Handling multiple different requests and responses in client-server Chat Program - networking

Question

I've successfully written a UDP client-server chat application but my way of handling requests and responses is hacky and not very scalable. The server basically listens for messages coming in and then runs some code depending on the message type:
if command == "CONN":
# handle new connection from client then send "OK"
if command == "MSG":
# send message to other connected clients
...
I'm happy with the design of the server but the client is really fiddly.
Here's a sample of the commands the client can send from the server:
Command Name | Argument | Outcome/Description
------------------------------------------------------------------------------
CONN | username | OK, ERR, or timeout if server isn't running
MSG | message | -
USRS | - | ["username1", "username2"]
QUIT | - | -
And receive from the server:
USRC | username | new user connected
USRD | username | user disconnected
MSG | username, message | print message from user
SHDW | - | server shut down
Basically I'm having trouble building a system that will handle these different sets of commands and responses. I know I have a state machine of sorts and can conceptualize a solution in my head, I just don't seem to be able to translate this to anything other than:
socket.send("CONN username")
if response == "OK":
# connected to the server ok
if response == "ERR":
# oops, there was a problem of sorts
# otherwise handle timeout
socket.send("USRS")
if response == "":
# no other users connected
else:
# print users
# start main listening loop
while True:
# send typed text as MSG
# handle any messages received from the server on separate thread
Any help appreciated and apologies for the weird python-esqe pseudocode.

Answer 1

To make things more scalable, your application could benefit from using multiple threads on both client side and the server side. Be sure to use locks when handling common data.
First, the client side could certainly benefit from using three threads. The first thread can listen for input from server (the recvfrom() call). The second thread can listen for input from the user and put these messages in a queue. The third thread can process messages from the queue and call socket.send() to send those messages to the server.
Since the server is handling multiple clients, it could also benefit from having threads to listen for messages from the client and to process them. Once again, you could use one thread to get messages from the client and then queue them. You can use second thread to process the received messages (make sure to store client information) and call sendto() to send responese; btw, recvfrom() does provide client information.