Code implementation of the Redis "Pattern: Reliable queue" - redis

The excellent redis documentation lists a Reliable queue pattern as a good candidate/example for the RPOPLPUSH function.
I understand "reliable queue" to be something with delivery patterns like Amazon SQS FIFO exactly once pattern.
Specifically, you have some N processes feeding into a queue, and some M workers working from the queue. What does this actually look like as an implementation?
I would venture something like:
Make the feeder process populating the work queue.
# feeder1
import redis
import datetime
import time
r = redis.Redis(host='localhost', port=6379, db=0)
while True:
now = datetime.datetime.now()
value_to_work_on = "f1:{}".format(now.second)
r.push('workqueue', value_to_work_on)
time.sleep(1)
Make another
# f2
import redis
import datetime
import time
r = redis.Redis(host='localhost', port=6379, db=0)
while True:
now = datetime.datetime.now()
value_to_work_on = "f2:{}".format(now.second)
r.push('workqueue', value_to_work_on)
time.sleep(1)
Now make the workers
# worker1
import redis
r = redis.Redis(host='localhost', port=6379, db=0)
def do_work(x):
print(x)
return True
while True:
todo = r.rpoplpush("workqueue" "donequeue")
if do_work(todo):
print("success")
else:
r.push("workqueue", todo)
# worker2 is exactly the same, just running elsewhere.
My questions are:
Is this generally what they mean in the documentation? If not, can you provide a fix as an answer?
This seems still incomplete and not really reliable. For example, should there be alternative lists for error vs complete queues? One for every possible error state? What happens if your Redis goes down during processing?

As #rainhacker pointed out in comments, it is now recommended to use Redis Streams for this instead of the recipe described in "Pattern: Reliable Queue"

Related

Passing a Queue with concurrent.futures regardless of executor type

Working up from threads to processes, I have switched to concurrent.futures, and would like to gain/retain flexibility in switching between a ThreadPoolExecutor and a ProcessPoolExecutor for various scenarios. However, despite the promise of a unified facade, I am having a hard time passing multiprocessing Queue objects as arguments on the futures.submit() when I switch to using a ProcessPoolExecutor:
import multiprocessing as mp
import concurrent.futures
def foo(q):
q.put('hello')
if __name__ == '__main__':
executor = concurrent.futures.ProcessPoolExecutor()
q = mp.Queue()
p = executor.submit(foo, q)
p.result()
print(q.get())
bumps into the following exception coming from multiprocessing's code:
RuntimeError: Queue objects should only be shared between processes through inheritance
which I believe means it doesn't like receiving the queue as an argument, but rather expects to (not in any OOP sense) "inherit it" on the multiprocessing fork rather than getting it as an argument.
The twist is that with bare-bones multiprocessing, meaning when not using it through the facade which concurrent.futures is ― there seems to be no such limitation, as the following code seamlessly works:
import multiprocessing as mp
def foo(q):
q.put('hello')
if __name__ == '__main__':
q = mp.Queue()
p = mp.Process(target=foo, args=(q,))
p.start()
p.join()
print(q.get())
I wonder what am I missing about this ― how can I make the ProcessPoolExecutor accept the queue as an argument when using concurrent.futures the same as it does when using the ThreadPoolExecutor or multiprocessing very directly like shown right above?

How to consume all messages from RabbitMQ queue using pika

I would like to write a daemon in Python that wakes up periodically to process some data queued up in a RabbitMQ queue.
When the daemon wakes up, it should consume all messages in the queue (or min(len(queue), N), where N is some arbitrary number) because it's better for the data to be processed in batches. Is there a way of doing this in pika, as opposed to passing in a callback that gets called on every message arrival?
Thanks.
Here is the code written using pika. A similar function can be written using basic.get
The below code will make use of channel.consume to start consuming messages. We break out/stop when the desired number of messages is reached.
I have set a batch_size to prevent pulling of huge number of messages at once. You can always change the batch_size to fit your needs.
from pika import BasicProperties, URLParameters
from pika.adapters.blocking_connection import BlockingChannel, BlockingConnection
from pika.exceptions import ChannelWrongStateError, StreamLostError, AMQPConnectionError
from pika.exchange_type import ExchangeType
import json
def consume_messages(queue_name: str):
msgs = list([])
batch_size = 500
q = channel.queue_declare(queue_name, durable=True, exclusive=False, auto_delete=False)
q_length = q.method.message_count
if not q_length:
return msgs
msgs_limit = batch_size if q_length > batch_size else q_length
try:
# Get messages and break out
for method_frame, properties, body in channel.consume(queue_name):
# Append the message
try:
msgs.append(json.loads(bytes.decode(body)))
except:
logger.info(f"Rabbit Consumer : Received message in wrong format {str(body)}")
# Acknowledge the message
channel.basic_ack(method_frame.delivery_tag)
# Escape out of the loop when desired msgs are fetched
if method_frame.delivery_tag == msgs_limit:
# Cancel the consumer and return any pending messages
requeued_messages = channel.cancel()
print('Requeued %i messages' % requeued_messages)
break
except (ChannelWrongStateError, StreamLostError, AMQPConnectionError) as e:
logger.info(f'Connection Interrupted: {str(e)}')
finally:
# Close the channel and the connection
channel.stop_consuming()
channel.close()
return msgs
You can use the basic.get API, which pulls messages from the brokers, instead of subscribin for the messages to be pushed

QThread doesn't appear to start; PyQt5, Python 2.7.9

SUMMARY
PyQt5 doesn't appear to be creating a new thread corresponding to QThread object, or I haven't established Slot/Signal linkage correctly. Please help me to isolate my problem.
I'm a relatively casual user of Python, but I've been asked to create a utility for another team that wraps some of their Python libraries (which themselves wrap C++) in a GUI. Because this utility is for another team, I can't change versions of compilers etc, or at least, not without providing a decent reason.
The utility is intended to provide an interface for debugging into some hardware that my colleagues are developing.
After examining the options, I decided to use Qt and the PyQt bindings. The steps I followed were:
Install Visual Studio 2010 SP1 (required because other team's libraries are compiled using this version of the MS compiler).
Install Python 2.7.9 (their version of Python)
Install qt-opensource-windows-x86-msvc2010-5.2.1.exe
Get source for SIP-4.18.zip and compile and install
Get source for PyQt-gpl-5.2.1.zip, compile and install
Try to build a PyQt application that wraps the other team's comms and translation libraries. Those libraries aren't asynchronous as far as I can tell, so I think that I need to separate that part of the application from the GUI.
The code that I've written produces the UI and is responsive in the sense that if I put break points in the methods that are called from the QAction objects, then those break points are appropriately triggered. My problem is that the Worker object that I create doesn't appear to move to a separate thread, (despite the call to moveToThread) because if I make the connection of type BlockingQueuedConnection instead of QueuedConnection then I get a deadlock. Breakpoints that I put on the slots in the Worker type are never triggered.
Here's the code::
import os
import sys
import time
from PyQt5.QtWidgets import QMainWindow, QTextEdit, QAction, QApplication, QStatusBar, QLabel, QWidget, QDesktopWidget, QInputDialog
from PyQt5.QtGui import QIcon
from PyQt5.QtCore import Qt, QThread, QObject, pyqtSignal, pyqtSlot
class Worker(QObject):
def __init__(self):
super(Worker, self).__init__()
self._isRunning = True
self._connectionId = ""
self._terminate = False
#pyqtSlot()
def cmd_start_running(self):
"""This slot is used to send a command to the HW asking for it to enter Running mode.
It will actually work by putting a command in a queue for the main_loop to get to
in its own serialised good time. All the other commands will work in a similar fashion
Up until such time as it is implemented, I will fake it."""
self._isRunning = True
pass
#pyqtSlot()
def cmd_stop_running(self):
"""This slot is used to send a command to the HW asking for it to enter Standby mode.
Up until such time as it is implemented, I will fake it."""
self._isRunning = False
#pyqtSlot()
def cmd_get_version(self):
"""This slot is used to send a command to the HW asking for its version string"""
pass
#pyqtSlot()
def cmd_terminate(self):
"""This slot is used to notify this object that it has to join the main thread."""
pass
#pyqtSlot()
def main_loop(self):
"""This slot is the main loop that is attached to the QThread object. It has sleep periods
that allow the messages on the other slots to be processed."""
while not self._terminate:
self.thread().sleep(1)
# While there is stuff on the wire, get it off, translate it, then
# signal it
# For the mean while, pretend that _isRunning corresponds to when
# RT streams will be
# being received from the HW.
if self._isRunning:
pass
# Search queue for commands, if any found, translate, then put on
# the wire
class DemoMainWindow(QMainWindow):
sgnl_get_version = pyqtSignal()
sgnl_start_running = pyqtSignal()
sgnl_stop_running = pyqtSignal()
sgnl_terminate = pyqtSignal()
def __init__(self):
super(DemoMainWindow, self).__init__()
self.initUI()
self._workerObject = Worker()
self._workerThread = QThread()
self._workerObject.moveToThread(self._workerThread)
self._workerThread.started.connect(self._workerObject.main_loop, type=Qt.QueuedConnection)
# I changed the following connection to type BlockingQueuedConnection,
# and got a Deadlock error
# reported, so I assume that there is already a problem before I get to
# this point.
# I understand that the default for 'type' (Qt.AutoConnection) is
# supposed to correctly infer that a QueuedConnection is required.
# I was getting desperate.
self.sgnl_get_version.connect(self._workerObject.cmd_get_version, type=Qt.QueuedConnection)
self.sgnl_start_running.connect(self._workerObject.cmd_start_running, type=Qt.QueuedConnection)
self.sgnl_stop_running.connect(self._workerObject.cmd_stop_running, type=Qt.QueuedConnection)
self.sgnl_terminate.connect(self._workerObject.cmd_terminate, type=Qt.QueuedConnection)
def initUI(self):
textEdit = QTextEdit()
self.setCentralWidget(textEdit)
lbl = QLabel(self.statusBar())
lbl.setText("HW Version: ")
self.statusBar().addPermanentWidget(lbl)
exitAction = QAction(QIcon('exit24.png'), 'Exit', self)
exitAction.setShortcut('Ctrl+Q')
exitAction.setStatusTip('Exit application')
exitAction.triggered.connect(self.close)
connectAction = QAction(QIcon('connect24.png'), 'Connect', self)
connectAction.setStatusTip('Connect to HW')
connectAction.triggered.connect(self.establishCanConnection)
enterRunningAction = QAction(QIcon('start24.png'), 'Start Running', self)
enterRunningAction.setStatusTip('Start Running')
enterRunningAction.triggered.connect(self.enterRunning)
enterStandbyAction = QAction(QIcon('stop24.png'), 'Stop Running', self)
enterStandbyAction.setStatusTip('Stop Running')
enterStandbyAction.triggered.connect(self.enterStandby)
self.statusBar()
menubar = self.menuBar()
fileMenu = menubar.addMenu('&File')
fileMenu.addAction(exitAction)
hwMenu = menubar.addMenu('&Hardware')
hwMenu.addAction(connectAction)
hwMenu.addAction(enterRunningAction)
hwMenu.addAction(enterStandbyAction)
toolbar = self.addToolBar('Exit')
toolbar.addAction(exitAction)
toolbar.addAction(connectAction)
toolbar.addAction(enterRunningAction)
toolbar.addAction(enterStandbyAction)
self.setGeometry(300, 300, 400, 350) # x, y, width, height
self.setWindowTitle('Demo Prog')
self.show()
def establishCanConnection(self):
iDlg = QInputDialog(self)
iDlg.setInputMode(QInputDialog.IntInput)
idInt, ok = iDlg.getInt(self, 'CAN ID Selection', 'HW ID:')
canID = '%s%d' % ('HW', idInt)
if ok:
self._workerThread.start()
pass
# this would be where the channel is established
def enterRunning(self):
self.sgnl_start_running.emit()
# this would be where the command to start running is sent from
def enterStandby(self):
self.sgnl_stop_running.emit()
# send the command to stop running
if __name__ == '__main__':
app = QApplication(sys.argv)
mainWindow = DemoMainWindow()
sys.exit(app.exec_())
Note that the call to start the _workerThread is in the establishCanConnection method, but that shouldn't be a problem, should it?
I used the procmon utility to check if more threads are created if establishCanConnection is run, and it appears that there are more threads, but I found it hard to relate which thread (if any of them) related to the QThread object.
Don't use BlockingQueuedConnection unless you really need it. If you don't know whether you need it or not, then you don't need it.
Cross-thread signals are queued in the event-loop of the receiving thread. If that thread is running code that blocks, it won't be able to process any events. Thus, if you send a signal with BlockingQueuedConnection to a thread that is blocked, you'll get a deadlock.
Your example uses a worker object that runs a blocking while loop, so it is subject to the deadlock problem outlined above. If you want to send signals to a thread that is blocked, you will need to arrange for the blocking code to periodically allow the thread to process its events, like this:
while not self._terminate:
self.thread().sleep(1)
QApplication.processEvents()
PS:
If you want to check that the worker is running in a different thread, you can print the return value of QThread.currentThread() or QThread.currentThreadId() (these functions are static, so you don't need an instance of QThread to call them).

Python Reddis Queue ValueError: Functions from the __main__ module cannot be processed by workers

I'm trying to enqueue a basic job in redis using python-rq, But it throws this error
"ValueError: Functions from the main module cannot be processed by workers"
Here is my program:
import requests
def count_words_at_url(url):
resp = requests.get(url)
return len(resp.text.split())
from rq import Connection, Queue
from redis import Redis
redis_conn = Redis()
q = Queue(connection=redis_conn)
job = q.enqueue(count_words_at_url, 'http://nvie.com')
print job
Break the provided code to two files:
count_words.py:
import requests
def count_words_at_url(url):
resp = requests.get(url)
return len(resp.text.split())
and main.py (where you'll import the required function):
from rq import Connection, Queue
from redis import Redis
from count_words import count_words_at_url # added import!
redis_conn = Redis()
q = Queue(connection=redis_conn)
job = q.enqueue(count_words_at_url, 'http://nvie.com')
print job
I always separate the tasks from the logic running those tasks to different files. It's just better organization. Also note that you can define a class of tasks and import/schedule tasks from that class instead of the (over-simplified) structure I suggest above. This should get you going..
Also see here to confirm you're not the first to struggle with this example. RQ is great once you get the hang of it.
Currently there is a bug in RQ, which leads to this error. You will not be able to pass functions in enqueue from the same file without explicitly importing it.
Just add from app import count_words_at_url above the enqueue function:
import requests
def count_words_at_url(url):
resp = requests.get(url)
return len(resp.text.split())
from rq import Connection, Queue
from redis import Redis
redis_conn = Redis()
q = Queue(connection=redis_conn)
from app import count_words_at_url
job = q.enqueue(count_words_at_url, 'http://nvie.com')
print job
The other way is to have the functions in a separate file and import them.

Multi Threading in scapy for sending packet

I need to send many packet at the same time in Scapy, for example i need to run this command at the same time:
send(IP(dst="69.69.69.69"),loop=1)
Is there any way to multi-thread this command? Or something to send packet in parallel?
Sorry about that, I am not an expert programmer.
import threading
import time
from scapy.all import *
LENGTH = 100 # min
life = 0
a=IP()
a.src="10.0.0.4"
a.dst="10.0.0.5"
def send_pkts():
global a
send(a)
global life
life = life +1
if life<LENGTH*60:
t=threading.Timer(1,send_pkts,())
t.start()
t=threading.Timer(1,send_pkts,())
t.start()