Terraform module library

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Build reusable Terraform modules for AWS, Azure, GCP, and OCI infrastructure following infrastructure-as-code best practices. Use when creating infrastructure modules, standardizing cloud provisioning, or implementing reusable IaC components.

SKILL.md

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Terraform Module Library

Production-ready Terraform module patterns for AWS, Azure, GCP, and OCI infrastructure.

Purpose

Create reusable, well-tested Terraform modules for common cloud infrastructure patterns across multiple cloud providers.

When to Use

Build reusable infrastructure components
Standardize cloud resource provisioning
Implement infrastructure as code best practices
Create multi-cloud compatible modules
Establish organizational Terraform standards

Module Structure

terraform-modules/
├── aws/
│   ├── vpc/
│   ├── eks/
│   ├── rds/
│   └── s3/
├── azure/
│   ├── vnet/
│   ├── aks/
│   └── storage/
├── gcp/
│   ├── vpc/
│   ├── gke/
│   └── cloud-sql/
└── oci/
    ├── vcn/
    ├── oke/
    └── object-storage/

Standard Module Pattern

module-name/
├── main.tf          # Main resources
├── variables.tf     # Input variables
├── outputs.tf       # Output values
├── versions.tf      # Provider versions
├── README.md        # Documentation
├── examples/        # Usage examples
│   └── complete/
│       ├── main.tf
│       └── variables.tf
└── tests/           # Terratest files
    └── module_test.go

AWS VPC Module Example

main.tf:

resource "aws_vpc" "main" {
  cidr_block           = var.cidr_block
  enable_dns_hostnames = var.enable_dns_hostnames
  enable_dns_support   = var.enable_dns_support

  tags = merge(
    {
      Name = var.name
    },
    var.tags
  )
}

resource "aws_subnet" "private" {
  count             = length(var.private_subnet_cidrs)
  vpc_id            = aws_vpc.main.id
  cidr_block        = var.private_subnet_cidrs[count.index]
  availability_zone = var.availability_zones[count.index]

  tags = merge(
    {
      Name = "${var.name}-private-${count.index + 1}"
      Tier = "private"
    },
    var.tags
  )
}

resource "aws_internet_gateway" "main" {
  count  = var.create_internet_gateway ? 1 : 0
  vpc_id = aws_vpc.main.id

  tags = merge(
    {
      Name = "${var.name}-igw"
    },
    var.tags
  )
}

variables.tf:

variable "name" {
  description = "Name of the VPC"
  type        = string
}

variable "cidr_block" {
  description = "CIDR block for VPC"
  type        = string
  validation {
    condition     = can(regex("^([0-9]{1,3}\\.){3}[0-9]{1,3}/[0-9]{1,2}$", var.cidr_block))
    error_message = "CIDR block must be valid IPv4 CIDR notation."
  }
}

variable "availability_zones" {
  description = "List of availability zones"
  type        = list(string)
}

variable "private_subnet_cidrs" {
  description = "CIDR blocks for private subnets"
  type        = list(string)
  default     = []
}

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

variable "tags" {
  description = "Additional tags"
  type        = map(string)
  default     = {}
}

outputs.tf:

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

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

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

Best Practices

Use semantic versioning for modules
Document all variables with descriptions
Provide examples in examples/ directory
Use validation blocks for input validation
Output important attributes for module composition
Pin provider versions in versions.tf
Use locals for computed values
Implement conditional resources with count/for_each
Test modules with Terratest
Tag all resources consistently

Reference: See references/aws-modules.md and references/oci-modules.md

Module Composition

module "vpc" {
  source = "../../modules/aws/vpc"

  name               = "production"
  cidr_block         = "10.0.0.0/16"
  availability_zones = ["us-west-2a", "us-west-2b", "us-west-2c"]

  private_subnet_cidrs = [
    "10.0.1.0/24",
    "10.0.2.0/24",
    "10.0.3.0/24"
  ]

  tags = {
    Environment = "production"
    ManagedBy   = "terraform"
  }
}

module "rds" {
  source = "../../modules/aws/rds"

  identifier     = "production-db"
  engine         = "postgres"
  engine_version = "15.3"
  instance_class = "db.t3.large"

  vpc_id     = module.vpc.vpc_id
  subnet_ids = module.vpc.private_subnet_ids

  tags = {
    Environment = "production"
  }
}

Testing

// 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) {
    terraformOptions := &terraform.Options{
        TerraformDir: "../examples/complete",
    }

    defer terraform.Destroy(t, terraformOptions)
    terraform.InitAndApply(t, terraformOptions)

    vpcID := terraform.Output(t, terraformOptions, "vpc_id")
    assert.NotEmpty(t, vpcID)
}

Related Skills

multi-cloud-architecture - For architectural decisions
cost-optimization - For cost-effective designs