fixed_queue.rb

require 'thread'
require 'logger'

POOL_SIZE = 10 # limit for HTTP parallel connections
QUEUE_SIZE = 5 # limit to consume only fixed amount of memory

queue = SizedQueue.new(QUEUE_SIZE)
logger = Logger.new($stdout)
END_MESSAGE = :done

threads = POOL_SIZE.times.map do |t|
  Thread.new do
    10.times do |i|
      # imagine HTTP request here I want to do in parallel and pushing result to the queue
      queue.push(:hello)
      logger.info "T#{t} pushed message #{i}"
      sleep rand
    end
    queue.push(END_MESSAGE)
    logger.info "T#{t} done"
  end
end

finished = 0
consumed = 0

loop do
  message = queue.pop
  if message == END_MESSAGE
    finished = finished + 1
    logger.info "done, finished #{finished} threads"
    break if finished == pool_size
  else
    consumed = consumed + 1
    logger.info "pop, consumed #{consumed} messages"
  end
end

logger.info "all done, consumed #{consumed} messages, finished #{finished} threads"