zero-mq: socket.recv() call is blocking

Question

I am trying to use zero-mq.My requirement is very simple.I want to be able to communicate between two peers in a network.I came across this program in the examples in the book.

$ pub_server.py

import zmq
import random
import sys
import time

port = "5556"
if len(sys.argv) > 1:
    port =  sys.argv[1]
    int(port)

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:%s" % port)

while True:
    topic = random.randrange(9999,10005)
    messagedata = random.randrange(1,215) - 80
    print "%d %d" % (topic, messagedata)
    socket.send("%d %d" % (topic, messagedata))
    time.sleep(1)

$sub_client.py

import sys
import zmq

port = "5556"
if len(sys.argv) > 1:
    port =  sys.argv[1]
    int(port)

# Socket to talk to server
context = zmq.Context()
socket = context.socket(zmq.SUB)

print "Collecting updates from weather server..."
socket.connect ("tcp://localhost:%s" % port)

# Subscribe to zipcode, default is NYC, 10001
topicfilter = "10001"
socket.setsockopt(zmq.SUBSCRIBE, topicfilter)

# Process 5 updates
total_value = 0
for update_nbr in range (5):
    string = socket.recv()
    topic, messagedata = string.split()
    total_value += int(messagedata)
    print ('{} {}'.format(topic, messagedata))

print('Avg data value for topic {} was {}'.format(topicfilter, (total_value/update_nbr)))

The problem I have with this model is that

string = socket.recv()

blocks till I recieve a message.I don't want this to happen.I want the messages to be queued up on the recieve side so that I can get it out of the queue (or something similar to this)

Is there some model in zero-mq that allows this?

Answer 1

zmq.Socket.recv will not block if you pass the zmq.NOBLOCK flag parameter.

The docs say:

If NOBLOCK is set, this method will raise a ZMQError with EAGAIN if a message is not ready.

zmq will queue messages that it receives and one message will be returned for each recv() call until this queue is exhausted after which ZMQError is raised.

zmq.Again used in the exmaples below is a wrapper for zmq.EAGAIN.

For example:

while True:
    try:
        #check for a message, this will not block
        message = socket.recv(flags=zmq.NOBLOCK)

        #a message has been received
        print "Message received:", message

    except zmq.Again as e:
        print "No message received yet"

    # perform other important stuff
    time.sleep(10)

The sub_client.py example could perhaps be written to use non-blocking behaviour like this:

import sys, time
import zmq

port = "5556"
if len(sys.argv) > 1:
    port =  sys.argv[1]
    int(port)

# Socket to talk to server
context = zmq.Context()
socket = context.socket(zmq.SUB)

print "Collecting updates from weather server..."
socket.connect ("tcp://localhost:%s" % port)

# Subscribe to zipcode, default is NYC, 10001
topicfilter = "10001"
socket.setsockopt(zmq.SUBSCRIBE, topicfilter)

# Process 5 updates
total_value = 0
received_value_count = 0
do_receive_loop = True
while do_receive_loop:
    try:
        #process all messages waiting on subscribe socket
        while True:
            #check for a message, this will not block
            string = socket.recv(flags=zmq.NOBLOCK)

            #message received, process it
            topic, messagedata = string.split()
            total_value += int(messagedata)
            print ('{} {}'.format(topic, messagedata))

            #check if we have all the messages we want
            received_value_count += 1
            if received_value_count > 4:
                do_receive_loop = False
                break

    except zmq.Again as e:
        #No messages waiting to be processed
        pass

    #Here we can do other stuff while waiting for messages
    #contemplate answer to 'The Last Question'
    time.sleep(15)
    print "INSUFFICIENT DATA FOR MEANINGFUL ANSWER"

print('Avg data value for topic {} was {}'.format(topicfilter, (total_value/5)))