I wanted to write a server that a client could connect to and receive periodic updates without having to poll. The problem I have experienced with asyncore is that if you do not return true when dispatcher.writable() is called, you have to wait until after the asyncore.loop has timed out (default is 30s).

The two ways I have tried to work around this is 1) reduce timeout to a low value or 2) query connections for when they will next update and generate an adequate timeout value. However if you refer to 'Select Law' in 'man 2 select_tut', it states, "You should always try to use select() without a timeout."

Is there a better way to do this? Twisted maybe? I wanted to try and avoid extra threads. I'll include the variable timeout example here:

#!/usr/bin/python

import time
import socket
import asyncore


# in seconds
UPDATE_PERIOD = 4.0

class Channel(asyncore.dispatcher):

    def __init__(self, sock, sck_map):
        asyncore.dispatcher.__init__(self, sock=sock, map=sck_map)
        self.last_update = 0.0  # should update immediately
        self.send_buf = ''
        self.recv_buf = ''

    def writable(self):
        return len(self.send_buf) > 0

    def handle_write(self):
        nbytes = self.send(self.send_buf)
        self.send_buf = self.send_buf[nbytes:]

    def handle_read(self):
        print 'read'
        print 'recv:', self.recv(4096)

    def handle_close(self):
        print 'close'
        self.close()

    # added for variable timeout
    def update(self):
        if time.time() >= self.next_update():
            self.send_buf += 'hello %f\n'%(time.time())
            self.last_update = time.time()

    def next_update(self):
        return self.last_update + UPDATE_PERIOD


class Server(asyncore.dispatcher):

    def __init__(self, port, sck_map):
        asyncore.dispatcher.__init__(self, map=sck_map)
        self.port = port
        self.sck_map = sck_map
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.bind( ("", port))
        self.listen(16)
        print "listening on port", self.port

    def handle_accept(self):
        (conn, addr) = self.accept()
        Channel(sock=conn, sck_map=self.sck_map)

    # added for variable timeout
    def update(self):
        pass

    def next_update(self):
        return None


sck_map = {}

server = Server(9090, sck_map)
while True:
    next_update = time.time() + 30.0
    for c in sck_map.values():
        c.update()  # <-- fill write buffers
        n = c.next_update()
        #print 'n:',n
        if n is not None:
            next_update = min(next_update, n)
    _timeout = max(0.1, next_update - time.time())

    asyncore.loop(timeout=_timeout, count=1, map=sck_map)