building-terraform-modules

Building Terraform Modules

Overview

Build reusable, production-ready Terraform modules with proper variable definitions, outputs, validation rules, documentation, and examples. Generate modules following HashiCorp's standard module structure for AWS, GCP, and Azure resources with security best practices, tagging conventions, and lifecycle management.

Prerequisites

• Terraform 1.0+ installed (terraform version)
• Cloud provider credentials configured for the target platform
• Understanding of the infrastructure resources the module will manage
• Familiarity with HCL syntax and Terraform module conventions
• terraform-docs installed for automated documentation generation (optional)

Instructions

1. Define the module scope: determine which cloud resources the module manages and its input/output contract
2. Create the standard module file structure: main.tf, variables.tf, outputs.tf, versions.tf, locals.tf
3. Write versions.tf with required_version and required_providers blocks pinned to stable versions
4. Define input variables in variables.tf with descriptions, types, defaults, and validation rules
5. Implement resources in main.tf using variables for all configurable values; use locals for computed values
6. Add meaningful outputs in outputs.tf for resource IDs, ARNs, endpoints, and connection strings
7. Implement security defaults: encryption enabled, public access blocked, least-privilege IAM, logging enabled
8. Create an examples/ directory with at least one complete usage example showing module invocation
9. Generate documentation with terraform-docs markdown table . > README.md
10. Validate the module with terraform init && terraform validate and test with terraform plan using the example

Output

• Module files: main.tf, variables.tf, outputs.tf, versions.tf, locals.tf
• Example usage in examples/basic/main.tf with realistic variable values
• Auto-generated README.md with inputs, outputs, and usage documentation
• .terraform-docs.yml configuration for documentation generation
• Optional: test files using Terratest or terraform test (HCL-based)

Error Handling

Error	Cause	Solution
variable validation failed	Input value does not meet validation rule	Check the validation block in variables.tf; adjust the value or the validation regex
provider not found	Missing or wrong provider source in versions.tf	Add the provider to required_providers with correct source and version constraint
circular dependency	Resources referencing each other in a loop	Refactor to break the cycle; use depends_on or separate into sub-modules
output references undeclared resource	Typo in resource name or resource removed	Verify resource names in main.tf match output references exactly
module source not found	Incorrect module path or registry reference	Verify the source path is relative (e.g., ./modules/vpc) or a valid registry address

Examples

• "Build a Terraform module for an AWS VPC with configurable CIDR, public/private subnets across 3 AZs, NAT gateway, and flow logs."
• "Create a GCP Cloud Run module with custom domain, IAM bindings, and auto-scaling configuration as input variables."
• "Generate a Terraform module for an S3 bucket with versioning, encryption, lifecycle rules, and access logging, with all settings as optional variables with secure defaults."

Resources

• Terraform module registry: https://registry.terraform.io/
• Module structure guide: https://developer.hashicorp.com/terraform/language/modules/develop/structure
• terraform-docs: https://terraform-docs.io/
• Module best practices: https://developer.hashicorp.com/terraform/language/modules/develop