Python : How to handle multiple clients and a server

Question

I am implementing a program with a server and multiple clients. All clients send data to the server and a server check out the step of each client. If all client's steps are the same, a server sends new data to all clients to do next step. And it continues this procedure again and again.

However, when I run my program, it cannot communicate each other. Here are my code. Would you give me some hints?

client & server

#client
from socket import *
from sys import *
import time
import stat, os
import glob

# set the socket parameters
host = "localhost"
port = 21567
buf = 1024
data = ''
addr = (host, port)

UDPSock = socket(AF_INET, SOCK_DGRAM)
UDPSock.settimeout(100)

def_msg = "=== TEST ==="

#FILE = open("test.jpg", "w+")
FILE = open("data.txt","w+")

while (1):
  #data, addr = UDPSock.recvfrom(buf)
  print "Sending"
  UDPSock.sendto(def_msg, addr)
  #time.sleep(3)
  data, addr = UDPSock.recvfrom(buf)

  if data == 'done':
   FILE.close()
   break
  FILE.write(data)

  print "Receiving"
  #time.sleep(3)

UDPSock.close()

# server program for nvt

from socket import *
import os, sys, time, glob
#import pygame
import stat

host = 'localhost'
port = 21567
buf = 1024
addr = (host, port)

print 'test server'

UDPSock = socket(AF_INET, SOCK_DGRAM)
UDPSock.bind(addr)

msg = "send txt file to all clients"

#FILE = open("cam.jpg", "r+")
FILE = open("dna.dat","r+")
sending_data = FILE.read()
FILE.close()

tmp_data = sending_data

while (1):
  #UDPSock.listen(1)
  #UDPSock.sendto(msg, addr)
  #FILE = open("gen1000.dat","r+")
  #sending_data = FILE.read()
  #FILE.close()


  #print 'client is at', addr
  data, addr = UDPSock.recvfrom(buf)
  #time.sleep(3)
  print data
  #msg = 'hello'

  # 
  tmp, sending_data = sending_data[:buf-6], sending_data[buf-6:]

  if len(tmp) < 1:
    msg = 'done'
    UDPSock.sendto(msg, addr)
    print "finished"
    sending_data = tmp_data

  UDPSock.sendto(tmp, addr)
  print "sending"
  #time.sleep(3)
UDPSock.close()

Is there any reason to use UDP? I hope you know what UDP is and how it works: en.wikipedia.org/wiki/User_Datagram_Protocol — Ivo Wetzel
– Ivo Wetzel, Commented Jan 10, 2011 at 19:09

John Riselvato · Accepted Answer · 2011-01-10 19:16:23Z

1

A server must perform the sequence socket(), bind(), listen(), accept() (possibly repeating the accept() to service more than one client), while a client only needs the sequence socket(), connect().

Your missing listen() i saw first. Listen for connections made to the socket.

Comments

John · Accepted Answer · 2011-01-10 23:17:40Z

1

Look at this: http://heather.cs.ucdavis.edu/~matloff/Python/PyNet.pdf

It's a very good Python networking tutorial including working examples of a client and server. Now, I'm not an expert on this, but it looks to me like your code is overcomplicated. And what's the deal with all the commented-out lines?

Quote from question:

#UDPSock.listen(1)
#UDPSock.sendto(msg, addr)
#FILE = open("gen1000.dat","r+")
#sending_data = FILE.read()
#FILE.close()

End quote

Those look like some pretty important lines to me.

Also, make sure the computers are connected. From a prompt run:

ping [IP]

where [IP] is the IP address of the other machine(Note: if you're not connected to the same LAN, this becomes much harder as you might then need port forwarding and possibly static IPs).

edited Jan 10, 2011 at 23:17

answered Jan 10, 2011 at 19:42

John

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1 Comment

gmauch Over a year ago

The document referenced on this answer is no longer available. Apart from that the rest of the answer seems more like a comment than an answer.

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