Confidently Ship Reusable OpenTofu and Terraform Modules

If there’s one thing we all care deeply about, it’s not fame, fortune or perfect HCL formatting; it’s reusability.

Whether you're a seasoned practitioner or new to Infrastructure as Code (IaC), Reusable modules are fast becoming the backbone of modern platform engineering. That's why modern platforms introduced Module Registries, central systems for publishing and consuming OpenTofu/Terraform modules across your organization. 

They promote the DRY principle ("Don't Repeat Yourself") by codifying best practices, reducing duplication, and helping teams ship faster by focusing on what’s unique to their workload

But as teams scale, so does the risk: a misconfigured or buggy module can break dozens of environments in seconds.

Enter testing for infrastructure modules.

Real-world example: How one bug could have taken out everything

A few years ago, a platform engineering team learned a painful lesson: one bad Terraform command can destroy everything.

This real incident describes how a single misconfigured module and an unguarded Terraform destroy wiped out an entire staging environment, dozens of services gone in minutes. Recovery took days.

Now imagine your team building a reusable VPC module. Without testing, a single overlooked bug, say, a missing region variable or a misconfigured ACL that leaves an S3 bucket public, could silently make it into your registry. Every environment using that module would be exposed.

Here’s how to prevent it:

Before publishing, the platform team runs an integration pipeline that provisions a real test workspace with actual cloud credentials. On the first run, the missing region is caught. On the second, the public S3 bucket is flagged. Both are fixed before the module ever touches the registry.

The single step of testing modules in isolation before release turns potential outages into harmless build failures, protecting every downstream environment.

Why test your modules?

When you publish a shared module to your registry, you're trusting that it works now, and will continue to work later. Without dedicated testing, it's easy to miss:

• Required variables that aren't defined
• Breaking changes introduced by new commits
• Inadvertent updates that change resource behaviour across all consumers
• Cross-module dependency conflicts
• Security misconfigurations that expose resources publicly

Testing modules addresses these risks by validating them in isolation before they’re promoted to the registry.

How module tests work

A dedicated Integration pipeline is added to your module’s development branch. This pipeline:

• Provisions a real test environment.
• Executes real init, plan, and apply commands using OpenTofu (or Terraform).
• Validates cross-module compatibility and runtime behaviour.

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steps:
   - step:
      type: tofu
      name: Plan VPC
      spec:
	  command: plan
	  framework: TOFU
	  workspace: vpc-test-sandbox
	  variables:
	     region: us-west-2
	     environment: test

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✅ Tip: You define test inputs just like consumers would, using actual variables, connectors, and real infrastructure.

Only after the module passes this pipeline should it be promoted to the main branch and published.

Key Benefits

• Confidence in Reuse: Prevent broken or misconfigured modules from being published to your shared registry
• Safe Promotion Workflow: Validate modules in feature branches before promoting to main
• Catch Critical Misconfigurations: Detect missing variables, logic bugs, or public exposure risks early
• Governance and Drift Control: Enforce testing standards for modules, protecting downstream workspaces
• Auditable Changes: Avoid disaster-level rollouts by validating before any code touches production
• Encourages Reusability: With confidence in test coverage, teams are more likely to standardize on shared modules

CI/CD Integration and Testing Approaches

Testing modules complements traditional IaC testing techniques like the following methods:

• tflint, validate
  
  • Syntax, formatting, and static checks
• Terratest
  
  • Unit tests & mocks
• Checkov, tfsec
  
  • Static security scanning
• Registry testing
  
  • Real-world integration and runtime testing

Example CI/CD flow:

stages:
  - validate        # Static analysis, linting
  - test-unit      # Unit tests, mocking
  - test-integration # Tests Modules
  - publish        # Promote to registry

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The integration testing stage spins up real infrastructure to validate that your modules work as expected before they reach production consumers.

Advanced Use Cases

Multi-Environment Testing

Test your modules across different cloud regions or account configurations to ensure portability:

matrix:
  environment:
    - dev-us-east-1
    - staging-eu-west-1
    - prod-ap-southeast-1

Dependency Chain Validation

Test complex module hierarchies where modules depend on outputs from other modules:

# Test network module first, then application module that depends on it
steps:
  - network-module-test
  - app-module-test:
      depends_on: network-module-test

Compliance and Security Validation

Integrate security scanning directly into your integration tests:

post_apply_checks:
  - security_scan: true
  - compliance_check: cis-benchmark
  - cost_estimation: true

Rollback Testing

Validate that module updates can be safely rolled back by testing both upgrade and downgrade paths.

Implementation Considerations

Setting up tests for your modules requires some initial overhead, but the investment pays dividends as your module ecosystem grows:

Resource Costs: Integration tests provision real infrastructure, so factor in cloud costs for test environments. Use short-lived resources and automated cleanup to minimize expenses.

Test Environment Management: Establish dedicated sandbox accounts or subscriptions for integration testing to avoid conflicts with production resources.

Pipeline Execution Time: Real infrastructure provisioning takes longer than unit tests, so optimize your pipeline for parallel execution where possible.

Where to find Module Registry and Module Testing

Testing modules is becoming a core best practice in the OpenTofu ecosystem. But finding a platform that natively integrates registry management and test pipelines can be challenging.

If you’re looking for a platform that natively integrates module registries with testing pipelines, Harness Infrastructure as Code Management (IaCM) has you covered:

• ✅ Manage reusable modules with a built-in Module Registry
• ✅ Validate them using Integration pipelines before publishing
• ✅ Promote only tested, trusted modules to your consumers
  
  

Check out how to create and register your IaC modules and configure module tests to get started with pipeline setup and test inputs.

Conclusion

If you value stability, reusability, and rapid iteration, then testing your modules is more than a nice-to-have; it’s your safeguard against chaos.

By combining traditional CI/CD validation with real infrastructure testing, you get the best of both worlds: fast feedback and real-world assurance.

Start small. Iterate. And as your registry grows, let testing give you the confidence to scale.

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