pool_monitor.rb

require 'monitor'

class ConnectionPool
  class Connection
  end

  def initialize
    @mutex = Monitor.new
    @condition = @mutex.new_cond
    @pool = {}
  end

  def current_connection_id
    Thread.current.object_id
  end
  
  def connection
    @mutex.synchronize do
      loop do
        if @pool[current_connection_id]
          return @pool[current_connection_id]
        else
          if @pool.size == 10
            @condition.wait_while {@pool.size == 10}
          else
            @pool[current_connection_id] = Connection.new
            return @pool[current_connection_id]
          end
        end
      end
    end
  end
  
  def check_in
    @mutex.synchronize do
      @pool.delete(current_connection_id)
      @condition.signal
    end
  end
end

pool = ConnectionPool.new

threads = []
200.times do
  threads << Thread.new do
    pool.connection
    i = 0
    while i < 1000000
      i+=1
    end
    pool.check_in
  end
end

threads.each {|t| t.join}