Need a thread-safe asynchronous message queue

Question

I'm looking for a Python class (preferably part of the standard language, rather than a 3rd party library) to manage asynchronous 'broadcast style' messaging.

I will have one thread which puts messages on the queue (the 'putMessageOnQueue' method must not block) and then multiple other threads which will all be waiting for messages, having presumably called some blocking 'waitForMessage' function. When a message is placed on the queue I want each of the waiting threads to get its own copy of the message.

I've looked at the built-in Queue class, but I don't think this is suitable because consuming messages seems to involve removing them from the queue, so only 1 client thread would see each one.

This seems like it should be a common use-case, can anyone recommend a solution?

Answer 1

I think the typical approach to this is to use a separate message queue for each thread, and push the message onto every queue which has previously registered an interest in receiving such messages.

Something like this ought to work, but it's untested code...

from time import sleep
from threading import Thread
from Queue import Queue

class DispatcherThread(Thread):

   def __init__(self, *args, **kwargs):
       super(DispatcherThread, self).__init__(*args, **kwargs)
       self.interested_threads = []

   def run(self):
       while 1:
           if some_condition:
               self.dispatch_message(some_message)
           else:
               sleep(0.1)

   def register_interest(self, thread):
       self.interested_threads.append(thread)

   def dispatch_message(self, message):
       for thread in self.interested_threads:
           thread.put_message(message)



class WorkerThread(Thread):

   def __init__(self, *args, **kwargs):
       super(WorkerThread, self).__init__(*args, **kwargs)
       self.queue = Queue()


   def run(self):

       # Tell the dispatcher thread we want messages
       dispatcher_thread.register_interest(self)

       while 1:
           # Wait for next message
           message = self.queue.get()

           # Process message
           # ...

   def put_message(self, message):
       self.queue.put(message)


dispatcher_thread = DispatcherThread()
dispatcher_thread.start()

worker_threads = []
for i in range(10):
    worker_thread = WorkerThread()
    worker_thread.start()
    worker_threads.append(worker_thread)

dispatcher_thread.join()

Answer 2

I think this is a more straight forward example (taken from the Queue example in Python Lib )

from threading import Thread
from Queue import Queue


num_worker_threads = 2

def worker():
    while True:
        item = q.get()
        do_work(item)
        q.task_done()

q = Queue()
for i in range(num_worker_threads):
     t = Thread(target=worker)
     t.daemon = True
     t.start()

for item in source():
    q.put(item)

q.join()       # block until all tasks are done