class Patient

  def initialize
    @ds = DATA_SOURCE.new(username, password)
  end
end

class DataSource

  def initialize(username, password)
    @u = username
    @p = password
  end

  def get_patients_from_backend_service
    @patients ||= BackendService.patient_list(@u, @p)
  end
end

class BackendService

  def patient_list(u, p)
    # http request
  end

end