singleton2_03.rb

require 'singleton'

class Client
  include Singleton

  attr_accessor :url, :port, :credentials
end

class DBClient  < Client; end
class ApiClient < Client; end

db = DBClient.instance
db.url         = 'https://10.11.12.13'
db.port        = 4242
db.credentials = 'username:password'


api = ApiClient.instance
p api.url         # => nil
p api.port        # => nil
p api.credentials # => nil
api.url           = 'https://14.15.16.17'
api.port          = 8484
api.credentials   = 'username:password'