Resource dependencies (implicit) in Terraform - Commands & Configuration

Introduction

When you create cloud resources with Terraform, some resources need others to be ready first. Terraform figures out these connections automatically without you telling it. This helps avoid errors and makes your setup smoother.

When you create a virtual machine that needs a network to be ready first.

When you set up a database that depends on a storage bucket being created.

When you deploy an application that requires a load balancer to exist before it starts.

When you want Terraform to handle the order of resource creation without extra instructions.

When you want to avoid errors caused by resources trying to use others that are not ready yet.

Config File - main.tf

main.tf

provider "aws" {
  region = "us-east-1"
}

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

resource "aws_subnet" "example_subnet" {
  vpc_id            = aws_vpc.example_vpc.id
  cidr_block        = "10.0.1.0/24"
  availability_zone = "us-east-1a"
}

resource "aws_instance" "example_instance" {
  ami           = "ami-0c94855ba95c71c99"
  instance_type = "t2.micro"
  subnet_id     = aws_subnet.example_subnet.id
}

This file creates three AWS resources:

• aws_vpc.example_vpc: A virtual private cloud network.
• aws_subnet.example_subnet: A subnet inside the VPC, which uses the VPC's ID to link itself.
• aws_instance.example_instance: A virtual machine that uses the subnet's ID.

Terraform automatically understands the order: VPC first, then subnet, then instance, because each resource uses the ID of the previous one.

Commands

This command sets up Terraform in the current folder by downloading necessary plugins and preparing the environment.

Terminal

terraform init

Expected Output

Initializing the backend...



Initializing provider plugins...

- Finding latest version of hashicorp/aws...

- Installing hashicorp/aws v4.0.0...

- Installed hashicorp/aws v4.0.0 (signed by HashiCorp)



Terraform has been successfully initialized!



You may now begin working with Terraform. Try running "terraform plan" to see any changes that are required for your infrastructure.

This command shows what Terraform will create or change without actually doing it. It helps you check your setup before applying.

Terminal

terraform plan

Expected Output

An execution plan has been generated and is shown below.

Resource actions are indicated with the following symbols:

  + create



Terraform will perform the following actions:



  # aws_vpc.example_vpc will be created

  + resource "aws_vpc" "example_vpc" {

      + cidr_block = "10.0.0.0/16"

      + id         = (known after apply)

    }



  # aws_subnet.example_subnet will be created

  + resource "aws_subnet" "example_subnet" {

      + cidr_block        = "10.0.1.0/24"

      + id                = (known after apply)

      + vpc_id            = (known after apply)

      + availability_zone = "us-east-1a"

    }



  # aws_instance.example_instance will be created

  + resource "aws_instance" "example_instance" {

      + ami           = "ami-0c94855ba95c71c99"

      + instance_type = "t2.micro"

      + subnet_id     = (known after apply)

      + id            = (known after apply)

    }



Plan: 3 to add, 0 to change, 0 to destroy.

This command creates the resources defined in the configuration. The flag skips asking for confirmation to speed up the process.

Terminal

terraform apply -auto-approve

Expected Output

aws_vpc.example_vpc: Creating...

aws_vpc.example_vpc: Creation complete after 3s [id=vpc-0abcd1234efgh5678]

aws_subnet.example_subnet: Creating...

aws_subnet.example_subnet: Creation complete after 2s [id=subnet-0abcd1234efgh5678]

aws_instance.example_instance: Creating...

aws_instance.example_instance: Still creating... [10s elapsed]

aws_instance.example_instance: Creation complete after 15s [id=i-0abcd1234efgh5678]



Apply complete! Resources: 3 added, 0 changed, 0 destroyed.

→

-auto-approve - Skip confirmation prompt to apply changes immediately

This command shows a graph of resource dependencies Terraform detected. It helps visualize the order Terraform will create resources.

Terminal

terraform graph

Expected Output

"digraph {

  compound = "true"

  newrank = "true"

  node [label="aws_vpc.example_vpc", shape=box]

  "aws_vpc.example_vpc";

  node [label="aws_subnet.example_subnet", shape=box]

  "aws_subnet.example_subnet";

  node [label="aws_instance.example_instance", shape=box]

  "aws_instance.example_instance";

  "aws_subnet.example_subnet" -> "aws_vpc.example_vpc";

  "aws_instance.example_instance" -> "aws_subnet.example_subnet";

}

"

Key Concept

Terraform automatically figures out which resources depend on others by looking at how you reference them, so it creates them in the right order without extra instructions.

Common Mistakes

Not referencing dependent resource IDs in later resources.

Terraform cannot detect the order to create resources, which may cause errors or resources trying to use others that don't exist yet.

Always use resource attributes (like IDs) from one resource inside another to let Terraform know about the dependency.

Trying to force dependencies manually when Terraform already detects them.

This can make the configuration more complex and harder to maintain without adding value.

Trust Terraform's implicit dependency detection unless you have a special case requiring explicit dependencies.

Summary

Terraform detects resource dependencies automatically by how resources reference each other.

Use resource attributes like IDs from one resource inside another to create implicit dependencies.

Commands: 'terraform init' prepares Terraform, 'terraform plan' shows what will happen, 'terraform apply' creates resources, and 'terraform graph' visualizes dependencies.