User `constraints` to specify the subdomain name for api portion and can introduce load balancing or increase number of api server needed based on its usage.

```ruby
Rails.application.routes.draw do
  namespace :api, path: '/', constraints: { subdomain: 'api' } do
    resources :users
  end
end
```

`namespace: api, path: '/'` => the `namespace` by default will put the namespace value in the url (e.g: `localhost:3000/api`), but when specify the `path` is '/', there will be no `/api` in the url.

This is good because we can group our codes in `api` folder and can use api subdomain.