Mutex Inventory

A mutex keeps shared state updates consistent when more than one thread changes the same object.

mutex_inventory.rb

orders_per_worker = 
stock = { "book" => 10 }
lock = Mutex.new

workers = 2.times.map do
  Thread.new do
    orders_per_worker.times do
      lock.synchronize do
        current = stock["book"]
        stock["book"] = current - 1
      end
    end
  end
end

workers.each(&:join)

sold = 10 - stock["book"]
puts "orders_per_worker=#{orders_per_worker}"
puts "sold=#{sold}"
puts "remaining=#{stock["book"]}"

orders_per_worker = 
stock = { "book" => 10 }
lock = Mutex.new

workers = 2.times.map do
  Thread.new do
    orders_per_worker.times do
      lock.synchronize do
        current = stock["book"]
        stock["book"] = current - 1
      end
    end
  end
end

workers.each(&:join)

sold = 10 - stock["book"]
puts "orders_per_worker=#{orders_per_worker}"
puts "sold=#{sold}"
puts "remaining=#{stock["book"]}"

orders_per_worker = 
stock = { "book" => 10 }
lock = Mutex.new

workers = 2.times.map do
  Thread.new do
    orders_per_worker.times do
      lock.synchronize do
        current = stock["book"]
        stock["book"] = current - 1
      end
    end
  end
end

workers.each(&:join)

sold = 10 - stock["book"]
puts "orders_per_worker=#{orders_per_worker}"
puts "sold=#{sold}"
puts "remaining=#{stock["book"]}"

mutex updates Wrap the read-modify-write section with `synchronize` so one thread updates the inventory at a time.