connection_pool.rb

require 'redis'
class Connpool
  DEFAULTS = { size: 5 }
  def initialize(options = {}, &block)
    raise ArgumentError, 'Conn pool requires a block' unless block

    options = DEFAULTS.merge(options)
    @size = options.fetch(:size)
    @available = Queue.new

    @size.times do
      @available.push block.call
    end
    @connection = nil
  end

  def with
    @connection = @available.pop
    yield conn
  ensure
    @available.push @connection
  end
end

$pool = Connpool.new(size: 2) { Redis.new }

t1 = Thread.new do
  $pool.with do |conn|
    sleep 20
    conn.set("name", "joy")
  end
end

t2 = Thread.new do
  $pool.with do |conn|
    sleep 20
    conn.set("age", "20")
  end
end

sleep 2
puts "after sleep"
$pool.with do |conn|
  puts "should block here"
  conn.set("hello", "word")
end

t1.join
t2.join