Depends_on for explicit dependencies in Terraform - Commands & Configuration

Introduction

Sometimes Terraform needs to know the order to create resources. Depends_on lets you tell Terraform to wait for one resource before starting another. This avoids errors when one resource needs another to exist first.

When you have two resources that must be created in a specific order but Terraform does not detect the dependency automatically.

When a resource uses data or output from another resource that is not directly referenced in its configuration.

When you want to ensure a resource is fully created before starting another that depends on it.

When you face errors due to race conditions during resource creation.

When you want to control the deployment order explicitly for clarity or troubleshooting.

Config File - main.tf

main.tf

resource "aws_s3_bucket" "my_bucket" {
  bucket = "example-bucket-terraform-12345"
  acl    = "private"
}

resource "aws_s3_bucket_object" "my_object" {
  bucket = aws_s3_bucket.my_bucket.bucket
  key    = "example.txt"
  content = "Hello, Terraform!"

  depends_on = [aws_s3_bucket.my_bucket]
}

This file creates an S3 bucket first.

The second resource uploads an object to that bucket.

The depends_on ensures the bucket is created before the object upload starts.

Commands

This command initializes Terraform in the current directory. It downloads necessary provider plugins and prepares 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 applies the configuration to create the resources. The -auto-approve flag skips manual approval to speed up the process.

Terminal

terraform apply -auto-approve

Expected Output

aws_s3_bucket.my_bucket: Creating...

aws_s3_bucket.my_bucket: Creation complete after 2s [id=example-bucket-terraform-12345]

aws_s3_bucket_object.my_object: Creating...

aws_s3_bucket_object.my_object: Creation complete after 1s [id=example.txt]



Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

→

-auto-approve - Automatically approve the apply without prompting

This command lists all resources Terraform is managing in the current state. It helps verify that both resources were created.

Terminal

terraform state list

Expected Output

aws_s3_bucket.my_bucket

aws_s3_bucket_object.my_object

Key Concept

If you remember nothing else from this pattern, remember: depends_on tells Terraform the exact order to create resources when it cannot figure it out by itself.

Common Mistakes

Not using depends_on when a resource depends on another but the dependency is not explicit in the config.

Terraform may try to create resources in parallel, causing errors if one resource needs the other to exist first.

Add depends_on with the resource that must be created first to ensure proper order.

Using depends_on unnecessarily when Terraform already detects the dependency.

This can make the configuration more complex and slower by forcing unnecessary waits.

Only use depends_on when Terraform does not detect the dependency automatically.

Summary

Use terraform init to prepare your working directory.

Use depends_on in resource blocks to explicitly set creation order.

Use terraform apply to create resources and terraform state list to verify them.

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