Server mediated inter-process communication.

server.py

import itertools
import os
import socket
import threading
import time

# This program illustrates a simple TCP server used to mediate inter-process
# communication. Three processes are created: a server, a producer, and a
# consumer. The server creates a thread to handle each incoming connection.

address = (socket.gethostname(), 5000)

if os.fork() != 0:
    # Create a TCP socket for the _actual_ server.
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind(address)
    server.listen(2)

    connections = []

    # Each connection will execute this function on a separate thread.
    def thread(connection):
        while True:
            # `socket.socket.recv` blocks. This is not a proble, however,
            # because the loop below is able to send messages even if the
            # thread that owns another connection is currently blocked waiting
            # a message.
            message = connection.recv(4096)

            for connection in connections:
                connection.send(message)

    while True:
        connection, address = server.accept()

        connections.append(connection)

        # Note the argument `args` requires a tuple and that `(connection, )`
        # is parsed as a tuple while `(connection)` is equivalent to
        # `connection` but with superfluous parenthesis.
        threading.Thread(target=thread, args=(connection, )).start()
else:
    if os.fork() == 0:
        # Create a TCP socket for the producer.
        server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        server.connect(address)

        # `itertools.count` returns an iterator that counts from zero to
        # infinity.
        for i in itertools.count():
            server.send(str(i).encode())

            time.sleep(1)
    else:
        # Create a TCP socket for the consumer.
        server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        server.connect(address)

        while True:
            print(server.recv(4096).decode())