terraform

---
name: terraform
description: Expert in Infrastructure as Code with Terraform. Use for creating, managing, and versioning infrastructure across cloud providers.
allowed-tools: Read, Write, Edit, Grep, Glob, Bash
---

# Terraform Infrastructure as Code Expert

## Purpose
Provide expert guidance on Terraform for infrastructure provisioning, state management, modules, and multi-cloud deployments.

## When to Use This Skill
- Writing Terraform configurations
- Creating reusable modules
- Managing infrastructure state
- Multi-cloud deployments
- Infrastructure refactoring
- CI/CD integration
- Best practices and patterns

## Project Structure
```
terraform/
├── main.tf
├── variables.tf
├── outputs.tf
├── versions.tf
├── terraform.tfvars
├── modules/
│   ├── vpc/
│   ├── compute/
│   └── database/
└── environments/
    ├── dev/
    ├── staging/
    └── production/
```

## Best Practices

### 1. Provider Configuration
```hcl
terraform {
  required_version = ">= 1.5"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }

  backend "s3" {
    bucket         = "my-terraform-state"
    key            = "production/terraform.tfstate"
    region         = "us-east-1"
    encrypt        = true
    dynamodb_table = "terraform-locks"
  }
}

provider "aws" {
  region = var.aws_region

  default_tags {
    tags = {
      Environment = var.environment
      ManagedBy   = "Terraform"
      Project     = var.project_name
    }
  }
}
```

### 2. Variables
```hcl
variable "environment" {
  description = "Environment name"
  type        = string
  validation {
    condition     = contains(["dev", "staging", "production"], var.environment)
    error_message = "Environment must be dev, staging, or production."
  }
}

variable "vpc_cidr" {
  description = "VPC CIDR block"
  type        = string
  default     = "10.0.0.0/16"
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro"
}

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

### 3. Outputs
```hcl
output "vpc_id" {
  description = "VPC ID"
  value       = aws_vpc.main.id
}

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

output "database_endpoint" {
  description = "Database endpoint"
  value       = aws_db_instance.main.endpoint
  sensitive   = true
}
```

### 4. Reusable Module
```hcl
# modules/vpc/main.tf
resource "aws_vpc" "main" {
  cidr_block           = var.cidr_block
  enable_dns_hostnames = true
  enable_dns_support   = true

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

resource "aws_subnet" "public" {
  count = length(var.public_subnet_cidrs)

  vpc_id                  = aws_vpc.main.id
  cidr_block              = var.public_subnet_cidrs[count.index]
  availability_zone       = var.availability_zones[count.index]
  map_public_ip_on_launch = true

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

# modules/vpc/variables.tf
variable "name" {
  description = "VPC name"
  type        = string
}

variable "cidr_block" {
  description = "VPC CIDR block"
  type        = string
}

variable "public_subnet_cidrs" {
  description = "Public subnet CIDR blocks"
  type        = list(string)
}

variable "availability_zones" {
  description = "Availability zones"
  type        = list(string)
}

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

# modules/vpc/outputs.tf
output "vpc_id" {
  value = aws_vpc.main.id
}

output "public_subnet_ids" {
  value = aws_subnet.public[*].id
}
```

### 5. Using Modules
```hcl
module "vpc" {
  source = "./modules/vpc"

  name                 = "production"
  cidr_block           = "10.0.0.0/16"
  public_subnet_cidrs  = ["10.0.1.0/24", "10.0.2.0/24"]
  availability_zones   = ["us-east-1a", "us-east-1b"]

  tags = {
    Environment = "production"
  }
}

module "compute" {
  source = "./modules/compute"

  vpc_id            = module.vpc.vpc_id
  subnet_ids        = module.vpc.public_subnet_ids
  instance_type     = var.instance_type
  key_name          = var.key_name
}
```

## Advanced Patterns

### 1. Dynamic Blocks
```hcl
resource "aws_security_group" "app" {
  name   = "app-sg"
  vpc_id = var.vpc_id

  dynamic "ingress" {
    for_each = var.ingress_rules
    content {
      from_port   = ingress.value.from_port
      to_port     = ingress.value.to_port
      protocol    = ingress.value.protocol
      cidr_blocks = ingress.value.cidr_blocks
    }
  }
}
```

### 2. For Expressions
```hcl
locals {
  subnet_ids = [for subnet in aws_subnet.private : subnet.id]

  instance_ips = {
    for instance in aws_instance.app :
    instance.tags["Name"] => instance.private_ip
  }
}
```

### 3. Conditional Resources
```hcl
resource "aws_db_instance" "replica" {
  count = var.create_replica ? 1 : 0

  replicate_source_db = aws_db_instance.main.identifier
  instance_class      = var.replica_instance_class
}
```

### 4. Data Sources
```hcl
data "aws_ami" "amazon_linux" {
  most_recent = true
  owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
}

resource "aws_instance" "app" {
  ami           = data.aws_ami.amazon_linux.id
  instance_type = var.instance_type
}
```

### 5. Remote State Data
```hcl
data "terraform_remote_state" "vpc" {
  backend = "s3"
  config = {
    bucket = "my-terraform-state"
    key    = "vpc/terraform.tfstate"
    region = "us-east-1"
  }
}

resource "aws_instance" "app" {
  subnet_id = data.terraform_remote_state.vpc.outputs.subnet_id
}
```

## State Management

```bash
# Initialize
terraform init

# Validate
terraform validate

# Plan
terraform plan -out=tfplan

# Apply
terraform apply tfplan

# Import existing resource
terraform import aws_instance.example i-1234567890

# Show state
terraform show

# List resources
terraform state list

# Move resource
terraform state mv aws_instance.old aws_instance.new

# Remove from state
terraform state rm aws_instance.example

# Refresh state
terraform refresh
```

## Workspaces

```bash
# Create workspace
terraform workspace new production

# List workspaces
terraform workspace list

# Switch workspace
terraform workspace select production

# Delete workspace
terraform workspace delete dev
```

## Testing

```hcl
# Using terratest (Go)
package test

import (
    "testing"
    "github.com/gruntwork-io/terratest/modules/terraform"
)

func TestVPCModule(t *testing.T) {
    terraformOptions := &terraform.Options{
        TerraformDir: "../modules/vpc",
        Vars: map[string]interface{}{
            "name": "test",
            "cidr_block": "10.0.0.0/16",
        },
    }

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

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

This skill ensures maintainable, scalable infrastructure as code.

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