Outputs for module communication in Terraform - Step-by-Step Execution

Process Flow - Outputs for module communication

Define output in child module

↓

Apply child module

↓

Output value stored

↓

Parent module calls child module

↓

Parent accesses output via module.<name>.<output>

↓

Use output in parent resources or variables

Outputs in Terraform modules let child modules share values with parent modules, enabling communication and reuse.

Execution Sample

Terraform

module "child" {
  source = "./child"
}

output "child_value" {
  value = module.child.example_output
}

Parent module calls child module and accesses its output named example_output.

Process Table

Step	Action	Evaluation	Result
1	Child module defines output 'example_output'	Set output value to 'Hello from child'	Output stored in child module state
2	Parent module calls child module	Child module initialized and applied	Child module resources created
3	Parent accesses module.child.example_output	Retrieve output value	'Hello from child' returned
4	Parent uses output in its own output 'child_value'	Assign value	'child_value' output set to 'Hello from child'
5	Terraform apply completes	Outputs available	Parent output 'child_value' shows 'Hello from child'

💡 All outputs resolved and available for parent module use after apply

Status Tracker

Variable	Start	After Step 1	After Step 3	Final
example_output	undefined	'Hello from child'	'Hello from child'	'Hello from child'
child_value	undefined	undefined	undefined	'Hello from child'

Key Moments - 2 Insights

Why can't the parent module access child module variables directly?

The parent module accesses values only through outputs defined in the child module, as shown in step 3 of the execution_table.

What happens if the child module does not define an output?

The parent module cannot retrieve any value from the child module, so referencing module.child.<output> will fail, as outputs must be explicitly declared (step 1).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'example_output' after step 1?

A undefined

B 'Hello from child'

C null

D Error

💡 Hint

Check the 'Result' column in row for step 1 in execution_table

At which step does the parent module retrieve the child's output value?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look for 'Parent accesses module.child.example_output' in execution_table

If the child module output value changed, which step would reflect the new value?

A Step 4

B Step 3

C Step 1

D Step 5

💡 Hint

Output value is set in child module definition at step 1

Concept Snapshot

Terraform module outputs:
- Defined in child module with 'output' block
- Provide values to parent module
- Accessed via module.<name>.<output>
- Enable communication between modules
- Must be declared explicitly to be accessible

Full Transcript

In Terraform, outputs allow a child module to share values with its parent module. The child module defines outputs using the output block. When the parent module calls the child module, it can access these outputs using the syntax module.<child_name>.<output_name>. This process happens during terraform apply, where the child module runs, sets its outputs, and the parent module retrieves them. Outputs enable modules to communicate and reuse values safely and clearly.