Question

I would like to check if a Consumer/Worker is present to consume a Message I am about to send.

If there isn’t any Worker, I would start some workers (both consumers and publishers are on a single machine) and then go about publishing Messages.

If there is a function like connection.check_if_has_consumers, I would implement it somewhat like this –

import pika
import workers

# code for publishing to worker queue
connection = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
channel = connection.channel()

# if there are no consumers running (would be nice to have such a function)
if not connection.check_if_has_consumers(queue="worker_queue", exchange=""):
    # start the workers in other processes, using python's `multiprocessing`
    workers.start_workers()

# now, publish with no fear of your queues getting filled up
channel.queue_declare(queue="worker_queue", auto_delete=False, durable=True)
channel.basic_publish(exchange="", routing_key="worker_queue", body="rockin",
                            properties=pika.BasicProperties(delivery_mode=2))
connection.close()

But I am unable to find any function with check_if_has_consumers functionality in pika.

Is there some way of accomplishing this, using pika? or maybe, by talking to The Rabbit directly?

I am not completely sure, but I really think RabbitMQ would be aware of the number of consumers subscribed to different queues, since it does dispatch messages to them and accepts acks

I just got started with RabbitMQ 3 hours ago… any help is welcome…

here is the workers.py code I wrote, if its any help….

import multiprocessing
import pika


def start_workers(num=3):
    """start workers as non-daemon processes"""
    for i in xrange(num):    
        process = WorkerProcess()
        process.start()


class WorkerProcess(multiprocessing.Process):
    """
    worker process that waits infinitly for task msgs and calls
    the `callback` whenever it gets a msg
    """
    def __init__(self):
        multiprocessing.Process.__init__(self)
        self.stop_working = multiprocessing.Event()

    def run(self):
        """
        worker method, open a channel through a pika connection and
        start consuming
        """
        connection = pika.BlockingConnection(
                              pika.ConnectionParameters(host='localhost')
                     )
        channel = connection.channel()
        channel.queue_declare(queue='worker_queue', auto_delete=False,
                                                    durable=True)

        # don't give work to one worker guy until he's finished
        channel.basic_qos(prefetch_count=1)
        channel.basic_consume(callback, queue='worker_queue')

        # do what `channel.start_consuming()` does but with stopping signal
        while len(channel._consumers) and not self.stop_working.is_set():
            channel.transport.connection.process_data_events()

        channel.stop_consuming()
        connection.close()
        return 0

    def signal_exit(self):
        """exit when finished with current loop"""
        self.stop_working.set()

    def exit(self):
        """exit worker, blocks until worker is finished and dead"""
        self.signal_exit()
        while self.is_alive(): # checking `is_alive()` on zombies kills them
            time.sleep(1)

    def kill(self):
        """kill now! should not use this, might create problems"""
        self.terminate()
        self.join()


def callback(channel, method, properties, body):
    """pika basic consume callback"""
    print 'GOT:', body
    # do some heavy lifting here
    result = save_to_database(body)
    print 'DONE:', result
    channel.basic_ack(delivery_tag=method.delivery_tag)

EDIT:

I have to move forward so here is a workaround that I am going to take, unless a better approach comes along,

So, RabbitMQ has these HTTP management apis, they work after you have turned on the management plugin and at middle of HTTP apis page there is

/api/connections – A list of all open connections.

/api/connections/name – An individual connection. DELETEing it will close the connection.

So, if I connect my Workers and my Produces both by different Connection names / users, I’ll be able to check if the Worker Connection is open… (there might be issues when worker dies…)

will be waiting for a better solution…

EDIT:

just found this in the rabbitmq docs, but this would be hacky to do in python:

shobhit@oracle:~$ sudo rabbitmqctl -p vhostname list_queues name consumers
Listing queues ...
worker_queue    0
...done.

so i could do something like,

subprocess.call("echo password|sudo -S rabbitmqctl -p vhostname list_queues name consumers | grep 'worker_queue'")

hacky… still hope pika has some python function to do this…

Thanks,

Answer 1

I was just looking into this as well. After reading through the source and docs I came across the following in channel.py:

@property
def consumer_tags(self):
    """Property method that returns a list of currently active consumers

    :rtype: list

    """
    return self._consumers.keys()

My own testing was successful. I used the following where my channel object is self._channel:

if len(self._channel.consumer_tags) == 0:
        LOGGER.info("Nobody is listening.  I'll come back in a couple of minutes.")
        ...