境界づけられたコンテキスト — ドメインを分割する

class Product < ApplicationRecord
  # カタログ情報
  has_many :product_images
  has_many :product_reviews
  has_many :product_tags
  validates :name, :description, presence: true
 
  # 在庫情報
  has_one :stock_info
  has_many :stock_reservations
  def available_quantity; end
  def reserve_stock!(quantity); end
 
  # 価格・税
  has_many :price_histories
  def current_price; end
  def price_with_tax; end
  def apply_discount(rate); end
 
  # 配送情報
  def shipping_weight; end
  def can_ship_to?(address); end
  def estimated_delivery_days; end
 
  # SEO
  def seo_title; end
  def seo_description; end
  def structured_data; end
 
  # 検索
  after_save :update_search_index
  def search_keywords; end
 
  # ... 2000行続く
end

# 注文処理コンテキスト — FreshCartの核心
module OrderContext
  class Order
    attr_reader :id, :customer_id, :status, :order_items
 
    def initialize(id:, customer_id:, delivery_address:)
      @id = id
      @customer_id = customer_id
      @delivery_address = delivery_address
      @status = OrderStatus::PENDING
      @order_items = []
      @domain_events = []
    end
 
    def confirm!
      validate_can_confirm!
      @status = OrderStatus::CONFIRMED
      @confirmed_at = Time.current
      record_event(OrderConfirmed.new(
        order_id: id,
        customer_id: customer_id,
        order_items: order_items,
        confirmed_at: @confirmed_at
      ))
      self
    end
 
    def total_amount
      order_items.sum(&:subtotal)
    end
 
    private
 
    def validate_can_confirm!
      raise InvalidStateTransition, "#{status}状態からは確定できません" unless pending?
      raise EmptyOrder, "注文商品が1つもありません" if order_items.empty?
    end
  end
end

# 在庫管理コンテキスト — 注文処理を支える
module InventoryContext
  class StockItem
    attr_reader :sku, :product_name, :total_quantity, :warehouse_location
 
    def initialize(sku:, product_name:, total_quantity:, warehouse_location:)
      @sku = sku
      @product_name = product_name
      @total_quantity = total_quantity
      @warehouse_location = warehouse_location
      @reservations = []
    end
 
    def available_quantity
      total_quantity - reserved_quantity
    end
 
    def reserve!(quantity:, order_id:)
      raise InsufficientStock, "在庫不足: #{available_quantity}" if available_quantity < quantity
 
      @reservations << StockReservation.new(
        order_id: order_id,
        quantity: quantity,
        reserved_at: Time.current
      )
      self
    end
 
    def release_reservation!(order_id:)
      reservation = @reservations.find { |r| r.order_id == order_id }
      raise ReservationNotFound, "引き当てが見つかりません" unless reservation
      @reservations.delete(reservation)
      self
    end
 
    private
 
    def reserved_quantity
      @reservations.sum(&:quantity)
    end
  end
end

# 通知コンテキスト — 汎用的な通知機能（外部サービスに委譲）
module NotificationContext
  class EmailNotification
    def initialize(provider: SendGridProvider.new)
      @provider = provider
    end
 
    def send(to:, template_id:, template_data:)
      @provider.send_email(
        to: to,
        template_id: template_id,
        data: template_data
      )
    end
  end
 
  class PushNotification
    def initialize(provider: FirebaseProvider.new)
      @provider = provider
    end
 
    def send(device_token:, title:, body:, data: {})
      @provider.send_push(
        token: device_token,
        notification: { title: title, body: body },
        data: data
      )
    end
  end
end

# NG: 注文処理コンテキストが在庫コンテキストのモデルを直接参照
module OrderContext
  class Order
    def confirm!
      # これはNG: 別コンテキストのオブジェクトを直接操作
      InventoryContext::StockItem.find_by(sku: product_sku).reserve!(quantity)
    end
  end
end
 
# OK: コンテキスト間の依存はサービス層を通じて行う
module OrderContext
  class ConfirmOrderUseCase
    def initialize(order_repository:, stock_reservation_service:)
      @order_repository = order_repository
      @stock_reservation_service = stock_reservation_service
    end
 
    def call(order_id:)
      order = @order_repository.find(order_id)
      order.confirm!
 
      # 別コンテキストへの操作はユースケース層で調整
      @stock_reservation_service.reserve_for_order(order)
 
      @order_repository.save(order)
    end
  end
end

# 注文処理コンテキストが商品カタログコンテキストのデータを使う際のACL
module OrderContext
  class CatalogAdapter
    def initialize(catalog_service: CatalogContext::ProductCatalogService.new)
      @catalog_service = catalog_service
    end
 
    # 外部の "CatalogContext::Product" を OrderContext の "OrderableProduct" に変換
    def find_orderable_product(product_id)
      catalog_product = @catalog_service.find_active_product(product_id)
      raise ProductNotAvailable, "商品が現在注文できません" unless catalog_product
 
      # 翻訳: カタログコンテキストの語彙 → 注文コンテキストの語彙
      OrderableProduct.new(
        id: catalog_product.id,
        name: catalog_product.display_name,        # display_name → name
        unit_price: Money.new(
          amount: catalog_product.price_jpy,       # price_jpy → unit_price
          currency: :jpy
        ),
        taxable: catalog_product.tax_category != 'food_exempt',
        available: catalog_product.currently_available?
      )
    end
  end
 
  # 注文コンテキスト内の語彙で定義された商品表現
  class OrderableProduct
    attr_reader :id, :name, :unit_price, :taxable
 
    def initialize(id:, name:, unit_price:, taxable:, available:)
      @id = id
      @name = name
      @unit_price = unit_price
      @taxable = taxable
      @available = available
      freeze
    end
 
    def price_with_tax
      taxable ? unit_price.multiply(1.1) : unit_price
    end
 
    def available?
      @available
    end
  end
end

# モノリス時のコンテキスト間通信（同一プロセス内）
module OrderContext
  class CatalogAdapter
    def initialize(catalog_service: CatalogContext::ProductCatalogService.new)
      @catalog_service = catalog_service  # 同一プロセス内のRubyオブジェクト
    end
  end
end
 
# マイクロサービス化後（HTTP通信に変わる）
module OrderContext
  class CatalogAdapter
    def initialize(catalog_client: CatalogServiceHttpClient.new)
      @catalog_client = catalog_client  # HTTP通信クライアントに差し替え
    end
    # インターフェースは同じ。ドメインロジックは変更なし
  end
end
 
# HTTP通信の実装のみが変わる
class CatalogServiceHttpClient
  def find_active_product(product_id)
    response = Faraday.get("#{ENV['CATALOG_SERVICE_URL']}/products/#{product_id}")
    return nil if response.status == 404
    CatalogProductDto.from_json(response.body)
  end
end

# 共有カーネルの例: Moneyは全コンテキストで使う
module SharedKernel
  class Money
    attr_reader :amount, :currency
 
    def initialize(amount:, currency: :jpy)
      @amount = Integer(amount)
      @currency = currency.to_sym
      freeze
    end
 
    def add(other)
      raise CurrencyMismatch unless same_currency?(other)
      Money.new(amount: amount + other.amount, currency: currency)
    end
 
    def multiply(factor)
      Money.new(amount: (amount * factor).ceil, currency: currency)
    end
 
    def ==(other)
      other.is_a?(Money) && amount == other.amount && currency == other.currency
    end
 
    def to_s
      "#{amount}#{currency.upcase}"
    end
 
    private
 
    def same_currency?(other)
      currency == other.currency
    end
  end
end

# コンテキストごとに異なるProductの表現
module CatalogContext
  class Product  # 約150行: 商品説明、画像、レビューのみ
    attr_reader :id, :name, :description, :images, :review_score
  end
end
 
module InventoryContext
  class StockItem  # 約120行: 在庫数、ロット、賞味期限のみ
    attr_reader :sku, :product_id, :quantity, :lot_number, :expiry_date
  end
end
 
module OrderContext
  class OrderableProduct  # 約80行: 注文に必要な情報のみ
    attr_reader :id, :name, :unit_price, :taxable
  end
end
 
module ShippingContext
  class ShippableItem  # 約100行: 配送に必要な情報のみ
    attr_reader :product_id, :weight_grams, :dimensions, :shipping_restrictions
  end
end