safe_cart.rb

require 'rubygems'
require 'redis'

redis = Redis.new

redis.set 'items', 10

threads = []

ARGV[0].to_i.times do |time|
  threads << Thread.new(time) do |time|
    redis = Redis.new
    items = redis.decr('items').to_i
    if items > 0
      p "##{time} bought! #{items}"
    elsif items < 0
      p redis.incr 'items'
    end
  end
end

threads.each(&:join)

I don't like the fact that "items" gets negative values