Depends_on for explicit dependencies in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of depends_on in Terraform?

depends_on explicitly tells Terraform that one resource must be created or updated before another. It controls the order of operations to avoid errors.

beginner

How does Terraform normally determine resource creation order without depends_on?

Terraform uses implicit dependencies by analyzing references between resources, like when one resource uses an output or attribute of another.

intermediate

When should you use depends_on in your Terraform configuration?

Use depends_on when Terraform cannot detect a dependency automatically but you know one resource must be created before another, such as with external resources or side effects.

intermediate

What happens if you omit depends_on when an explicit dependency is needed?

Terraform may try to create or update resources in the wrong order, causing errors or failures during deployment.

beginner

Example: How to use depends_on to make resource B wait for resource A?

resource "aws_instance" "A" {
  # config
}

resource "aws_instance" "B" {
  # config
  depends_on = [aws_instance.A]
}

What does depends_on do in Terraform?

A Changes resource configuration

B Specifies explicit resource creation order

C Deletes resources automatically

D Creates new resource types

depends_on explicitly sets the order Terraform creates or updates resources.

How does Terraform find dependencies without depends_on?

A By user input only

B By random order

C By analyzing resource references

D It does not find dependencies

Terraform detects implicit dependencies by checking if one resource uses another's outputs or attributes.

When is depends_on necessary?

A When Terraform cannot detect a dependency automatically

B Always, for every resource

C Only for deleting resources

D Never, it is deprecated

Use depends_on when implicit detection is not enough to ensure correct order.

What is a risk of missing a needed depends_on?

A Terraform will crash immediately

B Terraform will ignore the resource

C Resources will be duplicated

D Resources may be created in wrong order causing errors

Without explicit dependency, Terraform might create resources too early or late, causing failures.

Which syntax correctly uses depends_on?

A depends_on = [aws_instance.A]

B depends_on = aws_instance.A

C depends_on: aws_instance.A

D depends_on => aws_instance.A

depends_on expects a list of resources, so square brackets are required.

Explain in your own words why and how you would use depends_on in Terraform.

Think about cases where Terraform might not know which resource to create first.

Describe what could happen if you forget to add depends_on when it is needed.

Consider the impact on your infrastructure deployment process.

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