How to manage Terraform resource dependencies

Introduction

Managing dependencies in Terraform is crucial for automating and orchestrating your infrastructure efficiently. Terraform builds a dependency graph to determine the order in which resources should be created, updated, or destroyed. In this guide, we’ll explore how to manage these dependencies from basic explicit declarations to more advanced techniques, ensuring your infrastructure is provisioned in the correct order.

Understanding Implicit Dependencies

The simplest way to manage dependencies in Terraform is to let Terraform handle them implicitly. This is done by referencing attributes from other resources in your configuration. Here is a basic example:

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "example" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

In this scenario, Terraform automatically understands that the subnet depends on the VPC and will provision the VPC before the subnet.

Explicit Dependencies Using ‘depends_on’

While implicit dependencies are handy, sometimes you need more control. The depends_on argument allows you to explicitly state dependencies. An example:

resource "aws_instance" "my_instance" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
  depends_on    = [aws_vpc.main]
}

This ensures that the AWS instance is not created until the VPC is fully provisioned. Note that depends_on can accept multiple resources, creating a more complex dependency order when necessary.

Advanced Dependencies

For more advanced dependency management, you might use modules or even dynamic blocks to create dependencies. This is particularly useful in large-scale environments where dependencies are not as straightforward. Here’s an example using modules:

module "network" {
  source = "./modules/network"
  providers = {
    aws = aws
  }
}

module "instances" {
  source     = "./modules/instances"
  network_id = module.network.vpc_id
  providers = {
    aws = aws
  }
}

This structure allows you to encapsulate dependencies within modules, creating a clean and maintainable infrastructure as code. Notice how the instance module references the network module’s output, establishing a clear dependency.

Terraform Graph

To visually understand and diagnose your dependencies, Terraform provides a powerful tool called terraform graph. This command generates a visual representation of your dependency graph, illustrating how resources are interconnected. You can use this tool to ensure your dependencies are structured as intended.

Best Practices

• Use implicit dependencies whenever possible for simplicity.
• Reserve depends_on for situations where implicit dependencies cannot capture the necessary order.
• Leverage modules to encapsulate and manage dependencies in large projects.
• Regularly generate and review your Terraform graph to avoid cyclical dependencies and ensure optimal provisioning order.

Conclusion

Properly managing dependencies in Terraform ensures your infrastructure provisioning process is reliable and efficient. By utilizing implicit dependencies, explicit depends_on statements, and leveraging modules for complex scenarios, you can create a robust and maintainable infrastructure as code. Remember, regularly reviewing your dependency graph is key to maintaining a healthy infrastructure stack.