For demonstration purposes I've tried to create a simple Flask program, which starts a background process to process long running tasks. I planned the communication between the Flask app and the background process can be done using multiprocessing.Queue. It works fine on my own PC, but at here the queue's get waits forever even though there are some messages in the queue. Interesting: the qsize() can see the waiting messages, but get can't.
By the way, if I put a message in that queue next to starting the background process, the background can get it. When the q.put() placed in the routed function, the consumer process can't read out that message.
from flask import Flask
from multiprocessing import Process, Queue
...
def consumer(pq):
while True:
msg = pq.get()
print(f"Consumer: {msg}")
q = Queue()
p = Process(target=consumer, args=(q,))
p.run()
q.put("some message") # it is OK, consumer can get it
...
@app.route("/")
def index():
q.put("other message") # this increases the length of the queue, it visible
# in the consumer process, but the get() waits indefinitely,
# although pq.qsize() shows the correct length of the queue
return "Hello"
glenn
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PythonAnywhere staff
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