gistfile1.py

#!/usr/bin/env python 

"""

   Least-recently used (LRU) queue device
   Clients and workers are shown here in-process
 
   Author: Guillaume Aubert (gaubert) <guillaume(dot)aubert(at)gmail(dot)com>
  
"""

"""Extended by J2 Labs to help make it clear what's happening. 

Output looks like below:

$ ./lruqueue.py 
Initializing LRU Loop ]--------------------------
LRU Loop :: received READY from Worker-0
Worker-0 :: received <- HELLO -- sending -> OK
LRU Loop :: received READY from Worker-1
Worker-1 :: received <- HELLO -- sending -> OK
LRU Loop :: assigning Client-0 -> Worker-0
LRU Loop :: sending OK -> Client-0
Client-0 :: sending -> HELLO -- received <- OK
LRU Loop :: assigning Client-1 -> Worker-1
Worker-0 :: received <- HELLO -- sending -> OK
LRU Loop :: sending OK -> Client-1
Client-1 :: sending -> HELLO -- received <- OK
Worker-1 :: received <- HELLO -- sending -> OK
LRU Loop :: assigning Client-2 -> Worker-0
LRU Loop :: sending OK -> Client-2
Client-2 :: sending -> HELLO -- received <- OK
Worker-0 :: received <- HELLO -- sending -> OK
LRU Loop :: assigning Client-3 -> Worker-1
LRU Loop :: sending OK -> Client-3
Client-3 :: sending -> HELLO -- received <- OK
LRU Loop :: assigning Client-4 -> Worker-0
LRU Loop :: sending OK -> Client-4
Client-4 :: sending -> HELLO -- received <- OK



"""

import threading
import time
import zmq

NBR_CLIENTS = 5
NBR_WORKERS = 2

def worker_thread(worker_url, context, i):
    """ Worker using REQ socket to do LRU routing
    """
    socket = context.socket(zmq.REQ)
    identity = "Worker-%d" % (i)
    socket.setsockopt(zmq.IDENTITY, identity) #set worker identity
    socket.connect(worker_url)
    
    # Tell the borker we are ready for work
    socket.send("READY")
    try:
        while True:
            
            # python binding seems to eat empty frames
            address = socket.recv()
            request = socket.recv()
            
            print("%s :: received <- %s -- sending -> %s"
                  % (identity, request, "OK"))
               
            socket.send(address, zmq.SNDMORE)
            socket.send("", zmq.SNDMORE)
            socket.send("OK")

            
    except zmq.ZMQError, zerr:
        # context terminated so quit silently
        if zerr.strerror == 'Context was terminated':
            return
        else:
            raise zerr
    
        
def client_thread(client_url, context, i):
    """ Basic request-reply client using REQ socket
    """
    socket = context.socket(zmq.REQ)
    identity = "Client-%d" % (i)
    socket.setsockopt(zmq.IDENTITY, identity) #Set client identity. Makes tracing easier
    socket.connect(client_url)

    #  Send request, get reply
    socket.send("HELLO")
    reply = socket.recv()
    
    print("%s :: sending -> %s -- received <- %s"
          % (identity, "HELLO", reply))
    
    return
    
        
def main():
    """ main method """

    url_worker = "inproc://workers"
    url_client = "inproc://clients"
    client_nbr = NBR_CLIENTS
    
    # Prepare our context and sockets
    context = zmq.Context(1)
    frontend = context.socket(zmq.XREP)
    frontend.bind(url_client)
    backend = context.socket(zmq.XREP)
    backend.bind(url_worker)
    
    # create workers and clients threads
    for i in range(NBR_WORKERS):
        thread = threading.Thread(target=worker_thread, args=(url_worker, context, i, ))
        thread.start()
    
    for i in range(NBR_CLIENTS):
        thread_c = threading.Thread(target=client_thread, args=(url_client, context, i, ))
        thread_c.start()
    
    # Logic of LRU loop
    # - Poll backend always, frontend only if 1+ worker ready
    # - If worker replies, queue worker as ready and forward reply
    # to client if necessary
    # - If client requests, pop next worker and send request to it
    
    # Queue of available workers
    available_workers = 0
    workers_list      = []
    
    # init poller
    poller = zmq.Poller()
    
    # Always poll for worker activity on backend
    poller.register(backend, zmq.POLLIN)
    
    # Poll front-end only if we have available workers
    poller.register(frontend, zmq.POLLIN)
    
    while True:
        socks = dict(poller.poll())
    
        # Handle worker activity on backend
        if (backend in socks and socks[backend] == zmq.POLLIN):
            
            # Queue worker address for LRU routing
            worker_addr  = backend.recv()            
            
            assert available_workers < NBR_WORKERS
            
            # add worker back to the list of workers
            available_workers += 1
            workers_list.append(worker_addr)
            
            # Second frame is empty
            empty = backend.recv()
            assert empty == ""
            
            # Third frame is READY or else a client reply address
            client_addr = backend.recv()
            
            
            # If client reply, send rest back to frontend
            if client_addr != "READY":
                
                print("%s :: assigning %s -> %s"
                      % ("LRU Loop", client_addr, worker_addr))
            
                # Following frame is empty
                empty = backend.recv()
                assert empty == ""
                
                reply = backend.recv()
                
                frontend.send(client_addr, zmq.SNDMORE)
                frontend.send("", zmq.SNDMORE)
                frontend.send(reply)
                
                print("%s :: sending %s -> %s" % ("LRU Loop", reply, client_addr))
                
                client_nbr -= 1
                
                if client_nbr == 0:
                    break  # Exit after N messages
            else:
                print("%s :: received %s from %s"
                      % ("LRU Loop", client_addr, worker_addr))

    
        # poll on frontend only if workers are available
        if available_workers > 0:
            
            if (frontend in socks and socks[frontend] == zmq.POLLIN):
                # Now get next client request, route to LRU worker
                # Client request is [address][empty][request]
                client_addr = frontend.recv()
                
                empty = frontend.recv()
                assert empty == ""
                
                request = frontend.recv()
                
                #  Dequeue and drop the next worker address
                available_workers -= 1
                worker_id = workers_list.pop()
                 
                backend.send(worker_id, zmq.SNDMORE)
                backend.send("", zmq.SNDMORE)
                backend.send(client_addr, zmq.SNDMORE)
                backend.send(request)
                
    #out of infinite loop: do some housekeeping
    time.sleep (1)
    
    frontend.close()
    backend.close()
    context.term()

if __name__ == "__main__":
    print 'Initializing LRU Loop ]--------------------------'
    main()