A contrived example for cthulchu on the #ruby channel

tails.rb

require 'singleton'

class TailCounter
  include Singleton

  def initialize
    @tails = 0
    @lock = Mutex.new
  end

  def tails
    @lock.synchronize { @tails }
  end

  def increment n = 1
    @lock.synchronize { @tails += n }
  end

  def decrement n = 1
    @lock.synchronize { @tails -= n }
  end
end

class Dog
  TAIL_COUNTER = TailCounter.instance

  attr_reader :health
  attr_reader :tails

  def initialize health: :alive, tails: 1
    @health = health
    @tails = tails
    TAIL_COUNTER.increment @tails
  end

  def lose_a_tail
    TAIL_COUNTER.decrement
    @tails -= 1
  end

  def grow_a_tail
    TAIL_COUNTER.increment
    @tails += 1
  end

  def pass_away
    return if @health == :dead
    TAIL_COUNTER.decrement @tails
    @health = :dead
  end

  def revive
    return if @health == :alive
    TAIL_COUNTER.increment @tails
    @health = :alive
  end
end

fluffy_muffin = Dog.new tails: 0
glowing_green = Dog.new tails: 4
sam_the_floof = Dog.new tails: 1

TailCounter.instance.tails
#=> 5

sam_the_floof.lose_a_tail
#=> 0

TailCounter.instance.tails
#=> 4

glowing_green.pass_away
#=> :dead

TailCounter.instance.tails
#=> 0

glowing_green.revive
#=> :alive

TailCounter.instance.tails
#=> 4

glowing_green.grow_a_tail
#=> 5

fluffy_muffin.grow_a_tail
#=> 1

TailCounter.instance.tails
#=> 6