AWSマルチAZリソースのタグ付け戦略をTerraformで実装する方法｜default_tags・モジュール設計

この記事のポイント

・aws providerのdefault_tagsで全リソースに共通タグを自動付与できる
・環境・システム・コスト配分の3軸タグ体系でCost Explorer精度が向上する
・variablesとtfvarsを組み合わせてAZ別リソースのタグを一貫管理できる
・tags = {}上書きによってdefault_tagsが消えるトラブルへの対処法も解説する

# provider.tf（設定例）
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

provider "aws" {
  region = var.aws_region

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

# aws_vpc定義例（設定例）
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_support   = true
  enable_dns_hostnames = true

  tags = {
    Name = "${var.project_name}-vpc"
    Tier = "network"
  }
  # default_tagsのProject/Environment/ManagedBy/CostCenterが自動マージされる
}

# variables.tf（設定例）
variable "aws_region" {
  description = "AWSリージョン"
  type        = string
  default     = "ap-northeast-1"
}

variable "project_name" {
  description = "プロジェクト名（タグに使用）"
  type        = string
}

variable "environment" {
  description = "環境名 (dev/stg/prod)"
  type        = string
  validation {
    condition     = contains(["dev", "stg", "prod"], var.environment)
    error_message = "environmentはdev・stg・prodのいずれかを指定してください。"
  }
}

variable "cost_center" {
  description = "コスト配分コード"
  type        = string
}

# prod.tfvars（本番環境設定例）
aws_region   = "ap-northeast-1"
project_name = "webservice"
environment  = "prod"
cost_center  = "cc-infra-01"

# network.tf（設定例）
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_support   = true
  enable_dns_hostnames = true

  tags = {
    Name = "${var.project_name}-vpc"
    Tier = "network"
  }
}

resource "aws_subnet" "public_a" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.1.0/24"
  availability_zone = "${var.aws_region}a"

  tags = {
    Name = "${var.project_name}-public-subnet-1a"
    Tier = "public"
    AZ   = "${var.aws_region}a"
  }
}

resource "aws_subnet" "public_c" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.2.0/24"
  availability_zone = "${var.aws_region}c"

  tags = {
    Name = "${var.project_name}-public-subnet-1c"
    Tier = "public"
    AZ   = "${var.aws_region}c"
  }
}

# ec2.tf（設定例）
data "aws_ami" "amazon_linux_2023" {
  most_recent = true
  owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["al2023-ami-*-x86_64"]
  }
}

resource "aws_instance" "web_1a" {
  ami           = data.aws_ami.amazon_linux_2023.id
  instance_type = "t3.micro"
  subnet_id     = aws_subnet.public_a.id

  tags = {
    Name = "${var.project_name}-web-1a"
    Role = "web"
    AZ   = "${var.aws_region}a"
  }
  # default_tagsのProject/Environment/ManagedBy/CostCenterが自動付与される
}

resource "aws_instance" "web_1c" {
  ami           = data.aws_ami.amazon_linux_2023.id
  instance_type = "t3.micro"
  subnet_id     = aws_subnet.public_c.id

  tags = {
    Name = "${var.project_name}-web-1c"
    Role = "web"
    AZ   = "${var.aws_region}c"
  }
}

# 別リージョンのprovider aliasにもdefault_tagsを設定する例
provider "aws" {
  alias  = "us_east_1"
  region = "us-east-1"

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

# NGパターン（default_tagsが消えるケース）
variable "extra_tags" {
  type    = map(string)
  default = {}           # デフォルト{}はNG
}

# OKパターン（nullを使う）
variable "extra_tags" {
  type    = map(string)
  default = null         # nullにするとtagsを省略した扱いになる
}