fibers.rb

require 'socket'
require 'fiber'


#A list of readables and writables for IO.select, and which fiber is waiting for which IO
watch = {
  readable: {}, #{io => fiber}
  writable: {}
}

#Create 5 fibers that asynchronously fetch HTTP from httpbin.org/delay/{1,2,3,4,5}
#httpbin.org/delay will respond after n secs
fibers = 5.times.map do |n|
  Fiber.new do
    sock = TCPSocket.new('httpbin.org', 80)
    #Add the socket to IO.select's writables with this fiber as the watcher
    watch[:writable][sock] = Fiber.current
    #Yield from this fiber so that other fibers can run while we're waiting
    Fiber.yield
    #Fiber has now been resumed by the IO loop, and sock is writable
    sock.write "GET /delay/#{n} HTTP/1.0\r\nHost: httpbin.org\r\n\r\n"
    res = ''
    loop do
      #Yield until sock is readable
      watch[:readable][sock] = Fiber.current
      Fiber.yield
      if data = sock.read_nonblock(1024) rescue break
        res << data 
      else
        break #done reading response
      end
    end
    p res
  end
end.each do |f|
  f.resume #Start fiber
end

loop do
  readables, writables, _ = IO.select watch[:readable].keys, watch[:writable].keys

  readables.each do |readable|
    if f = watch[:readable].delete(readable)
      f.resume(readable)
    end
  end

  writables.each do |writable|
    if f = watch[:writable].delete(writable)
      f.resume(writable)
    end
  end
end