class StateTrack
  def initialize
    @state = :initialized
  end

  # Expected state transition:
  # :initialized -> :starting -> :running -> :shuttingdown -> :terminated
  def process_event(ev, *args)
    case [ev, @state]
    when [:on_start, :initialized]
      @state = :starting
    when [:on_run, :starting]
      @state = :running
    when [:on_shutdown, :running]
      @state = :shuttingdown
    when [:on_terminate, :shuttingdown]
      @state = :terminated
    else
      raise "Unknown state transition: #{ev} #{@state}"
    end
  end
end

s = new StateTrack

s.process_event(:on_start)
s.process_event(:on_run)
s.process_event(:on_shutdown)
s.process_event(:on_terminate)

s2 = new StateTrack
s.process_event(:on_start)
s.process_event(:on_shutdown) # raise error