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How would you write a reusable module for a standard service (as you did to cut provisioning time 70%)?

Infrastructure as Code (Terraform, Ansible) · Intermediate level

Answer

For a reusable service module, I define a clean interface, encode secure defaults, expose only necessary knobs, create outputs needed by downstream systems, and add examples and tests. The goal is to turn repeated manual provisioning into a small, approved module call that is fast, consistent, and safe.

Technical explanation

A service module should encode defaults for logging, monitoring, tags, encryption, IAM boundaries, and deployment patterns.

Expose sizing and optional features as typed inputs with validation.

Prove reuse through examples for dev and prod and a test that provisions the minimal valid service.

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: How would you write a reusable module for a standard service (as you did to cut provisioning time 70%)?

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

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• 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?

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