Creating a child module in Terraform - Visual Walkthrough

Process Flow - Creating a child module

Create child module folder

↓

Write child module files (main.tf, variables.tf)

↓

Reference child module in root module

↓

Run terraform init

↓

Run terraform apply

↓

Child module resources created

This flow shows how to create a child module folder with configuration, reference it in the root module, initialize Terraform, and apply to create resources.

Execution Sample

Terraform

module "child_example" {
  source = "./modules/child"
  name   = "example"
}

# Child module main.tf
resource "null_resource" "example" {
  triggers = { name = var.name }
}

# Child module variables.tf
variable "name" {
  type = string
}

This code defines a child module in a subfolder and calls it from the root module passing a variable.

Process Table

Step	Action	Evaluation	Result
1	Create child module folder and files	Files created: main.tf, variables.tf	Child module ready
2	Write resource in child module main.tf	Resource null_resource with trigger name	Resource defined
3	Reference child module in root module	module block with source and name variable	Module call ready
4	Run terraform init	Initialize modules and providers	Modules and providers initialized
5	Run terraform apply	Apply configuration	Child module resource created
6	Verify resource	Check terraform state	Resource present in state
7	Exit	No more steps	Execution complete

💡 All steps completed, child module created and applied successfully

Status Tracker

Variable	Start	After Step 3	After Step 5	Final
var.name	undefined	"example"	"example"	"example"
module.child_example	undefined	module call defined	resource created	resource created

Key Moments - 3 Insights

Why do we need to run 'terraform init' after adding a child module?

Because 'terraform init' downloads and initializes modules and providers, making the child module available for use as shown in step 4 of the execution_table.

How does the root module pass variables to the child module?

The root module passes variables by setting arguments inside the module block, like 'name = "example"' in step 3, which the child module accesses via 'var.name'.

What happens if the child module folder or files are missing?

Terraform will fail during 'terraform apply' because it cannot find the module source (local path validated during plan phase), stopping execution at step 5.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the child module resource actually created?

A Step 5

B Step 3

C Step 4

D Step 2

💡 Hint

Refer to step 5 where 'terraform apply' runs and the resource is created.

According to variable_tracker, what is the value of var.name after step 3?

A null

B undefined

C "example"

D "child_example"

💡 Hint

Check the 'var.name' row under 'After Step 3' column in variable_tracker.

If the child module folder is missing, which step in execution_table will fail?

A Step 2

B Step 5

C Step 3

D Step 4

💡 Hint

Look at step 5 where 'terraform apply' runs; missing local folder causes failure during module loading in the plan phase.

Concept Snapshot

Creating a child module in Terraform:
- Create a folder with module files (main.tf, variables.tf).
- Define resources inside the child module.
- Reference the child module in root with 'module' block and 'source'.
- Pass variables via module arguments.
- Run 'terraform init' to initialize modules.
- Run 'terraform apply' to create resources.

Full Transcript

To create a child module in Terraform, first make a folder for the module and add configuration files like main.tf and variables.tf. Inside, define the resources you want. Then, in your root module, add a module block that points to the child module folder using the source argument. Pass any needed variables inside this block. Next, run 'terraform init' to download and prepare the module. Finally, run 'terraform apply' to create the resources defined in the child module. This process helps organize infrastructure code into reusable parts.