Depends_on for explicit dependencies in Terraform - Mini Project: Build & Apply

Depends_on for explicit dependencies

📖 Scenario: You are setting up a simple cloud infrastructure using Terraform. You want to create two resources: a virtual network and a virtual machine. The virtual machine must only be created after the virtual network is ready.

🎯 Goal: Build a Terraform configuration where the virtual machine resource explicitly depends on the virtual network resource using depends_on.

📋 What You'll Learn

Create a virtual network resource named my_vnet with address space 10.0.0.0/16.

Create a virtual machine resource named my_vm.

Use depends_on in the virtual machine resource to explicitly depend on the virtual network resource.

💡 Why This Matters

🌍 Real World

In real cloud projects, some resources must be created only after others are ready. Using depends_on ensures Terraform respects these dependencies.

💼 Career

Cloud engineers and DevOps professionals use depends_on to control resource creation order and avoid deployment errors.

1

Create the virtual network resource

Create a Terraform resource block named azurerm_virtual_network.my_vnet with the address space set to ["10.0.0.0/16"].

Terraform

# Your code here to create azurerm_virtual_network.my_vnet

Hint

Use resource "azurerm_virtual_network" "my_vnet" { ... } and set address_space to ["10.0.0.0/16"].

2

Create the virtual machine resource

Add a Terraform resource block named azurerm_virtual_machine.my_vm with a placeholder configuration (name, location, resource group). Do not add depends_on yet.

Terraform

resource "azurerm_virtual_network" "my_vnet" {
  name                = "myVnet"
  address_space       = ["10.0.0.0/16"]
  location            = "East US"
  resource_group_name = "myResourceGroup"
}

# Your code here to create azurerm_virtual_machine.my_vm without depends_on

Hint

Create resource "azurerm_virtual_machine" "my_vm" { ... } with basic properties like name, location, and resource_group_name.

3

Add depends_on to the virtual machine

Inside the azurerm_virtual_machine.my_vm resource block, add depends_on = [azurerm_virtual_network.my_vnet] to explicitly depend on the virtual network.

Terraform

resource "azurerm_virtual_network" "my_vnet" {
  name                = "myVnet"
  address_space       = ["10.0.0.0/16"]
  location            = "East US"
  resource_group_name = "myResourceGroup"
}

resource "azurerm_virtual_machine" "my_vm" {
  name                  = "myVM"
  location              = "East US"
  resource_group_name   = "myResourceGroup"
  network_interface_ids = []
  vm_size               = "Standard_DS1_v2"
  # Other required VM settings omitted for brevity
  # Add depends_on here
}

Hint

Add depends_on = [azurerm_virtual_network.my_vnet] inside the my_vm resource block.

4

Complete the Terraform configuration

Add the required provider block for AzureRM with version constraint "~> 3.0" and a resource group resource named myResourceGroup with location East US.

Terraform

resource "azurerm_virtual_network" "my_vnet" {
  name                = "myVnet"
  address_space       = ["10.0.0.0/16"]
  location            = "East US"
  resource_group_name = "myResourceGroup"
}

resource "azurerm_virtual_machine" "my_vm" {
  name                  = "myVM"
  location              = "East US"
  resource_group_name   = "myResourceGroup"
  network_interface_ids = []
  vm_size               = "Standard_DS1_v2"
  depends_on            = [azurerm_virtual_network.my_vnet]
  # Other required VM settings omitted for brevity
}

# Add provider and resource group below

Hint

Add provider "azurerm" { version = "~> 3.0" } and a resource group resource named myResourceGroup with location East US.

Practice

1. What is the main purpose of depends_on in Terraform?

easy

A. To delete resources automatically

B. To define the resource type

C. To set resource properties

D. To explicitly specify the order in which resources are created

Solution

1. Step 1: Understand resource creation order

   Terraform creates resources in parallel by default, but sometimes one resource must be created before another.

2. Step 2: Role of depends_on

   The depends_on attribute explicitly tells Terraform to wait for one resource before creating another.

3. Final Answer:

   To explicitly specify the order in which resources are created -> Option D

4. Quick Check:

   depends_on controls creation order = A [OK]

Hint: depends_on sets creation order explicitly [OK]

Common Mistakes:

• Confusing depends_on with resource type definition
• Thinking depends_on sets resource properties
• Assuming depends_on deletes resources

2. Which of the following is the correct syntax to add depends_on for a resource named aws_instance.example?

easy

A. depends_on = aws_instance.example

B. depends_on = {aws_instance.example}

C. depends_on = [aws_instance.example]

D. depends_on = ["aws_instance.example"]

Solution

1. Step 1: Understand depends_on syntax

   The depends_on attribute expects a list of resource references.

2. Step 2: Correct list format

   Resources must be inside square brackets as unquoted references, like [aws_instance.example].

3. Final Answer:

   depends_on = [aws_instance.example] -> Option C

4. Quick Check:

   depends_on requires list of resource references = B [OK]

Hint: Use list of unquoted resource references for depends_on [OK]

Common Mistakes:

• Omitting square brackets around resource reference
• Using curly braces instead of square brackets
• Not using a list format

3. Given the following Terraform snippet, what will happen during apply?

resource "aws_s3_bucket" "bucket" {
  bucket = "my-bucket"
}

resource "aws_s3_bucket_policy" "policy" {
  bucket = aws_s3_bucket.bucket.id
  policy = "..."
  depends_on = [aws_s3_bucket.bucket]
}

medium

A. The bucket will be created before the bucket policy

B. The bucket policy will be created before the bucket

C. Both resources will be created simultaneously

D. Terraform will throw a syntax error

Solution

1. Step 1: Analyze depends_on usage

   The bucket policy resource explicitly depends on the bucket resource using depends_on, so the bucket must exist first.

2. Step 2: Understand creation order

   Terraform will create the bucket first, then the bucket policy, ensuring the policy applies to an existing bucket.

3. Final Answer:

   The bucket will be created before the bucket policy -> Option A

4. Quick Check:

   depends_on enforces bucket before policy = B [OK]

Hint: depends_on enforces creation order, bucket before policy [OK]

Common Mistakes:

• Assuming simultaneous creation despite depends_on
• Thinking depends_on reverses order
• Confusing resource references with strings

4. You wrote this Terraform code but get an error:

resource "aws_instance" "web" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

resource "aws_eip" "ip" {
  instance = aws_instance.web.id
  depends_on = aws_instance.web
}

What is the problem?

medium

A. depends_on should be a list of resource references

B. instance attribute is incorrect

C. Resource names cannot be used in depends_on

D. The AMI ID is invalid

Solution

1. Step 1: Check depends_on syntax

   The depends_on attribute must be a list, even if it has one resource.

2. Step 2: Identify error in code

   Here, depends_on = aws_instance.web is missing square brackets, causing a syntax error.

3. Final Answer:

   depends_on should be a list of resource references -> Option A

4. Quick Check:

   depends_on needs list syntax = A [OK]

Hint: Always use square brackets for depends_on list [OK]

Common Mistakes:

• Using single resource without list brackets
• Confusing depends_on with resource attributes
• Ignoring Terraform syntax requirements

5. You have two resources: aws_db_instance.db and aws_ecs_service.ecs. The ECS service must start only after the database is fully created. How should you configure depends_on in aws_ecs_service.ecs to ensure this?

hard

A. depends_on = aws_db_instance.db

B. depends_on = [aws_db_instance.db]

C. depends_on = ["aws_db_instance.db"]

D. depends_on = {"aws_db_instance.db"}

Solution

1. Step 1: Understand depends_on format

   Terraform requires depends_on to be a list of resource references.

2. Step 2: Correct usage for ECS service

   To make ECS depend on the DB, use depends_on = [aws_db_instance.db] inside the ECS resource block.

3. Final Answer:

   depends_on = [aws_db_instance.db] -> Option B

4. Quick Check:

   depends_on list of resource references for explicit dependency = A [OK]

Hint: Use list of unquoted resource references for depends_on [OK]

Common Mistakes:

• Omitting square brackets around resource reference
• Omitting square brackets
• Using curly braces instead of brackets