Resource Dependencies in Terraform – A Smart Way to Manage

Resource dependencies in Terraform are crucial when building complex infrastructure where certain resources must be created before others. Understanding how dependencies work ensures that Terraform provisions and destroys infrastructure in the correct order—avoiding unexpected errors and ensuring smooth automation.

In this blog, you’ll learn how Terraform handles dependencies between resources both automatically and manually, and how to implement them in real-world scenarios.

What Are Resource Dependencies in Terraform?

Terraform operates by reading your configuration files and figuring out how to create resources. But sometimes, resources are linked—one might rely on the output of another.

In such cases, Terraform uses resource dependencies to determine the right sequence. There are two types:

• Implicit Dependencies
• Explicit Dependencies

Let’s explore both in detail.

Implicit Dependencies: Let Terraform Decide

Implicit dependencies occur naturally when you link one resource to another using reference expressions. For example, if a file resource’s content depends on the output of a random_pet resource, Terraform automatically understands the order:

1. Create random_pet first.
2. Then, create the file with the pet name as content.

You don’t need to declare any special instruction. Terraform evaluates the relationship and handles the provisioning accordingly.

The same logic applies when destroying resources—but in reverse. Terraform deletes the file first and then removes the pet name generator.

This hands-off method is clean and reduces manual overhead, making your code more readable.

Explicit Dependencies: When You Need More Control

Sometimes, a resource might depend on another, but there’s no direct link or reference between them. In such situations, Terraform cannot infer the order by itself.

To handle this, use the depends_on argument inside a resource block.

Example Scenario:

Suppose you have a configuration that doesn’t use any reference from one resource to another, but you still want one to wait for the other to be created first. You can manually specify the dependency like this:

resource "local_file" "example" {
  filename = "/root/example.txt"
  content  = "This file depends on a pet name"

  depends_on = [random_pet.my_pet]
}

Here, the local file has no direct reference to random_pet.my_pet, but we want Terraform to wait for it to be created first. By using depends_on, you explicitly instruct Terraform about the sequence.

When to Use Explicit Dependencies

While implicit dependencies work well in most scenarios, explicit dependencies are helpful when:

• A resource relies on the side effect of another resource.
• There’s no direct attribute reference, but a logical dependency exists.
• You want to ensure a specific order of execution to avoid race conditions or errors.

It’s best practice not to overuse depends_on, as it adds complexity and can make your configuration harder to maintain.

Use it only when Terraform cannot infer the dependency on its own.

Common Pitfalls with Dependencies

Although dependencies streamline the infrastructure lifecycle, misuse can cause:

• Unnecessary delays in provisioning due to forced sequencing.
• Unexpected errors when destroying resources if not properly linked.
• Difficulty in debugging if the dependency chain is too complex or undocumented.

To avoid these, always:

• Use implicit dependencies where possible.
• Comment on explicit dependencies to document the rationale.
• Test changes with terraform plan before applying them.

Conclusion

Mastering resource dependencies in Terraform is key to managing infrastructure that’s both robust and scalable. Implicit dependencies help keep code clean and efficient, while explicit dependencies offer control when Terraform can’t determine the order automatically.

By understanding when and how to use each type, you can avoid errors, prevent misordered deployments, and ensure your Terraform configurations are production-ready.

Frequently Asked Questions (FAQs)