es.md

Aggregate

class Ship
  def load_cargo(name)
    apply :cargo_loaded, :name => name
  end

  def paint(color)
    apply :painted, :color => color
  end

  private
  def apply_cargo_loaded(e)
    @cargo = e[:name]
  end

  def apply_painted(e)
    @paint_color = e[:color]
  end
end

Scenario

• Two LoadCargo commands(C1, C2) for Ship aggregate "abc" come in at the same time.
• The command handlers(async) both load up the ship to the latest version (let's say 5).
• C1 completes before C2 (making the aggregate version 6 at persistence level).
• When C2 commits, a ConcurrencyException is thrown due to C2 only knowing about events up to version 5.
• The ship's event stream repopulates up to 6.
• The uncommitted events are compared against the newly loaded events. Is this comparison better as a blacklist or whitelist?
• If there is a conflict, throw ConflictException and C2 never completes, otherwise, attempt C2 again.