把訂單流程從單體 Rails 拆成獨立服務

create table orders (
  id           uuid primary key default gen_random_uuid(),
  customer_id  uuid not null,
  total_cents  bigint not null check (total_cents >= 0),
  status       text not null default 'pending',
  created_at   timestamptz not null default now(),
  updated_at   timestamptz not null default now()
);

create table order_outbox (
  id          bigserial primary key,
  aggregate   uuid not null,           -- order id
  event_type  text not null,
  payload     jsonb not null,
  created_at  timestamptz not null default now(),
  sent_at     timestamptz                  -- relay 寫回
);

create index outbox_unsent
  on order_outbox (id) where sent_at is null;

class OrderWriter
  def create(params)
    legacy = Order.create!(params)         # 主路徑、必成功

    # 影子寫入：失敗只記 log、不擋使用者
    Thread.new do
      begin
        shadow = OrdersClient.create(
          customer_id: legacy.customer_id,
          total_cents: legacy.total_cents,
          idempotency_key: legacy.id     # 用舊 id 當去重
        )
        ShadowDiff.record(legacy, shadow)
      rescue => e
        Metrics.incr("orders.shadow.error",
                     tags: [e.class.name])
      end
    end

    legacy
  end
end