Question

I am trying to implement a simple UDP client and server. Server should receive a message and return a transformed one.

My main technique for server is to listen UDP messages in a loop, then spawn multiprocessing.Process for each incoming message and send the reply within each Process instance:

class InputProcessor(Process):
    ...

    def run(self):
        output = self.process_input()
        self.sock.sendto(output, self.addr) # send a reply

if __name__ == "__main__":
    print "serving at %s:%s" % (UDP_IP, UDP_PORT)

    sock = socket.socket(socket.AF_INET,    # Internet
                         socket.SOCK_DGRAM) # UDP
    sock.bind((UDP_IP,UDP_PORT))

    while True:
        data, addr = sock.recvfrom(1024) # buffer size is 1024 bytes
        print "received message: %s from %s:%s" % (data, addr[0], addr[1])
        p = InputProcessor(sock, data, addr)
        p.start()

In test client, I do something like this:

def send_message(ip, port, data):
    sock = socket.socket(socket.AF_INET,    # Internet
                         socket.SOCK_DGRAM) # UDP
    print "sending: %s" % data
    sock.sendto(data, (ip, port))
    sock.close()


for i in xrange(SECONDS*REQUESTS_PER_SECOND):
    data = generate_data()
    p = multiprocessing.Process(target=send_message, args=(UDP_IP,
                                                           UDP_PORT,
                                                           data))
    p.start()
    time.sleep(1/REQUESTS_PER_SECOND)

The problem I am having with the code above is that when REQUESTS_PER_SECOND becomes higher than certain value (~50), it seems some client processes receive responses destinated to different processes, i.e. process #1 receives response for process #2, and vice versa.

Please criticize my code as much as possible, due to I am new to network programming and may miss something obvious. Maybe it’s even worth and better for some reason to use Twisted, hovewer, I am highly interested in understanding the internals. Thanks.

Answer 1

Thanks guys a lot! It seems I’ve found why my code failed before. I was using multiprocessing.Manager().dict() within client to check if the results from server are correct. However, I didn’t use any locks to wrap a set of write operations to that dict(), thus got a lot of errors though the output from server was correct.

Shortly, in client, I was doing incorrect checks for correct server responses.