Why Depends_on for explicit dependencies in Terraform? - Purpose & Use Cases

The Big Idea

What if your cloud setup could build itself in the perfect order without you worrying about mistakes?

The Scenario

Imagine you are building a house and you try to install the windows before the walls are finished. You have to keep track of every step yourself to avoid mistakes.

The Problem

Manually managing the order of building resources is slow and confusing. You might forget a step or do things in the wrong order, causing errors and delays.

The Solution

Using depends_on in Terraform lets you clearly tell the system which resources must be ready before others start. This avoids mistakes and makes the process smooth and automatic.

Before vs After

✗ Before

resource "aws_instance" "app" {
  # no explicit order
}
resource "aws_eip" "ip" {
  # no explicit order
}

✓ After

resource "aws_instance" "app" {
  # instance details
}
resource "aws_eip" "ip" {
  depends_on = [aws_instance.app]
  # ensures IP created after instance
}

What It Enables

You can build complex infrastructure safely by controlling the exact order resources are created.

Real Life Example

When creating a database and a server, you want the database ready before the server connects. depends_on makes sure this happens automatically.

Key Takeaways

Manual ordering of resources is error-prone and slow.

depends_on explicitly sets resource creation order.

This leads to reliable and predictable infrastructure builds.

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