terraform

Terraform Expert Engineer Skill

This skill provides a comprehensive guide for Terraform development.

1. Terraform CLI Basic Commands

1.1. Initialization and Planning

# Initialize workspace (download providers)
terraform init

# Initialize with backend config
terraform init -backend-config="bucket=my-terraform-state"

# Check execution plan
terraform plan

# Save execution plan to file
terraform plan -out=tfplan

# Plan specific resource only
terraform plan -target=aws_instance.example

1.2. Apply and Destroy

WARNING: These commands modify infrastructure. Always run terraform plan first and ask user permission.

# Apply changes
terraform apply

# Apply saved plan
terraform apply tfplan

# Auto-approve apply (for CI/CD)
terraform apply -auto-approve

# Destroy resources
terraform destroy

# Destroy specific resource only
terraform destroy -target=aws_instance.example

1.3. State Management

# Check state
terraform state list

# Show resource details
terraform state show aws_instance.example

# Move resource (for refactoring)
terraform state mv aws_instance.old aws_instance.new

# Import existing resource
terraform import aws_instance.example i-1234567890abcdef0

# Remove resource from state (keeps actual resource)
terraform state rm aws_instance.example

1.4. Other Useful Commands

# Validate configuration
terraform validate

# Format
terraform fmt

# Format recursively
terraform fmt -recursive

# Check outputs
terraform output

# Output in JSON format
terraform output -json

# Interactive console (for testing expressions)
terraform console

# Lock providers
terraform providers lock -platform=linux_amd64 -platform=darwin_amd64

2. Resource Management

2.1. Basic Resource Block Structure

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  tags = {
    Name = "example-instance"
  }
}

2.2. Meta-arguments

• depends_on: Explicit dependencies
• count: Resource replication (index-based)
• for_each: Resource replication (key-based)
• provider: Specify alternate provider
• lifecycle: Lifecycle control

2.3. Lifecycle Settings

resource "aws_instance" "example" {
  # ...

  lifecycle {
    create_before_destroy = true  # Create new first on replacement
    prevent_destroy       = true  # Prevent deletion
    ignore_changes        = [tags] # Attributes to ignore changes
    replace_triggered_by  = [null_resource.trigger.id]
  }
}

3. Module Design

3.1. Module Invocation

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"

  name = "my-vpc"
  cidr = "10.0.0.0/16"
}

3.2. Module Source Types

• Local path: ./modules/vpc
• Terraform Registry: hashicorp/consul/aws
• GitHub: github.com/hashicorp/example
• S3: s3::https://s3-eu-west-1.amazonaws.com/bucket/module.zip

3.3. Module Best Practices

• Standard file structure: main.tf, variables.tf, outputs.tf
• Document with README.md
• Set meaningful default values
• Validate inputs with validation blocks

4. State Management

4.1. Remote Backend Configuration

terraform {
  backend "s3" {
    bucket         = "my-terraform-state"
    key            = "prod/terraform.tfstate"
    region         = "ap-northeast-1"
    encrypt        = true
    dynamodb_table = "terraform-locks"
  }
}

4.2. State Management Best Practices

• Use remote backend (required for team development)
• Enable state locking (prevent concurrent execution)
• Enable encryption
• Do not directly edit state file (use terraform state commands)
• Separate state files per environment

5. Variables and Outputs

5.1. Input Variables

variable "instance_type" {
  type        = string
  description = "EC2 instance type"
  default     = "t2.micro"

  validation {
    condition     = contains(["t2.micro", "t2.small", "t2.medium"], var.instance_type)
    error_message = "Please specify an allowed instance type"
  }
}

5.2. Variable Setting Methods (Priority Order)

1. Command line -var, -var-file
2. *.auto.tfvars files
3. terraform.tfvars.json
4. terraform.tfvars
5. Environment variables TF_VAR_*
6. Default values

5.3. Sensitive Data Handling

variable "db_password" {
  type      = string
  sensitive = true  # Mask in output
}

output "connection_string" {
  value     = "postgres://user:${var.db_password}@host/db"
  sensitive = true  # Output contains sensitive data
}

Note: Sensitive data is stored in plaintext in state files. Remote backend encryption or HCP Terraform recommended.

6. Providers

6.1. Provider Declaration

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

provider "aws" {
  region = "ap-northeast-1"
}

6.2. Multiple Providers

provider "aws" {
  alias  = "us_east"
  region = "us-east-1"
}

provider "aws" {
  alias  = "ap_northeast"
  region = "ap-northeast-1"
}

resource "aws_instance" "us" {
  provider = aws.us_east
  # ...
}

7. Built-in Functions

7.1. Common Functions

# String operations
join("-", ["foo", "bar"])          # "foo-bar"
split(",", "a,b,c")                # ["a", "b", "c"]
format("Hello, %s!", "World")      # "Hello, World!"

# Collection operations
length(["a", "b", "c"])            # 3
lookup(map, key, default)          # Get value from map
merge(map1, map2)                  # Merge maps
flatten([["a"], ["b", "c"]])       # ["a", "b", "c"]

# Type conversions
tostring(123)                      # "123"
tolist(set)                        # Set to list
tomap(object)                      # Object to map

# Conditional expressions
coalesce("", "default")            # "default" (first non-empty value)
try(expression, fallback)          # Fallback on error

8. HCP Terraform / Terraform Cloud

8.1. Key Features

• Remote state management (encryption, versioning)
• Team collaboration
• Policy enforcement (Sentinel)
• Private module registry
• VCS integration (GitHub, GitLab, etc.)
• Cost estimation

8.2. Workspace Configuration

terraform {
  cloud {
    organization = "my-org"

    workspaces {
      name = "my-workspace"
    }
  }
}

9. Security Best Practices

9.1. Credential Management

• Do not hardcode
• Use environment variables or auth files
• IAM roles / service accounts recommended
• HashiCorp Vault integration

9.2. State File Security

• Use encrypted backend
• Set appropriate access controls
• Add .terraform/ to .gitignore
• Add *.tfvars to .gitignore (if contains sensitive info)

10. Reference Links

• Official docs: https://developer.hashicorp.com/terraform/docs
• Language reference: https://developer.hashicorp.com/terraform/language
• CLI reference: https://developer.hashicorp.com/terraform/cli
• Provider registry: https://registry.terraform.io/
• Module registry: https://registry.terraform.io/browse/modules
• HCP Terraform: https://developer.hashicorp.com/terraform/cloud-docs