I am writing multiprocess program. There are four class: Main, Worker, Request and Ack. The Main class is the entry point of program. It will create the sub-process called Worker to do some jobs. The main process put the Request into JoinableQueue, and than Worker get request from queue. When Worker finished the request, it will put the ACK into queue. The part of code shown as below:
Main:
class Main():
def __init__(self):
self.cmd_queue = JoinableQueue()
self.worker = Worker(self.cmd_queue)
def call_worker(self, cmd_code):
if self.cmd_queue.empty() is True:
request = Request(cmd_code)
self.cmd_queue.put(request)
self.cmd_queue.join()
ack = self.cmd_queue.get()
self.cmd_queue.task_done()
if ack.value == 0:
return True
else:
return False
else:
# TODO: Error Handling.
pass
def run_worker(self):
self.worker.start()
Worker:
class Worker(Process):
def __init__(self, cmd_queue):
super(Worker, self).__init__()
self.cmd_queue = cmd_queue
...
def run(self):
while True:
ack = Ack(0)
try:
request = self.cmd_queue.get()
if request.cmd_code == ReqCmd.enable_handler:
self.enable_handler()
elif request.cmd_code == ReqCmd.disable_handler:
self.disable_handler()
else:
pass
except Exception:
ack.value = -1
finally:
self.cmd_queue.task_done()
self.cmd_queue.put(ack)
self.cmd_queue.join()
It often works normally. But Main process stuck at self.cmd_queue.join(), and the Worker stuck at self.cmd_queue.join() sometimes. It is so weird! Does anyone have any ideas? Thanks
There's nothing weird in the above issue: you shouldn't call queue's join within a typical single worker process activity because
Queue.join()
Blocks until all items in the queue have been gotten and
processed.
Such a calls where they are in your current implementation will make the processing pipeline wait.
Usually queue.join() is called in the main (supervisor) thread after initiating/starting all threads/workers.
https://docs.python.org/3/library/queue.html#queue.Queue.join
Related
Suppose I have some consumer daemon threads that constantly take objects from a queue whenever the main thread puts them there and performs some long operation (a couple of seconds) with them.
The problem is that whenever the main thread is done, the daemon threads are killed before they finish processing whatever is left in the queue.
I know that one way to solve this could be to wait for the daemon threads to finish processing whatever is left in the queue and then exit, but I am curious if there is any way for the daemon threads to "clean up" after themselves (i.e. finish processing whatever is left in the queue) when the main thread exits, without explicitly having the main thread tell the daemon threads to start cleaning up.
The motivation behind this is that I made a python package that has a logging handler class that puts items into a queue whenever the user tries to log something (e.g. with logging.info("message")), and the handler has a daemon thread that sends the logs over the network. I'd prefer if the daemon thread could clean up by itself, so users of the package wouldn't have to manually make sure to make their main thread wait for the log handler to finish its processing.
Minimal working example
# this code is in my package
class MyHandler(logging.Handler):
def __init__(self, level):
super().__init__(level=level)
self.queue = Queue()
self.thread = Thread(target=self.consume, daemon=True)
self.thread.start()
def emit(self, record):
# This gets called whenever the user does logging.info, or similar
self.queue.put(record)
def consume(self):
while True:
record = self.queue.get()
send(record) # send record over network, can take a few seconds (assume it never raises)
self.queue.task_done()
# This is user's main code
# user will have to keep a reference to the handler for later. I want to avoid this.
my_handler = MyHandler()
# set up logging
logging.basicConfig(..., handlers=[..., my_handler])
# do some stuff...
logging.info("this will be sent over network")
# some more stuff...
logging.error("also sent over network")
# even more stuff
# before exiting must wait for handler to finish sending
# I don't want user to have to do this
my_hanler.queue.join()
You can use threading.main_thread.join() which will wait until shutdown like so:
import threading
import logging
import queue
class MyHandler(logging.Handler):
def __init__(self, level):
super().__init__(level=level)
self.queue = queue.Queue()
self.thread = threading.Thread(target=self.consume) # Not daemon
# Shutdown thread
threading.Thread(
target=lambda: threading.main_thread().join() or self.queue.put(None)
).start()
self.thread.start()
def emit(self, record):
# This gets called whenever the user does logging.info, or similar
self.queue.put(record)
def consume(self):
while True:
record = self.queue.get()
if record is None:
print("cleaning")
return # Cleanup
print(record) # send record over network, can take a few seconds (assume it never raises)
self.queue.task_done()
Quick test code:
logging.getLogger().setLevel(logging.INFO)
logging.getLogger().addHandler(MyHandler(logging.INFO))
logging.info("Hello")
exit()
You can use atexit to wait until the daemon thread shuts down:
import queue, threading, time, logging, atexit
class MyHandler(logging.Handler):
def __init__(self, level):
super().__init__(level=level)
self.queue = queue.Queue()
self.thread = threading.Thread(target=self.consume, daemon=True)
# Right before main thread exits, signal cleanup and wait until done
atexit.register(lambda: self.queue.put(None) or self.thread.join())
self.thread.start()
def emit(self, record):
# This gets called whenever the user does logging.info, or similar
self.queue.put(record)
def consume(self):
while True:
record = self.queue.get()
if record is None: # Cleanup requested
print("cleaning")
time.sleep(5)
return
print(record) # send record over network, can take a few seconds (assume it never raises)
self.queue.task_done()
# Test code
logging.getLogger().setLevel(logging.INFO)
logging.getLogger().addHandler(MyHandler(logging.INFO))
logging.info("Hello")
I have a producer thread that produces data from a serial connection and puts them into multiple queues that will be used by different consumer threads. However, I'd like to be able to add in additional queues (additional consumers) from the main thread after the producer thread has already started running.
I.e. In the code below, how could I add a Queue to listOfQueues from the main thread while this thread is running? Can I add in a method such as addQueue(newQueue) to this class which appends to it listOfQueues? This doesn't seem likely as the thread will be in the run method. Can I create some sort of Event similar to the stop event?
class ProducerThread(threading.Thread):
def __init__(self, listOfQueues):
super(ProducerThread, self).__init__()
self.listOfQueues = listOfQueues
self._stop_event = threading.Event() # Flag to be set when the thread should stop
def run(self):
ser = serial.Serial() # Some serial connection
while(not self.stopped()):
try:
bytestring = ser.readline() # Serial connection or "producer" at some rate
for q in self.listOfQueues:
q.put(bytestring)
except serial.SerialException:
continue
def stop(self):
'''
Call this function to stop the thread. Must also use .join() in the main
thread to fully ensure the thread has completed.
:return:
'''
self._stop_event.set()
def stopped(self):
'''
Call this function to determine if the thread has stopped.
:return: boolean True or False
'''
return self._stop_event.is_set()
Sure, you can simply have an append function that adds to your list. E.g.
def append(self, element):
self.listOfQueues.append(element)
That will work even after your thread's start() method has been called.
Edit: for non thread-safe procedures you can use a lock, e.g.:
def unsafe(self, element):
with self.lock:
# do stuff
You would then also need to add the lock inside your run method, e.g.:
with lock:
for q in self.listOfQueues:
q.put(bytestring)
Any code acquiring a lock will wait for the lock to be released elsewhere.
I am trying to implement a Python (2.6.x/2.7.x) thread pool that would check for network connectivity(ping or whatever), the entire pool threads must be killed/terminated when the check is successful.
So I am thinking of creating a pool of, let's say, 10 worker threads. If any one of them is successful in pinging, the main thread should terminate all the rest.
How do I implement this?
This is not a compilable code, this is just to give you and idea how to make threads communicate..
Inter process or threads communication happens through queues or pipes and some other ways..here I'm using queues for communication.
It works like this.. I'll send ip addresses in in_queue and add response to out_queue, my main thread monitors out_queue and if it gets desired result, it marks all the threads to terminate.
Below is the pinger thread definition..
import threading
from Queue import Queue, Empty
# A thread that pings ip.
class Pinger(threading.Thread):
def __init__(self, kwargs=None):
threading.Thread.__init__(self)
self.kwargs = kwargs
self.stop_pinging = False
def run(self):
ip_queue = self.kwargs.get('in_queue')
out_queue = self.kwargs.get('out_queue')
while not self.stop_pinging:
try:
data = ip_quque.get(timeout=1)
ping_status = ping(data)
# This is pseudo code, you've to takecare of
# your own ping.
if ping_status:
out_queue.put('success')
# you can even break here if you don't want to
# continue after one success
else:
out_queue.put('failure')
if ip_queue.empty()
break
except Empty, e:
pass
Here is the main thread block..
# Create the shared queue and launch both thread pools
in_queue = Queue()
out_queue = Queue()
ip_list = ['ip1', 'ip2', '....']
# This is to add all the ips to the queue or you can
# customize to add through some producer way.
for ip in ip_list:
in_queue.put(ip)
pingerer_pool = []
for i in xrange(1, 10):
pingerer_worker = Pinger(kwargs={'in_queue': in_queue, 'out_queue': out_queue}, name=str(i))
pingerer_pool.append(pinger_worker)
pingerer_worker.start()
while 1:
if out_queue.get() == 'success':
for pinger in pinger_pool:
pinger_worker.stop_pinging = True
break
Note: This is a pseudo code, you should make this workable as you like.
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?
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()
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
I am developing a multi-threaded application in python. I have following scenario.
There are 2-3 producer threads which communicate with DB and get some data in large chunks and fill them up in a queue
There is an intermediate worker which breaks large chunks fetched by producer threads into smaller ones and fill them up in another queue.
There are 5 consumer threads which consume queue created by intermediate worker thread.
objects of data sources are accessed by producer threads through their API. these data sources are completely separate. So these producer understands only presence or absence of data which is supposed to be given out by data source object.
I create threads of these three types and i make main thread wait for completion of these threads by calling join() on them.
Now for such a setup I want a common error handler which senses failure of any thread, any exception and decides what to do. For e.g if I press ctrl+c after I start my application, main thread dies but producer, consumer threads continue to run. I would like that once ctrl+c is pressed entire application should shut down. Similarly if some DB error occurs in data source module, then producer thread should get notified of that.
This is what I have done so far:
I have created a class ThreadManager, it's object is passed to all threads. I have written an error handler method and passed it to sys.excepthook. This handler should catch exceptions, error and then it should call methods of ThreadManager class to control the running threads. Here is snippet:
class Producer(threading.Thread):
....
def produce():
data = dataSource.getData()
class DataSource:
....
def getData():
raise Exception("critical")
def customHandler(exceptionType, value, stackTrace):
print "In custom handler"
sys.excepthook = customHandler
Now when a thread of producer class calls getData() of DataSource class, exception is thrown. But this exception is never caught by my customHandler method.
What am I missing? Also in such scenario what other strategy can I apply? Please help. Thank you for having enough patience to read all this :)
What you need is a decorator. In essence you are modifying your original function and putting in inside a try-except:
def exception_decorator(func):
def _function(*args):
try:
result = func(*args)
except:
print('*** ESC default handler ***')
os._exit(1)
return result
return _function
If your thread function is called myfunc, then you add the following line above your function definition
#exception_decorator
def myfunc():
pass;
Can't you just catch "KeyboardInterrupt" when pressing Ctrl+C and do:
for thread in threading.enumerate():
thread._Thread__stop()
thread._Thread__delete()
while len(threading.enumerate()) > 1:
time.sleep(1)
os._exit(0)
and have a flag in each threaded class which is self.alive
you could theoretically call thread.alive = False and have it stop gracefully?
for thread in threading.enumerate():
thread.alive = False
time.sleep(5) # Grace period
thread._Thread__stop()
thread._Thread__delete()
while len(threading.enumerate()) > 1:
time.sleep(1)
os._exit(0)
example:
import os
from threading import *
from time import sleep
class worker(Thread):
def __init__(self):
self.alive = True
Thread.__init__(self)
self.start()
def run(self):
while self.alive:
sleep(0.1)
runner = worker()
try:
raw_input('Press ctrl+c!')
except:
pass
for thread in enumerate():
thread.alive = False
sleep(1)
try:
thread._Thread__stop()
thread._Thread__delete()
except:
pass
# There will always be 1 thread alive and that's the __main__ thread.
while len(enumerate()) > 1:
sleep(1)
os._exit(0)
Try going about it by changing the internal system exception handler?
import sys
origExcepthook = sys.excepthook
def uberexcept(exctype, value, traceback):
if exctype == KeyboardInterrupt:
print "Gracefully shutting down all the threads"
# enumerate() thingie here.
else:
origExcepthook(exctype, value, traceback)
sys.excepthook = uberexcept