Terraform Modules Deep Dive for PowerShell Developers - Part 6

Master Terraform modules from a PowerShell perspective - learn best practices for creating, structuring, and maintaining reusable infrastructure components.

Posted Aug 26, 2025

By Morten Mynster

13 min read

📚 Series Navigation:
Part 1: Getting Started with Terraform for PowerShell People
Part 2: Resources, Variables, and State in Terraform
Part 3: Advanced Terraform and PowerShell Integration
Part 4: Advanced State Management and Collaboration
Part 5: Testing Terraform Code
Part 6: Terraform Modules Deep Dive ← You are here
Part 7: CI/CD with GitHub Actions (September 2)

Mastering Terraform Modules for PowerShell Users

With comprehensive testing strategies from Part 5 now mastered, it’s time to focus on creating reusable, production-ready infrastructure components. In this part, we’ll dive deep into Terraform modules - the equivalent of PowerShell modules for infrastructure code.

As PowerShell professionals, you already understand the power of modularity from building PowerShell modules, functions, and reusable scripts. Terraform modules serve the same purpose for infrastructure code, allowing us to create standardized, testable, and maintainable components. Now that you have testing expertise, we can build modules that are thoroughly validated and enterprise-ready.

PowerShell Modules vs. Terraform Modules

Let’s compare the module concepts:

PowerShell Module Structure

  
MyPowerShellModule/
├── MyPowerShellModule.psd1     # Module manifest
├── MyPowerShellModule.psm1     # Module implementation
├── Public/                     # Public functions
│   ├── New-AzureResource.ps1
│   └── Get-AzureResource.ps1
├── Private/                    # Private functions
│   └── helpers.ps1
└── Tests/                      # Tests for the module
    └── MyModule.Tests.ps1

Terraform Module Structure

my-terraform-module/
├── main.tf           # Main module implementation
├── data.tf           # Data sources
├── variables.tf      # Input variables definition
├── outputs.tf        # Output values definition
├── terraform.tf       # Required providers and versions
├── README.md         # Documentation
└── examples/         # Example usage
    └── basic/
        ├── main.tf
        └── variables.tf

Creating Your First Terraform Module

Let’s create a reusable module for a web application:

  
# Create module directory structure
New-Item -ItemType Directory -Path ".\modules\resource-group" -Force
New-Item -ItemType File -Path ".\modules\resource-group\main.tf"
New-Item -ItemType File -Path ".\modules\resource-group\variables.tf"
New-Item -ItemType File -Path ".\modules\resource-group\outputs.tf"
New-Item -ItemType File -Path ".\modules\resource-group\terraform.tf"
New-Item -ItemType File -Path ".\main.tf"

Module Files

  
# modules/resource-group/variables.tf
variable "name" {
  description = "Name of the web application"
  type        = string
}

variable "location" {
  description = "Azure region where resources will be created"
  type        = string
}

  
# modules/resource-group/main.tf
resource "azurerm_resource_group" "rg" {
  name     = "example"
  location = "West Europe"
}

  
# modules/resource-group/outputs.tf
output "id" {
  description = "Id of the resourcegroup"
  value       = azurerm_resource_group.rg.id
}

  
# modules/resource-group/terraform.tf
terraform {
  required_providers {
    azurerm = {
      source  = "hashicorp/azurerm"
      version = "4.41.0"
    }
  }
  required_version = ">= 1.6.0"
}

Using the Module

  
# main.tf
provider "azurerm" {
  features {}
}


module "resource-group" {
  source              = "./modules/resource-group"
  name                = "rg-from-mod"
  location            = "West Europe"
}

Module Design Patterns

Input Variables (Like PowerShell Parameters)

In PowerShell functions, we use parameters with validation:

  
function New-rg{
    [CmdletBinding()]
    param (
        [Parameter(Mandatory=$true)]
        [string]$Name,

        [Parameter(Mandatory=$true)]
        [ValidateSet("westus", "eastus", "northeurope")]
        [string]$Location
    )

    # Function logic here
}

In Terraform, we use input variables with validation:

  
variable "name" {
  description = "Name of the resource group"
  type        = string

  validation {
    condition     = length(var.name) > 3 && length(var.name) <= 60
    error_message = "Resource group name must be between 4 and 60 characters."
  }
}

variable "location" {
  description = "Azure region where resources will be created"
  type        = string

  validation {
    condition     = contains(["West US", "East US", "North Europe", "West Europe"], var.location)
    error_message = "Allowed values are: West US, East US, North Europe, West Europe"
  }
}

Nested Modules (Like PowerShell Functions Calling Other Functions)

In PowerShell, we often have functions that call other functions:

  
function New-WebInfrastructure {
    param (
        [string]$Name,
        [string]$Location
    )

    $resourceGroup = New-AzResourceGroup -Name "$Name-rg" -Location $Location
    $storageAccount = New-StorageAccount -Name $Name -ResourceGroup $resourceGroup.ResourceGroupName
    $webApp = New-WebApp -Name $Name -Location $Location -ResourceGroup $resourceGroup.ResourceGroupName

    return @{
        ResourceGroup = $resourceGroup
        StorageAccount = $storageAccount
        WebApp = $webApp
    }
}

In Terraform, we use nested modules:

  
module "storage_account" {
  source = "./modules/storage_account"
  name   = "mystorage"
  location = "West Europe"
}

Where the storage_account module might look like:

  
# modules/storage_account/main.tf
module "resource_group" {
  source              = "./modules/resource-group"
  name                = "rg-from-mod"
  location            = "West Europe"
}

module "storage" {
  source              = "../storage"
  name                = "${var.name}store"
  resource_group_name = module.resource_group.name
  location            = module.resource_group..location
}

output "resource_group" {
  value = module.resource_group
}

output "storage" {
  value = module.storage
}

Module Best Practices

1. Consistent Structure (Like PowerShell Module Structure)

Just as PowerShell modules have a recommended structure, so do Terraform modules:

module-name/
├── README.md           # Documentation
├── main.tf             # Main resources
├── data.tf             # Data sources
├── variables.tf        # Input variables
├── outputs.tf          # Output values
├── terraform.tf         # Provider requirements
├── examples/           # Example usage
│   └── basic/
│       └── main.tf
└── test/               # Tests
    └── module_test.tftest

2. Use Local Values for Derived Variables (Like PowerShell Local Variables)

In PowerShell:

  
function Deploy-Resources {
    param ($baseName, $env)

    $resourceGroupName = "$baseName-$env-rg"
    $storageAccountName = ($baseName + $env).ToLower() -replace "[^a-z0-9]", ""

    # Use the local variables
}

In Terraform:

  
locals {
  resource_group_name  = "${var.base_name}-${var.environment}-rg"
  storage_account_name = lower(replace("${var.base_name}${var.environment}", "/[^a-z0-9]/", ""))
}

module "resource_group" {
  source              = "./modules/resource-group"
  name     = local.resource_group_name
  location = var.location
}

module "storage" {
  source              = "../storage"
  name                = local.storage_account_name
  resource_group_name = module.resource_group.name
  location            = module.resource_group..location
  account_tier             = "Standard"
  account_replication_type = "LRS"
}

3. Version Your Modules (Like PowerShell Module Versioning)

In PowerShell, we version our modules in the manifest:

  
# MyModule.psd1
@{
    ModuleVersion = '1.2.0'
    # ...
}

In Terraform, we can use Git tags for versions and reference them:

  
module "resource_group" {
  source  = "git::https://github.com/myorg/terraform-modules.git//modules/resource-group?ref=v1.2.0"
  name    = "example"
  # ...
}

Or use the Terraform Registry:

Terraform Registry:.

  
module "vnet" {
  source  = "Azure/vnet/azurerm"
  version = "2.6.0"
  # ...
}

Creating a Module Library (Like PowerShell Gallery)

While PowerShell has the PowerShell Gallery, Terraform has module registries.

Private Module Registry

For organizations, you can set up:

Azure DevOps Artifacts
GitHub Packages
Terraform Cloud Private Registry

Setting up a Simple Git-Based Registry

Create a repository structure:

terraform-modules/
├── modules/
│   ├── resource-group/
│   │   ├── main.tf
│   │   └── ...
│   ├── storage/
│   │   ├── main.tf
│   │   └── ...
│   └── network/
│       ├── main.tf
│       └── ...
└── README.md

Reference modules in your Terraform configurations:

  
module "resource_group" {
  source = "git::https://github.com/myorg/terraform-modules.git//modules/resource-group?ref=v1.0.0"
  # ...
}

Module Testing Integration

As covered in Part 5, testing is crucial for reliable modules. Here’s how to integrate testing into your module development workflow:

Testing Module Structure

modules/resource-group/
├── main.tf
├── variables.tf
├── outputs.tf
├── terraform.tf
├── README.md
├── examples/
│   ├── basic/
│   │   └── main.tf
│   └── advanced/
│       └── main.tf
└── tests/
    ├── unit.tftest.hcl
    ├── integration.tftest.hcl
    └── variables.auto.tfvars

Unit Tests for Modules

  
# modules/storage-account/tests/unit.tftest.hcl

# Mock provider for fast unit testing
override_provider "azurerm" {
  features {}
}

# Mock the resource group that the storage account depends on
override_resource {
  target = azurerm_resource_group.main
  values = {
    name     = "test-rg"
    location = "East US"
    id       = "/subscriptions/test/resourceGroups/test-rg"
  }
}

variables {
  name                = "teststorageacct"
  resource_group_name = "test-rg"
  location           = "East US"
  account_tier       = "Standard"
  replication_type   = "LRS"
  tags = {
    Environment = "Test"
    Department  = "IT"
  }
}

run "test_storage_account_creation" {
  command = plan

  assert {
    condition     = azurerm_storage_account.main.name == var.name
    error_message = "Storage account name should match input variable"
  }

  assert {
    condition     = azurerm_storage_account.main.account_tier == var.account_tier
    error_message = "Storage account tier should match input variable"
  }

  assert {
    condition     = azurerm_storage_account.main.account_replication_type == var.replication_type
    error_message = "Storage account replication type should match input variable"
  }
}

run "test_blob_properties" {
  command = plan

  assert {
    condition     = azurerm_storage_account.main.blob_properties[0].delete_retention_policy[0].days == 7
    error_message = "Default blob retention policy should be 7 days"
  }

  assert {
    condition     = azurerm_storage_account.main.min_tls_version == "TLS1_2"
    error_message = "Minimum TLS version should be 1.2 for security"
  }
}

run "test_output_values" {
  command = plan

  assert {
    condition     = output.storage_account_id != ""
    error_message = "Storage account ID output should not be empty"
  }

  assert {
    condition     = can(regex("^https://.*\\.blob\\.core\\.windows\\.net/$", output.primary_blob_endpoint))
    error_message = "Primary blob endpoint should follow Azure pattern"
  }
}

Integration Tests for Modules

  
# modules/storage-account/tests/integration.tftest.hcl

variables {
  name              = "integrationtest"
  resource_group_name = "integration-test-rg"
  location         = "East US"
  account_tier     = "Standard"
  replication_type = "GRS"  # Using geo-redundant storage for integration test
  tags = {
    Environment = "Integration"
    Project     = "Testing"
  }
  allow_blob_public_access = false
  enable_https_traffic_only = true
}

# Create test resource group first
run "setup_resource_group" {
  command = apply

  module {
    source = "../examples/prerequisites"
  }
}

run "test_storage_account_deployment" {
  command = apply

  depends_on = [run.setup_resource_group]

  assert {
    condition     = azurerm_storage_account.main.account_replication_type == var.replication_type
    error_message = "Storage account should use GRS replication as specified"
  }

  assert {
    condition     = azurerm_storage_account.main.allow_blob_public_access == false
    error_message = "Public blob access should be disabled for security"
  }
}

run "test_container_creation" {
  command = apply
  depends_on = [run.test_storage_account_deployment]

  # Test that the containers are created successfully
  assert {
    condition     = length(azurerm_storage_container.containers) >= 1
    error_message = "At least one storage container should be created"
  }

  assert {
    condition     = azurerm_storage_container.containers["data"].name == "data"
    error_message = "A container named 'data' should exist"
  }
}

run "test_network_rules" {
  command = plan

  # Test that network rules are properly configured
  assert {
    condition     = azurerm_storage_account.main.network_rules[0].default_action == "Deny"
    error_message = "Network rules should deny by default for security"
  }

  assert {
    condition     = length(azurerm_storage_account.main.network_rules[0].ip_rules) > 0
    error_message = "At least one IP rule should be configured"
  }
}

PowerShell Module Testing Helpers

  
# scripts/Test-TerraformModule.ps1
function Test-TerraformModule {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory)]
        [string]$ModulePath,

        [ValidateSet("unit", "integration", "all")]
        [string]$TestType = "all",

        [switch]$Verbose
    )

    Push-Location $ModulePath
    try {
        Write-Host "Testing Terraform module: $ModulePath" -ForegroundColor Cyan

        switch ($TestType) {
            "unit" {
                Write-Host "Running unit tests..." -ForegroundColor Yellow
                $result = terraform test tests/unit.tftest.hcl
                if ($LASTEXITCODE -ne 0) {
                    throw "Unit tests failed"
                }
            }
            "integration" {
                Write-Host "Running integration tests..." -ForegroundColor Yellow
                $result = terraform test tests/integration.tftest.hcl
                if ($LASTEXITCODE -ne 0) {
                    throw "Integration tests failed"
                }
            }
            "all" {
                Test-TerraformModule -ModulePath $ModulePath -TestType "unit" -Verbose:$Verbose
                Test-TerraformModule -ModulePath $ModulePath -TestType "integration" -Verbose:$Verbose
            }
        }

        Write-Host "All tests passed for module: $ModulePath" -ForegroundColor Green

    } catch {
        Write-Error "Module testing failed: $_"
        throw
    } finally {
        Pop-Location
    }
}

# Test all modules in a directory
function Test-AllTerraformModules {
    param(
        [string]$ModulesPath = "./modules",

        [ValidateSet("unit", "integration", "all")]
        [string]$TestType = "all"
    )

    $modules = Get-ChildItem -Path $ModulesPath -Directory
    $results = @()

    foreach ($module in $modules) {
        try {
            Test-TerraformModule -ModulePath $module.FullName -TestType $TestType
            $results += @{ Module = $module.Name; Status = "Passed" }
        } catch {
            $results += @{ Module = $module.Name; Status = "Failed"; Error = $_.Exception.Message }
        }
    }

    # Summary
    Write-Host "`nTest Results Summary:" -ForegroundColor Blue
    foreach ($result in $results) {
        $color = if ($result.Status -eq "Passed") { "Green" } else { "Red" }
        Write-Host "  $($result.Module): $($result.Status)" -ForegroundColor $color
        if ($result.Error) {
            Write-Host "    Error: $($result.Error)" -ForegroundColor Red
        }
    }

    $passed = ($results | Where-Object { $_.Status -eq "Passed" }).Count
    $total = $results.Count
    Write-Host "`nOverall: $passed/$total modules passed" -ForegroundColor $(if ($passed -eq $total) { "Green" } else { "Red" })

    return $results
}

Module Versioning and Lifecycle Management

For this i would personally use dependabot to keep watch over new version and automatically create PR’s for your module repository.

I will not cover the exact process but i believe this guide from GitHub should cover most circumstances:

Enabling dependabot for your repository.

Conclusion and Next Steps

In this sixth part of our PowerShell-to-Terraform series, you’ve mastered creating enterprise-grade, reusable infrastructure modules:

What We’ve Accomplished:

Module Architecture: Structured, testable modules following PowerShell development patterns
Testing Integration: Comprehensive module testing using the native testing framework from Part 5
Lifecycle Management: Versioning, publishing, and maintaining modules at enterprise scale

PowerShell Developer Advantages: Your PowerShell module development expertise translates perfectly to Terraform modules:

Similar project structure and organization principles
Familiar parameter validation and input/output patterns
Testing approaches that build on your Pester experience
Version management strategies that mirror PowerShell Gallery patterns

Module Maturity Achieved:

Capability	PowerShell Modules	Terraform Modules	Enterprise Benefits
Code Organization	Functions + Manifests	Resources + Variables	Reusable infrastructure patterns
Parameter Validation	Parameter attributes	Variable validation	Type-safe infrastructure inputs
Testing	Pester	`.tftest.hcl` + Pester	Automated validation & regression
Versioning	Semantic versioning	Git tags + registries	Controlled releases & rollbacks
Distribution	PowerShell Gallery	Module registries	Team sharing & standardization
Documentation	Comment-based help	README + examples	Self-documenting infrastructure

Advanced Patterns Mastered:

Module Factories: Dynamic infrastructure generation based on configuration
Composition Strategies: Building complex systems from simple, tested modules
Integration Testing: End-to-end validation using real infrastructure components

Coming Next: In Part 7 Terraform CICD With GitHub Actions, our final chapter, we’ll bring everything together by implementing comprehensive CI/CD pipelines that automatically test, validate, and deploy your modules and infrastructure using GitHub Actions - creating a complete end-to-end automation workflow.

You now have the skills to build and maintain enterprise-grade infrastructure module libraries Congratulations!

Infrastructure, Terraform

This post is licensed under CC BY 4.0 by the author.