Lab: depends_on and Implicit Dependencies

Objective

By the end of this lab, you will have:

• Created a configuration with implicit dependencies through references.
• Created a configuration with explicit dependencies through depends_on.
• Visualized the dependency graph with terraform graph.
• Recognised when depends_on is appropriate.

Requirements

• A Linux or macOS workstation with shell access.
• The Terraform CLI 1.9.x or later installed.
• Graphviz (optional, for visualising the graph).

Scenario

You have a configuration with two resources where one depends on the other. The dependency can be implicit (through a reference) or explicit (through depends_on). The choice has implications for clarity and correctness.

Tasks

Task 1: Create the working directory

mkdir -p ~/rb-deps-lab
cd ~/rb-deps-lab

Task 2: Create the implicit-dependency configuration

Create main.tf:

terraform {
  required_version = ">= 1.9.0"
  required_providers {
    local = {
      source  = "hashicorp/local"
      version = "~> 2.5"
    }
  }
}

resource "local_file" "readme" {
  filename = "${path.module}/README.md"
  content  = "Initial content.\n"
}

resource "local_file" "followup" {
  filename = "${path.module}/followup.md"
  content  = "local_file.readme.content = ${local_file.readme.content}"
}

The local_file.followup references local_file.readme.content. The dependency is implicit.

Task 3: Apply the configuration

terraform init
terraform apply

Verify the files:

cat README.md
cat followup.md

The followup.md file contains the content of README.md.

Task 4: Visualize the dependency graph

terraform graph > graph.dot

(If Graphviz is installed:)

dot -Tpng graph.dot > graph.png

The graph shows local_file.readme -> local_file.followup.

Task 5: Create the explicit-dependency configuration

Edit main.tf to use depends_on:

resource "local_file" "followup" {
  filename = "${path.module}/followup.md"
  content  = "Followup content.\n"

  depends_on = [local_file.readme]
}

The content does not reference local_file.readme.content. The dependency is explicit.

Task 6: Apply the configuration

terraform apply

The plan shows both resources being updated. The followup resource is updated after the readme resource.

Task 7: Visualize the dependency graph

terraform graph > graph.dot

The graph shows the same dependency: local_file.readme -> local_file.followup.

Task 8: Recognise when depends_on is appropriate

The depends_on is appropriate when:

• The dependency is on a side effect (e.g. an IAM role).
• The reference is to a known-after-apply attribute.
• The implicit dependency is not strong enough.

The depends_on is not appropriate when:

• The dependency is already implicit (a reference to the resource).
• The dependency is to an unrelated resource.

Validation

The lab is successful if:

• The implicit-dependency configuration created both files.
• The explicit-dependency configuration created both files.
• The dependency graph shows the right relationship.

Expected Outcome

At the end of the lab:

+---------------------------------+
| ~/rb-deps-lab/                     |
|   .terraform/                    |
|   .terraform.lock.hcl            |
|   README.md                       |
|   followup.md                     |
|   main.tf                        |
+---------------------------------+

The followup.md file contains the content of README.md.

Cleanup

cd ~/rb-deps-lab
terraform destroy
rm -rf .terraform .terraform.lock.hcl terraform.tfstate*

What You Learned

You learned the dependency mechanism:

• Implicit dependencies are created by references. Terraform infers the dependency from the expression.
• Explicit dependencies are created by depends_on. The operator declares the dependency.
• Most dependencies should be implicit. The reference is the documentation.
• depends_on is for side effects. When the dependency is on a side effect, the reference is not enough.