Terragrunt DRY Terraform Guide

environments/
├── dev/
│   ├── main.tf      # 90% identical to staging
│   ├── variables.tf
│   └── backend.tf   # Different state bucket
├── staging/
│   ├── main.tf      # 90% identical to production
│   ├── variables.tf
│   └── backend.tf
└── production/
    ├── main.tf
    ├── variables.tf
    └── backend.tf

live/
├── terragrunt.hcl          # Root config (backend, provider)
├── dev/
│   └── terragrunt.hcl      # Environment-specific values
├── staging/
│   └── terragrunt.hcl
└── production/
    └── terragrunt.hcl

modules/
└── app-infrastructure/     # Single Terraform module
    ├── main.tf
    ├── variables.tf
    └── outputs.tf

# live/terragrunt.hcl
remote_state {
  backend = "s3"
  generate = {
    path      = "backend.tf"
    if_exists = "overwrite"
  }
  config = {
    bucket         = "myorg-terraform-state"
    key            = "${path_relative_to_include()}/terraform.tfstate"
    region         = "eu-west-1"
    encrypt        = true
    dynamodb_table = "terraform-locks"
  }
}

generate "provider" {
  path      = "provider.tf"
  if_exists = "overwrite"
  contents  = <<EOF
provider "aws" {
  region = "eu-west-1"
}
EOF
}

# live/dev/terragrunt.hcl
include "root" {
  path = find_in_parent_folders()
}

terraform {
  source = "../../modules/app-infrastructure"
}

inputs = {
  environment    = "dev"
  instance_type  = "t3.small"
  instance_count = 1
  enable_cdn     = false
}

# live/production/terragrunt.hcl
include "root" {
  path = find_in_parent_folders()
}

terraform {
  source = "../../modules/app-infrastructure"
}

inputs = {
  environment    = "production"
  instance_type  = "t3.large"
  instance_count = 3
  enable_cdn     = true
}

# live/production/app/terragrunt.hcl
dependency "vpc" {
  config_path = "../vpc"
}

dependency "database" {
  config_path = "../database"
}

inputs = {
  vpc_id     = dependency.vpc.outputs.vpc_id
  subnet_ids = dependency.vpc.outputs.private_subnet_ids
  db_url     = dependency.database.outputs.connection_string
}

# Apply everything in the right order
cd live/production
terragrunt run-all apply

# Apply a single environment
cd live/dev
terragrunt apply

# Apply all environments
cd live
terragrunt run-all apply

# Plan across all environments
terragrunt run-all plan

# Destroy in reverse dependency order
terragrunt run-all destroy

# live/terragrunt.hcl
locals {
  environment = basename(get_terragrunt_dir())
  
  env_config = {
    dev = {
      instance_type = "t3.small"
      min_size      = 1
    }
    staging = {
      instance_type = "t3.medium"
      min_size      = 2
    }
    production = {
      instance_type = "t3.large"
      min_size      = 3
    }
  }
}

inputs = local.env_config[local.environment]

terraform {
  before_hook "validate" {
    commands = ["apply", "plan"]
    execute  = ["tflint", "--init"]
  }

  after_hook "notify" {
    commands = ["apply"]
    execute  = ["slack-notify", "Terraform apply completed"]
  }
}