conn.rb

require 'thread'

class ConnectionPool
  
  class Connection
  end

  def initialize
    @mutex = Mutex.new
    @pool = {}
  end

  def current_connection_id
    Thread.current.object_id
  end
  
  def connection
    @mutex.synchronize do
      if @pool[current_connection_id]
        puts "Found existing connection"
        @pool[current_connection_id]
      else
        if @pool.size == 10
          raise "Out of resources"
        else
          puts "Creating new connection"
          @pool[current_connection_id] = Connection.new
          puts @pool.size
        end
      end
    end
  end
end

pool = ConnectionPool.new

11.times do
  Thread.new do
    puts pool.connection
    pool.connection
  end.join
end

conn_checkin.rb

require 'thread'

class ConnectionPool
  
  class Connection
  end

  def initialize
    @mutex = Mutex.new
    @pool = {}
  end

  def current_connection_id
    Thread.current.object_id
  end
  
  def connection
    @mutex.synchronize do
      if @pool[current_connection_id]
        puts "Found existing connection"
        @pool[current_connection_id]
      else
        if @pool.size == 10
          raise "Out of resources"
        else
          puts "Creating new connection"
          @pool[current_connection_id] = Connection.new
          puts @pool.size
        end
      end
    end
  end
  
  def check_in
    @pool.delete(current_connection_id)
  end
end

pool = ConnectionPool.new

20.times do
  Thread.new do
    puts pool.connection
    pool.connection
    pool.check_in
  end.join
end

conn_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
          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}

fibers.rb

f = Fiber.new do
  0.upto(100) do |num|
    Fiber.yield num
  end
end

while (i = f.resume) <= 10
  puts i
end

handler_fibers.rb

require 'fiber'

class Handler
  class Model
    def initialize(params)
    end
  end
  
  def initialize
    @worker = Fiber.new do |blk|
      loop do
        blk.call
        @handler.transfer
      end
    end
    
    @handler = Fiber.new do
      loop do
        create(nil)
      end
    end
  end
  
  def create(params)
    Model.new(params)
    blk = lambda {}
    @worker.transfer blk
  end
  
  def run
    @handler.transfer
  end
end

Handler.new.run

queue.rb

require 'thread'

class Handler
  class Model
    def initialize(params)
    end
  end
  
  def initialize
    @queue = Queue.new
    @worker = Thread.new do
      loop do
        while blk = @queue.pop
          blk.call
        end
      end
    end
  end
  
  def create(params)
    Model.new(params)
    @queue << lambda do
      # do some long-ish calculation
    end
  end
  
  def run
    loop do
      create(nil)
    end
  end
end

Handler.new.run