Depends_on for explicit dependencies in Terraform - Step-by-Step Execution

Process Flow - Depends_on for explicit dependencies

Start Terraform Plan

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Identify Resources

↓

Check depends_on

↓

If depends_on present

↓

Wait for dependencies

↓

Create dependent resource

↓

If no depends_on

↓

Create resource normally

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Continue to next resource

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End Plan

Terraform checks if a resource has depends_on to wait for other resources before creating it, ensuring correct order.

Execution Sample

Terraform

resource "aws_instance" "web" {
  depends_on = [aws_security_group.sg]
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

This resource waits for the security group to be created before launching the instance.

Process Table

Step	Resource	depends_on	Dependency Status	Action	Result
1	aws_security_group.sg	none	No dependencies	Create security group	Security group created
2	aws_instance.web	aws_security_group.sg	Dependency created	Create instance	Instance created
3	End	N/A	All resources created	Finish plan	Terraform apply complete

💡 All resources created respecting depends_on dependencies

Status Tracker

Resource	Initial State	After Step 1	After Step 2	Final State
aws_security_group.sg	Not created	Created	Created	Created
aws_instance.web	Not created	Waiting	Created	Created

Key Moments - 2 Insights

Why does aws_instance.web wait before creation?

Because it has depends_on aws_security_group.sg, so Terraform waits until the security group is created (see execution_table step 2).

What happens if depends_on is missing but resource uses another resource?

Terraform may create resources in parallel, causing errors if the dependency is required. depends_on forces order (see execution_table step 1 vs 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the dependency status of aws_instance.web at step 2?

A Dependency created

B No dependencies

C Waiting for dependencies

D Dependency failed

💡 Hint

Check the 'Dependency Status' column at step 2 in execution_table

At which step does Terraform finish applying all resources?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Step' column and 'Result' column for completion in execution_table

If depends_on was removed from aws_instance.web, what would likely happen?

A Instance creation waits for security group anyway

B Instance and security group may be created in parallel

C Terraform throws an error immediately

D Security group creation waits for instance

💡 Hint

Depends_on controls creation order; without it, resources may create in parallel (see key_moments)

Concept Snapshot

depends_on in Terraform forces one resource to wait for another before creation.
Syntax: depends_on = [resource]
Use it to fix creation order when implicit dependencies are not detected.
Without depends_on, Terraform may create resources in parallel.
Always use depends_on for explicit dependency control.

Full Transcript

Terraform uses depends_on to explicitly control resource creation order. When a resource has depends_on, Terraform waits for the listed resources to be created first. This avoids errors from resources being created too early. In the example, aws_instance.web waits for aws_security_group.sg before creation. The execution table shows the security group created first, then the instance. Without depends_on, Terraform might create both at the same time, causing failures. depends_on is a simple way to tell Terraform about dependencies it might not detect automatically.

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