Terraform Module Library

---
name: terraform-module-library
description: "Production-ready Terraform module patterns for AWS, Azure, and GCP infrastructure."
risk: critical
source: community
date_added: "2026-02-27"
---
## Supporting material availability

This catalog copy contains the inline guidance below, but does not include the following supporting paths. They are external prerequisites, not bundled files:

- `<support-root>/assets/vpc-module/`
- `<support-root>/assets/rds-module/`
- `<support-root>/references/azure-modules.md`
- `<support-root>/references/gcp-modules.md`

Ask for the original supporting material or an existing installation before using a step that depends on these paths; this catalog does not identify a verified download source for them. Resolve each path relative to that installation, inspect its contents, and verify any applicable license and execution prerequisites. Do not run a command or claim a referenced procedure was followed until its required file exists. Do not fabricate the missing scripts or treat an unrelated file with the same name as equivalent. Independent steps fully documented here can proceed within the user’s authorized scope; report any dependent step as unavailable.


# Terraform Module Library

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

## Do not use this skill when

- The task is unrelated to terraform module library
- You need a different domain or tool outside this scope

## Instructions

- Clarify goals, constraints, and required inputs.
- Apply relevant best practices and validate outcomes.
- Provide actionable steps and verification.

## Purpose

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

## Use this skill when

- 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/
```

## 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:**
```hcl
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:**
```hcl
variable "name" {
  description = "Name of the VPC"
  type        = string
}

variable "cidr_block" {
  description = "CIDR block for VPC"
  type        = string
  validation {
    condition     = can(cidrnetmask(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 "enable_dns_support" {
  type    = bool
  default = true
}

variable "create_internet_gateway" {
  type    = bool
  default = false
}

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

**outputs.tf:**
```hcl
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

1. **Use semantic versioning** for modules
2. **Document all variables** with descriptions
3. **Provide examples** in examples/ directory
4. **Use validation blocks** for input validation
5. **Output important attributes** for module composition
6. **Pin provider versions** in versions.tf
7. **Use locals** for computed values
8. **Implement conditional resources** with count/for_each
9. **Test modules** with Terratest
10. **Tag all resources** consistently

## Module Composition

```hcl
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"
  }
}
```

## Reference Files

- `assets/vpc-module/` - Complete VPC module example
- `assets/rds-module/` - RDS module example
- `references/aws-modules.md` - AWS module patterns
- `references/azure-modules.md` - Azure module patterns
- `references/gcp-modules.md` - GCP module patterns

## Testing

```go
// 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

## Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

The VPC fragment does not create routing/NAT or a full private network. Validate every subnet CIDR, non-overlap, VPC containment and sufficient availability-zone entries before planning; route-table/NAT decisions require a separate design. The RDS module and other missing assets cannot be executed from this catalog. Terratest `InitAndApply`/`Destroy` create/delete billable infrastructure: use an authorized disposable account/workspace, immutable provider/module pins, cost limits and task-owned cleanup. Confirm current engine/provider compatibility rather than treating historical versions as current recommendations.