Re-run OpenTofu:

opentofu init
opentofu plan
opentofu apply
# Confirm with "yes"

Verify that /root/key.txt contains your PEM-encoded private key.

Cleanup

When you’re done, destroy both resources:

opentofu destroy
# Confirm with "yes"

Explicit Dependency with depends_on

Create a new directory (e.g., /root/explicit-dependency) and add main.tf:

resource "local_file" "krill" {
  filename = "/root/krill.txt"
  content  = "krill"
}

resource "local_file" "whale" {
  filename   = "/root/whale.txt"
  content    = "whale"
  depends_on = [local_file.krill]
}

Here, whale will only be created after krill, illustrating an explicit dependency without attribute references.

Apply this configuration:

opentofu init
opentofu plan
opentofu apply
# Enter "yes" to proceed

Links and References

• OpenTofu CLI Documentation
• Terraform TLS Provider
• OpenTofu GitHub Repository