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

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Check depends_on

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If depends_on present

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Wait for dependencies

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Create dependent resource

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If no depends_on

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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.