Interview › Infrastructure as Code (Terraform, Ansible)

What is the difference between a root module and a child module?

Infrastructure as Code (Terraform, Ansible) · Basic level

Answer

The root module is the Terraform directory where you run terraform plan and apply. A child module is called from another module using a module block. The root module wires environment-specific inputs, providers, and backends; child modules package reusable infrastructure logic.

Technical explanation

The root module owns backend configuration and the environment's entrypoint.

Child modules should be reusable and should not usually configure remote state backends.

Provider configuration can be passed from root to child modules for multi-account or multi-region setups.

Keep Terraform's ownership boundary clear: one state should own a resource or field, and other tools should consume published outputs instead of modifying it.

Use fmt, validate, linting, policy checks, plan review, and state locking before production applies.

Design for small blast radius by splitting state around lifecycle, permissions, and recovery boundaries.

Hands-on example

1. Write a reusable module for: What is the difference between a root module and a child module?

2. Create modules/s3_bucket with variables, locals, resource, and outputs:

variable "name" { type = string }

variable "tags" { type = map(string) default = {} }

locals { common_tags = merge(var.tags, { managed_by = "terraform" }) }

resource "aws_s3_bucket" "this" { bucket = var.name tags = local.common_tags }

output "bucket_id" { value = aws_s3_bucket.this.id }

3. Call it from the root module and pass the output to another module:

module "logs" { source = "../../modules/s3_bucket" name = "company-prod-logs" tags = var.tags }

module "app" { source = "../../modules/app" log_bucket_id = module.logs.bucket_id }

4. Add validation, README examples, and a minimal Terratest or plan test before releasing v1.0.0.

More Infrastructure as Code (Terraform, Ansible) interview questions

• What is Infrastructure as Code, and what problems does it solve over click-ops?
• What is the difference between declarative and imperative IaC, and where do Terraform and Ansible fall?
• What is the difference between configuration management and provisioning?
• What is Terraform, and what is the core plan/apply workflow?
• What does terraform init do?
• What is the Terraform state file, and why is it critical?
• Why should state be stored remotely, and what backend would you use on AWS?
• What is state locking, and why does it matter for teams?

← All Infrastructure as Code (Terraform, Ansible) questions