limited_polling_workers.rb

class Job
  def self.fetch
    return new if rand(100) > 85 # only return a job 15% of the time
  end

  attr_reader :count

  def initialize
    @count = rand(5) # some random data
  end
end


class Worker
  def run
    @starting = true
    @terminating = false
    @thread = Thread.new do
      work(poll)
    end
    self
  end

  def join
    return unless defined?(@thread)
    @terminating = true
    @thread.join
  end

  def starting?
    @starting
  end

  def done?
    defined?(@thread) && ! @thread.status
  end

  private

  def poll
    until @terminating || job = Job.fetch
      sleep 0.25 # polling delay
    end
    job
  end

  def work(job)
    @starting = false
    job&.count&.times do
      break if @terminating
      sleep 0.25 # fake work
    end
  end
end

#
#  The main body of script
#

Signal.trap("SIGINT") { throw :done }

MAX_WORKERS = 10
workers = []

catch :done do
  loop do

    workers.delete_if {|w| w.done? }

    if workers.none?{|w| w.starting? } && workers.count < MAX_WORKERS
      workers << Worker.new.run
    end

    sleep 0.25
  end
end

workers.each(&:join)