Client(1) to Server to Client(2) in Python 3.x (Sockets?)

Question

I've been working with a project that involves sending information to a public server (to demonstrate how key-exchange schemes work) and then sending it to a specific client. There is only two clients.

I'm hoping to get pushed in the right direction on how to get information from client(1) to the server, then have the server redirect that information to client(2). I've messed with the code somewhat, getting comfortable with how to send and recieve information from the server, but I have no idea (~2 hours of research so far) how to differentiate clients and send information to specific clients My current server code (pretty much unchanged from the python3 docs:

import socketserver

class MyTCPHandler(socketserver.BaseRequestHandler):
"""
The RequestHandler class for our server.

It is instantiated once per connection to the server, and must
override the handle() method to implement communication to the
client.
"""

def handle(self):
    # self.request is the TCP socket connected to the client
    self.data = self.request.recv(1024).strip()
    print("{} wrote:".format(self.client_address[0]))
    print(self.data)
    # just send back the same data, but upper-cased
    self.request.sendall(self.data.upper())

if __name__ == "__main__":
    HOST, PORT = "localhost", 9999

# Create the server, binding to localhost on port 9999
server = socketserver.TCPServer((HOST, PORT), MyTCPHandler)

# Activate the server; this will keep running until you
# interrupt the program with Ctrl-C
server.serve_forever()

My client code (pretty much unchanged from the python3 docs:

import socket
import time

data = "matt is ok"

def contactserver(data):
    HOST, PORT = "localhost", 9999
    # Create a socket (SOCK_STREAM means a TCP socket)
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

     # Connect to server and send data
    sock.connect((HOST, PORT))
    sock.sendall(bytes(data, "utf-8"))

   # Receive data from the server and shut down
    received = str(sock.recv(1024), "utf-8")
    print("Sent:     {}".format(data))
    print("Received: {}".format(received))
    return format(received)

while True:
    k = contactserver('banana')
    time.sleep(1)
print(k)

Answer 1

First, a base socketserver.TCPServer can't even talk to two clients at the same time. As the docs explain:

These four classes process requests synchronously; each request must be completed before the next request can be started.

As the same paragraph tells you, you can solve that problem by using a forking or threading mix-in. That's pretty easy.

---

But there's a bigger problem. A threaded socketserver server creates a separate, completely independent, object for each connected client, and has no means of communicating between them, or even letting them find out about each other. So, what can you do?

You can always build it yourself. Put some kind of shared data somewhere, and some kind of synchronization on it, and all of the threads can talk to each other the same way any threads can, socketserver or otherwise.

For your design, a queue has all the magic built in for everything we need: client 1 can put a message on the queue (whether client 2 has shown up yet or not), and client 2 can get a message off the same queue (automatically waiting around if the message isn't there yet), and it's all automatically synchronized.

The big question is: how does the server know who's client 1 and who's client 2? Unless you want to switch based on address and port, or add some kind of "login" mechanism, the only rule I can think of is that whoever connects first is client 1, whoever connects second is client 2, and anyone who connects after that, who cares, they don't belong here. So, we can use a simple shared flag with a Lock on it.

Putting it all together:

class MyTCPHandler(socketserver.ThreadingMixIn, socketserver.BaseRequestHandler):
    q = queue.queue()
    got_first = False
    got_first_lock = threading.Lock()
    def handle_request(self):
        with MyTCPHandler.got_first_lock:
            if MyTCPHandler.got_first:
                 first = False
            else:
                 first = True
                 MyTCPHandler.got_first = True
        if first:
            self.data = self.request.recv(1024).strip()
            print("{} wrote:".format(self.client_address[0]))
            print(self.data)
            # just send back the same data, but upper-cased
            self.request.sendall(self.data.upper())
            # and also queue it up for client 2
            MyTCPHandler.q.put(self.data)
        else:
            # get the message off the queue, waiting if necessary
            self.data = MyTCPHandler.q.get()
            self.request.sendall(self.data)

If you want to build a more complicated chat server, where everyone talks to everyone… well, that gets a bit more complicated, and you're stretching socketserver even farther beyond its intended limits.

I would suggest either (a) dropping to a lower level and writing a threaded or multiplexing server manually, or (b) going to a higher-level, more-powerful framework that can more easily handle interdependent clients.

The stdlib comes with a few alternatives for writing servers, but all of them suck except for asyncio—which is great, but unfortunately brand new (it requires 3.4, which is still in beta, or can be installed as a back-port for 3.3). If you don't want to skate on the bleeding edge, there are some great third-party choices like twisted or gevent. All of these options have a higher learning curve than socketserver, but that's only to be expected from something much more flexible and powerful.