multithreading - Python multithreaded ZeroMQ REQ-REP

Question

I am looking to implement a REQ-REP pattern with Python and ZeroMQ using multithreading.

With Python, I can create a new thread when a new client connects to the server. This thread will handle all communications with that particular client, until the socket is closed:

# Thread that will handle client's requests
class ClientThread(threading.Thread):
    # Implementation...
    def __init__(self, socket):
        threading.Thread.__init__(self)
        self.socket = socket
    def run(self):
        while keep_alive:
            # Thread can receive from client
            data = self.socket.recv(1024)
            # Processing...
            # And send back a reply
            self.socket.send(reply)

while True:
    # The server accepts an incoming connection
    conn, addr = sock.accept()
    # And creates a new thread to handle the client's requests
    newthread = ClientThread(conn)
    # Starting the thread
    newthread.start()

Is it possible to do the same[*] using ZeroMQ? I have seen some examples of multithreading with ZeroMQ and Python, but in all of them a pool of threads is created with a fixed number of threads at the beginning and it seems to be more oriented to load balancing.

[*] Notice what I want is to keep the connection between a client and its thread alive, as the thread is expecting multiple REQ messages from the client and it will store information that must be kept between messages (i.e.: a variable counter that increments its value on a new REQ message; so each thread has its own variable and no other client should ever be able to access that thread). New client = new thread.

Answer 1

Yes, ZeroMQ is a powerful can-do toolbox

However, the major surprise will be, that ZeroMQ <socket>-s are by far more structured than their plain counterparts, you use in the sample.

{ aZmqContext -> aZmqSocket -> aBehavioralPrimitive }

ZeroMQ builds a remarkable, abstraction-rich framework, under a hood of a "singleton" ZMQ-Context, which is (and shall remain) the only thing used as "shared".

Threads shall not "share" any other "derived" objects, the less any their state, as there is a strong distributed-responsibility framework architecture implemented, both in the sake of clean-design and a high performance & low-latency.

For all ZMQ-Socket-s one shall rather imagine a much smarter, layered sub-structure, where one receives off-loaded worries about I/O-activities ( managed inside ZMQ-Context responsibility -- thus keep-alive issues, timing issues and fair-queue buffering / select-polling issues simply cease to be visible for you ... ), with one sort of a formal communication pattern behaviour ( given by a chosen ZMQ-Socket-type archetype ).

Finally

ZeroMQ and similarly nanomsg libraries are rather LEGO-like projects, that empower you, as an architect & designer, more than one typically realises at the very beginning.

One thus can focus on distributed-system behaviour, as opposed to lose time and energy on solving just-another-socket-messaging-[nightmare].

( Definitely worth a look into both books from Pieter Hintjens, co-father of the ZeroMQ. There you find plenty Aha!-moments on this great subject. )

... and as a cherry on a cake -- you get all of this as a Transport-agnostic, universal environment, whether passing some messages on inproc://, other over ipc:// and also in parallel listening / speaking over tcp:// layers.

EDIT#12014-08-19 17:00 [UTC+0000]

Kindly check comments below and further review your -- both elementary and advanced -- design-options for a <trivial-failure-prone>-spin-off processing, for a <load-balanced>-REP-worker queueing, for a <scale-able>-distributed processing and a <fault-resilient_mode>-REP-worker binary-start shaded processing.

No heap of mock-up SLOC(s), no single code-sample will do a One-Size-Fits-All.

This is exponentially valid in designing distributed messaging systems.

"""REQ/REP modified with QUEUE/ROUTER/DEALER add-on ---------------------------

   Multithreaded Hello World server

   Author: Guillaume Aubert (gaubert) <guillaume(dot)aubert(at)gmail(dot)com>

"""
import time
import threading
import zmq

print "ZeroMQ version sanity-check: ", zmq.__version__

def aWorker_asRoutine( aWorker_URL, aContext = None ):
    """Worker routine"""
    #Context to get inherited or create a new one trick------------------------------
    aContext = aContext or zmq.Context.instance()
    
    # Socket to talk to dispatcher --------------------------------------------------
    socket = aContext.socket( zmq.REP )
    
    socket.connect( aWorker_URL )
    
    while True:

        string  = socket.recv()

        print( "Received request: [ %s ]" % ( string ) )
        
        # do some 'work' -----------------------------------------------------------
        time.sleep(1)

        #send reply back to client, who asked --------------------------------------
        socket.send( b"World" )

def main():
    """Server routine"""

    url_worker = "inproc://workers"
    url_client = "tcp://*:5555"

    # Prepare our context and sockets ------------------------------------------------
    aLocalhostCentralContext = zmq.Context.instance()

    # Socket to talk to clients ------------------------------------------------------
    clients = aLocalhostCentralContext.socket( zmq.ROUTER )
    clients.bind( url_client )

    # Socket to talk to workers ------------------------------------------------------
    workers = aLocalhostCentralContext.socket( zmq.DEALER )
    workers.bind( url_worker )

    # --------------------------------------------------------------------||||||||||||--
    # Launch pool of worker threads --------------< or spin-off by one in OnDemandMODE >
    for i in range(5):
        thread = threading.Thread( target = aWorker_asRoutine, args = ( url_worker, ) )
        thread.start()

    zmq.device( zmq.QUEUE, clients, workers )

    # ----------------------|||||||||||||||------------------------< a fair practice >--
    # We never get here but clean up anyhow
    clients.close()
    workers.close()
    aLocalhostCentralContext.term()

if __name__ == "__main__":
    main()