Member Avatar for NewbieXcellence

Hello... I've been working on a simple server using the sockets module and threads (just for fun btw ;p). I want to be able to connect to it by specifying a "channel" and then anything the client sends is recieved by all clients connected to the same "channel". Now I'm thinking I could make this happen by using a globally accessable list of sockets that are associated with each channel, so each client thread on the server can iterate through the list and send the same message to each client socket, but that sounds like a whole lot of iteration/workload for the server? I figured there must be a better solution to this, where perhaps each client thread monitors a shared interface of some kind? I wondered about using something like a UDP broadcast or some other kind of send-once type of method... any ideas how I could set this up? Any ideas would be greatly appreciated!!

Thanks, John

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raised the right idea and pointed toward a pub/sub way of thinking, but a few clarifications make the model practical. There is no TCP primitive that writes once and magically delivers to many separate TCP streams — the server accepts each client (the listening socket returns a distinct socket per connection) and normally maintains a map of channel_name -> set(client_sockets). Iterating that set to deliver a message is the standard approach; the important design choices are how to do those writes without blocking and how to scale when the number of clients grows.

A compact pattern (non-blocking sockets + simple dispatcher) looks like this:

import socket, select
from collections import defaultdict, deque

listen = socket.socket(); listen.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
listen.bind(('0.0.0.0', 9000)); listen.listen(100); listen.setblocking(False)

channels = defaultdict(set)   # channel -> set(sockets)
outq = {}                     # sock -> deque()

inputs = [listen]; outputs = []

while True:
    r, w, _ = select.select(inputs, outputs, inputs)
    for s in r:
        if s is listen:
            client, _ = listen.accept(); client.setblocking(False)
            inputs.append(client); outq[client] = deque()
        else:
            data = s.recv(4096)
            if not data:
                # cleanup omitted
                continue
            # protocol: "JOIN <chan>" or "MSG <chan> <payload>"
            cmd, rest = data.split(b' ', 1)
            if cmd == b'JOIN':
                channels[rest.strip().decode()].add(s)
            else:
                chan, payload = rest.split(b' ', 1)
                for peer in channels[chan.decode()]:
                    if peer is not s:
                        outq[peer].append(payload); outputs.append(peer) if peer not in outputs else None
    for s in w:
        if outq[s]:
            s.send(outq[s].popleft())
        if not outq[s] and s in outputs: outputs.remove(s)

Notes and tradeoffs: thread-per-client is simplest for small deployments but fails at scale (context-switching, blocking writes). For larger scale, use an event loop (select/poll/epoll, or asyncio), or introduce a message broker (Redis pub/sub, ZeroMQ, RabbitMQ) so several worker processes can share load. UDP multicast can broadcast on a LAN but is lossy and not suitable across the Internet or through NAT. For browser clients, WebSockets or a broker-backed service are better.

Troubleshooting: the "connection already in use" symptom usually comes from binding the same local address/port twice or reusing one socket object for multiple connect() calls. Each client needs its own socket; the server needs one listening socket and calls accept() to get per-client sockets. Use non-blocking sends, per-client outgoing queues, detect and remove dead sockets, and consider SO_REUSEADDR for quick restarts.

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Member Avatar for Beat_Slayer

How about all the clients read from the 'channel' they are associated, and post to the 'channel' socket.

The server has only receive message, and associate to the proper channel.

Cheers and Happy coding.

Member Avatar for NewbieXcellence

Yeah, that's what I was hoping I could do but I didn't think I could connect multiple connections to one socket ? I tried something like that at first but I got a "connection already in use" type message when I tried to connect the second client. Perhaps I am mis-understanding how to implement this with sockets.... I'll try and search around some more to see if I am missing something here!
-Thanks for the idea btw!

Edited by NewbieXcellence because: n/a
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