nada.rb

define array_scaling

  #
  # making sure some initial fields are set

  precondition
    server_array_href type: string, empty: false

  #
  # body of the process

  repeat

    server_array list_instances
      # sets the 'instances' workitem field

    make_scaling_decision
      # has to be provided by the user, as a participant

    subprocess 'scale_${scaling_direction}', if: '${scaling_count} != 0'
      # 'scale_up' or 'scale_down'

    wait '${scaling_rest_time}'
      # this field was computed by 'make_scaling_decision'

  #
  # default/suggested implementations for the subflows
  #
  # may be overriden by the user

  define scale_up

    citerator times: '$f:scaling_count', merge_type: union
      server_array launch_next_instance, id: '${v:i}'

  define scale_down

    server_array select_oldest_instances

    citerator on: '${oldest_instances}', to: instance_href
      instance terminate