farm.py

"""
Simple task farm, with routed replies in pyzmq

For http://stackoverflow.com/questions/7809200/implementing-task-farm-messaging-pattern-with-zeromq

Note that things are run in threads to keep stuff in one file, there is no
reason they need to be.


License: Public Domain
"""

import os
import time
import random
import zmq
import threading

ctx = zmq.Context.instance()
client_iface = "tcp://127.0.0.1:5555"
engine_iface = "tcp://127.0.0.1:5556"

def scheduler():
    """ROUTER-DEALER queue device, for load-balancing requests from clients
    across engines, and routing replies to the originating client."""
    router = ctx.socket(zmq.ROUTER)
    router.bind(client_iface)
    dealer = ctx.socket(zmq.DEALER)
    # this is optional, it just makes identities more obvious when they appear
    dealer.setsockopt(zmq.IDENTITY, b'Controller.DEALER')
    dealer.bind(engine_iface)
    # the remainder of this function can be entirely replaced with
    # zmq.device(zmq.QUEUE, router, dealer)
    # but this shows what is really going on:
    poller = zmq.Poller()
    poller.register(router, zmq.POLLIN)
    poller.register(dealer, zmq.POLLIN)
    while True:
        evts = dict(poller.poll())
        # poll() returns a list of tuples [(socket, evt), (socket, evt)]
        # dict(poll()) turns this into {socket:evt, socket:evt}
        if router in evts:
            msg = router.recv_multipart()
            # ROUTER sockets prepend the identity of the sender, for routing replies
            client = msg[0]
            print "Controller.ROUTER received %s, relaying via DEALER" % msg
            dealer.send_multipart(msg)
        if dealer in evts:
            msg = dealer.recv_multipart()
            client = msg[0]
            print "Controller.DEALER received %s, relaying via ROUTER" % msg
            router.send_multipart(msg)

def process_request(msg):
    """process the message (reverse letters)"""
    return [ part[::-1] for part in msg ]

def engine(id):
    """The engine - receives messages, performs some action, and sends a reply,
    preserving the leading two message parts as routing identities
    """
    s = ctx.socket(zmq.ROUTER)
    s.connect(engine_iface)
    while True:
        msg = s.recv_multipart()
        print "engine %s recvd message:" % id, msg
        # note that the first two parts will be [b'Controller.ROUTER', b'Client.<id>']
        # these are needed for the reply to propagate up to the right client
        idents, request = msg[:2], msg[2:]
        reply = idents + process_request(request)
        
        print "engine %s sending reply:" % id, reply
        s.send_multipart(reply)

def client(id, n):
    """The client - sends messages, and receives replies after they
    have been processed by the """
    s = ctx.socket(zmq.DEALER)
    s.identity = "Client.%s" % id
    s.connect(client_iface)
    for i in range(n):
        print
        msg = ["hello", "world", str(random.randint(10,100))]
        print "client %s sending :" % id, msg
        s.send_multipart(msg)
        msg = s.recv_multipart()
        print "client %s received:" % id, msg
        time.sleep(0.1)

if __name__ == '__main__':
    st = threading.Thread(target=scheduler)
    st.daemon=True
    st.start()
    engines = []
    for i in range(4):
        t = threading.Thread(target=engine, args=(i,))
        t.daemon=True
        t.start()
        engines.append(t)
    # now start a few clients, and fire off some requests
    clients = []
    for i in range(3):
        t = threading.Thread(target=client, args=(i,12))
        t.start()
        # remove this t.join() to allow clients to be run concurrently.
        # this will work just fine, but the print-statements will
        # be less easy to follow
        t.join()
        clients.append(t)