Terraform Module Library

1---
2name: terraform-module-library
3description: Build reusable Terraform modules for AWS, Azure, and GCP infrastructure following infrastructure-as-code best practices. Use when creating infrastructure modules, standardizing cloud provisioning, or implementing reusable IaC components.
4---
5 
6# Terraform Module Library
7 
8Production-ready Terraform module patterns for AWS, Azure, and GCP infrastructure.
9 
10## Do not use this skill when
11 
12- The task is unrelated to terraform module library
13- You need a different domain or tool outside this scope
14 
15## Instructions
16 
17- Clarify goals, constraints, and required inputs.
18- Apply relevant best practices and validate outcomes.
19- Provide actionable steps and verification.
20- If detailed examples are required, open `resources/implementation-playbook.md`.
21 
22## Purpose
23 
24Create reusable, well-tested Terraform modules for common cloud infrastructure patterns across multiple cloud providers.
25 
26## Use this skill when
27 
28- Build reusable infrastructure components
29- Standardize cloud resource provisioning
30- Implement infrastructure as code best practices
31- Create multi-cloud compatible modules
32- Establish organizational Terraform standards
33 
34## Module Structure
35 
36```
37terraform-modules/
38├── aws/
39│   ├── vpc/
40│   ├── eks/
41│   ├── rds/
42│   └── s3/
43├── azure/
44│   ├── vnet/
45│   ├── aks/
46│   └── storage/
47└── gcp/
48    ├── vpc/
49    ├── gke/
50    └── cloud-sql/
51```
52 
53## Standard Module Pattern
54 
55```
56module-name/
57├── main.tf          # Main resources
58├── variables.tf     # Input variables
59├── outputs.tf       # Output values
60├── versions.tf      # Provider versions
61├── README.md        # Documentation
62├── examples/        # Usage examples
63│   └── complete/
64│       ├── main.tf
65│       └── variables.tf
66└── tests/           # Terratest files
67    └── module_test.go
68```
69 
70## AWS VPC Module Example
71 
72**main.tf:**
73```hcl
74resource "aws_vpc" "main" {
75  cidr_block           = var.cidr_block
76  enable_dns_hostnames = var.enable_dns_hostnames
77  enable_dns_support   = var.enable_dns_support
78 
79  tags = merge(
80    {
81      Name = var.name
82    },
83    var.tags
84  )
85}
86 
87resource "aws_subnet" "private" {
88  count             = length(var.private_subnet_cidrs)
89  vpc_id            = aws_vpc.main.id
90  cidr_block        = var.private_subnet_cidrs[count.index]
91  availability_zone = var.availability_zones[count.index]
92 
93  tags = merge(
94    {
95      Name = "${var.name}-private-${count.index + 1}"
96      Tier = "private"
97    },
98    var.tags
99  )
100}
101 
102resource "aws_internet_gateway" "main" {
103  count  = var.create_internet_gateway ? 1 : 0
104  vpc_id = aws_vpc.main.id
105 
106  tags = merge(
107    {
108      Name = "${var.name}-igw"
109    },
110    var.tags
111  )
112}
113```
114 
115**variables.tf:**
116```hcl
117variable "name" {
118  description = "Name of the VPC"
119  type        = string
120}
121 
122variable "cidr_block" {
123  description = "CIDR block for VPC"
124  type        = string
125  validation {
126    condition     = can(regex("^([0-9]{1,3}\\.){3}[0-9]{1,3}/[0-9]{1,2}$", var.cidr_block))
127    error_message = "CIDR block must be valid IPv4 CIDR notation."
128  }
129}
130 
131variable "availability_zones" {
132  description = "List of availability zones"
133  type        = list(string)
134}
135 
136variable "private_subnet_cidrs" {
137  description = "CIDR blocks for private subnets"
138  type        = list(string)
139  default     = []
140}
141 
142variable "enable_dns_hostnames" {
143  description = "Enable DNS hostnames in VPC"
144  type        = bool
145  default     = true
146}
147 
148variable "tags" {
149  description = "Additional tags"
150  type        = map(string)
151  default     = {}
152}
153```
154 
155**outputs.tf:**
156```hcl
157output "vpc_id" {
158  description = "ID of the VPC"
159  value       = aws_vpc.main.id
160}
161 
162output "private_subnet_ids" {
163  description = "IDs of private subnets"
164  value       = aws_subnet.private[*].id
165}
166 
167output "vpc_cidr_block" {
168  description = "CIDR block of VPC"
169  value       = aws_vpc.main.cidr_block
170}
171```
172 
173## Best Practices
174 
1751. **Use semantic versioning** for modules
1762. **Document all variables** with descriptions
1773. **Provide examples** in examples/ directory
1784. **Use validation blocks** for input validation
1795. **Output important attributes** for module composition
1806. **Pin provider versions** in versions.tf
1817. **Use locals** for computed values
1828. **Implement conditional resources** with count/for_each
1839. **Test modules** with Terratest
18410. **Tag all resources** consistently
185 
186## Module Composition
187 
188```hcl
189module "vpc" {
190  source = "../../modules/aws/vpc"
191 
192  name               = "production"
193  cidr_block         = "10.0.0.0/16"
194  availability_zones = ["us-west-2a", "us-west-2b", "us-west-2c"]
195 
196  private_subnet_cidrs = [
197    "10.0.1.0/24",
198    "10.0.2.0/24",
199    "10.0.3.0/24"
200  ]
201 
202  tags = {
203    Environment = "production"
204    ManagedBy   = "terraform"
205  }
206}
207 
208module "rds" {
209  source = "../../modules/aws/rds"
210 
211  identifier     = "production-db"
212  engine         = "postgres"
213  engine_version = "15.3"
214  instance_class = "db.t3.large"
215 
216  vpc_id     = module.vpc.vpc_id
217  subnet_ids = module.vpc.private_subnet_ids
218 
219  tags = {
220    Environment = "production"
221  }
222}
223```
224 
225## Reference Files
226 
227- `assets/vpc-module/` - Complete VPC module example
228- `assets/rds-module/` - RDS module example
229- `references/aws-modules.md` - AWS module patterns
230- `references/azure-modules.md` - Azure module patterns
231- `references/gcp-modules.md` - GCP module patterns
232 
233## Testing
234 
235```go
236// tests/vpc_test.go
237package test
238 
239import (
240    "testing"
241    "github.com/gruntwork-io/terratest/modules/terraform"
242    "github.com/stretchr/testify/assert"
243)
244 
245func TestVPCModule(t *testing.T) {
246    terraformOptions := &terraform.Options{
247        TerraformDir: "../examples/complete",
248    }
249 
250    defer terraform.Destroy(t, terraformOptions)
251    terraform.InitAndApply(t, terraformOptions)
252 
253    vpcID := terraform.Output(t, terraformOptions, "vpc_id")
254    assert.NotEmpty(t, vpcID)
255}
256```
257 
258## Related Skills
259 
260- `multi-cloud-architecture` - For architectural decisions
261- `cost-optimization` - For cost-effective designs
262