Terraform Modules: Building Reusable Infrastructure Components

modules/
└── vpc/
    ├── main.tf          # Primary resource definitions
    ├── variables.tf     # Input variable declarations
    ├── outputs.tf       # Output value declarations
    ├── versions.tf      # Provider and Terraform version constraints
    ├── locals.tf        # Local values and computed expressions
    ├── data.tf          # Data source definitions
    ├── README.md        # Module documentation
    ├── examples/        # Example configurations
    │   ├── basic/
    │   │   └── main.tf
    │   └── complete/
    │       └── main.tf
    └── tests/           # Test configurations
        └── vpc_test.go

# variables.tf

variable "vpc_name" {
  description = "Name of the VPC. Used for tagging and naming resources."
  type        = string

  validation {
    condition     = length(var.vpc_name) <= 32
    error_message = "VPC name must be 32 characters or less."
  }
}

variable "cidr_block" {
  description = "CIDR block for the VPC."
  type        = string
  default     = "10.0.0.0/16"

  validation {
    condition     = can(cidrhost(var.cidr_block, 0))
    error_message = "Must be a valid CIDR block."
  }
}

variable "availability_zones" {
  description = "List of availability zones for subnet distribution."
  type        = list(string)
  default     = []
}

variable "enable_dns_hostnames" {
  description = "Enable DNS hostnames in the VPC."
  type        = bool
  default     = true
}

variable "tags" {
  description = "Map of tags to apply to all resources."
  type        = map(string)
  default     = {}
}

# outputs.tf

output "vpc_id" {
  description = "The ID of the VPC."
  value       = aws_vpc.main.id
}

output "vpc_cidr_block" {
  description = "The CIDR block of the VPC."
  value       = aws_vpc.main.cidr_block
}

output "public_subnet_ids" {
  description = "List of public subnet IDs."
  value       = aws_subnet.public[*].id
}

output "private_subnet_ids" {
  description = "List of private subnet IDs."
  value       = aws_subnet.private[*].id
}

output "nat_gateway_ips" {
  description = "List of Elastic IPs for NAT Gateways."
  value       = aws_eip.nat[*].public_ip
  sensitive   = false
}

# modules/s3-bucket/main.tf

resource "aws_s3_bucket" "this" {
  bucket = var.bucket_name

  tags = merge(
    var.tags,
    {
      Name        = var.bucket_name
      Environment = var.environment
      ManagedBy   = "Terraform"
    }
  )
}

resource "aws_s3_bucket_versioning" "this" {
  bucket = aws_s3_bucket.this.id

  versioning_configuration {
    status = var.enable_versioning ? "Enabled" : "Suspended"
  }
}

resource "aws_s3_bucket_server_side_encryption_configuration" "this" {
  bucket = aws_s3_bucket.this.id

  rule {
    apply_server_side_encryption_by_default {
      sse_algorithm     = var.sse_algorithm
      kms_master_key_id = var.kms_key_arn
    }
    bucket_key_enabled = var.bucket_key_enabled
  }
}

resource "aws_s3_bucket_public_access_block" "this" {
  bucket = aws_s3_bucket.this.id

  block_public_acls       = var.block_public_access
  block_public_policy     = var.block_public_access
  ignore_public_acls      = var.block_public_access
  restrict_public_buckets = var.block_public_access
}

resource "aws_s3_bucket_lifecycle_configuration" "this" {
  count  = length(var.lifecycle_rules) > 0 ? 1 : 0
  bucket = aws_s3_bucket.this.id

  dynamic "rule" {
    for_each = var.lifecycle_rules
    content {
      id     = rule.value.id
      status = rule.value.enabled ? "Enabled" : "Disabled"

      filter {
        prefix = rule.value.prefix
      }

      expiration {
        days = rule.value.expiration_days
      }

      dynamic "transition" {
        for_each = rule.value.transitions
        content {
          days          = transition.value.days
          storage_class = transition.value.storage_class
        }
      }
    }
  }
}

# main.tf - Root module consuming the S3 module# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputs# Access module outputsss module outputs
output "bucket_arn" {
  value = module.app_bucket.bucket_arn
}

# modules/application-stack/main.tf# A composed module that creates a complete application infrastructure# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values# Outputs expose composed values expose composed values
output "cluster_endpoint" {
  value = module.eks.cluster_endpoint
}

output "database_endpoint" {
  value = module.rds.endpoint
}

# 1. Create repository with correct naming# Example: terraform-aws-vpc# 2. Structure your module# 3. Create a semantic version tag# 3. Create a semantic version tag# 3. Create a semantic version tag# 3. Create a semantic version tag# 3. Create a semantic version tag# 3. Create a semantic version tag# 4. Sign in to registry.terraform.io with GitHub# 4. Sign in to registry.terraform.io with GitHub# 5. Select "Publish" → "Module"# 6. Select your repository# 6. Select your repository# Using published moduleule
module "vpc" {
  source  = "your-org/vpc/aws"
  version = "~> 1.0"
  
  # ... variables
}

# Exact version# Pessimistic constraint (recommended)# Pessimistic constraint (recommended)# Pessimistic constraint (recommended)# Allows 5.1.x but not 5.2.0# Range constraint# Range constraint# Range constraint# Range constraint# Range constraint# Range constraint# For Git sources, use ref# For Git sources, use ref# For Git sources, use ref# For Git sources, use ref Git sources, use ref
module "vpc" {
  source = "git::https://github.com/org/terraform-aws-vpc.git?ref=v1.2.0"
}

// tests/vpc_test.go
package test

import (
    "testing"

    "github.com/gruntwork-io/terratest/modules/terraform"
    "github.com/stretchr/testify/assert"
)

func TestVpcModule(t *testing.T) {
    t.Parallel()

    terraformOptions := terraform.WithDefaultRetryableErrors(t, &terraform.Options{
        TerraformDir: "../examples/basic",
        Vars: map[string]interface{}{
            "vpc_name":   "test-vpc",
            "cidr_block": "10.0.0.0/16",
        },
    })

    // Clean up resources after test
    defer terraform.Destroy(t, terraformOptions)

    // Deploy the module
    terraform.InitAndApply(t, terraformOptions)

    // Validate outputs
    vpcId := terraform.Output(t, terraformOptions, "vpc_id")
    assert.NotEmpty(t, vpcId)

    cidrBlock := terraform.Output(t, terraformOptions, "vpc_cidr_block")
    assert.Equal(t, "10.0.0.0/16", cidrBlock)
}