Implement a Cache in Ruby:

class Cache
  def initialize
    @hash = {}
  end

  def [](key)
    @hash[key]
  end

  def []=(key, value)
    @hash[key] = value
  end
end

Usage:

cache = Cache.new

if user = cache["1"]
  user = cache["1"] = User.find("1")
end

user

To make it thread safe, add a Mutex to protect the class from accessing by other threads:

require "thread"

class Cache
  def initialize
    @mutex = Mutex.new
    @hash = {}
  end

  def [](key)
    @mutex.synchronize { @hash[key] }
  end

  def []=(key, value)
    @mutex.synchronize { @hash[key] = value }
  end
end