CODEWITHSUNDEEP
pythonpython-3.xflaskzerorpc
Christian Benseler
Christian Benseler

Reputation: 8065

Use zerorpc inside Flask app throws error "operation would block forever"

I have a RPC Server using zerorpc in Python, written this way

import zerorpc
from service import Service

print('RPC server - loading')
def main():
    print('RPC server - main')
    s = zerorpc.Server(Service())
    s.bind("tcp://*:4242")
    s.run()

if __name__ == "__main__" : main()

It works fine when I create a client

import zerorpc, sys

client_rpc = zerorpc.Client()
client_rpc.connect("tcp://127.0.0.1:4242")
name = sys.argv[1] if len(sys.argv) > 1 else "dude"
print(client_rpc.videos('138cd9e5-3c4c-488a-9b6f-49907b55a040.webm'))

and runs it. The print() outputs what this 'videos' function returns.

But when I try to use it this same code inside route from a Flask app, I receive the following error:

File "src/gevent/__greenlet_primitives.pxd", line 35, in gevent.__greenlet_primitives._greenlet_switch gevent.exceptions.LoopExit: This operation would block forever Hub:

The flask method/excerpt

import zerorpc, sys

client_rpc = zerorpc.Client()
client_rpc.connect("tcp://127.0.0.1:4242")

@app.route('/videos', methods=['POST'])
def videos():

    global client_rpc
    client_rpc.videos('138cd9e5-3c4c-488a-9b6f-49907b55a040.webm')

I can't find out what might be happening. I'm quite new to Python and I understand that this may have some relation with Flask and how it handles the thread, but I can't figure out how to solve it.

Upvotes: 0

Views: 568

Answers (1)

bombela
bombela

Reputation: 633

zerorpc depends on gevent, which provides async IO with cooperative coroutines. This means your flask application must use gevent for all IO operations.

In your specific case, you are likely starting your application with a standard blocking IO WSGI server.

Here is a snippet using the WSGI server from gevent:

import zerorpc
from gevent.pywsgi import WSGIServer

app = Flask(__name__)

client_rpc = zerorpc.Client()
client_rpc.connect("tcp://127.0.0.1:4242")

@app.route('/videos', methods=['POST'])
def videos():
    global client_rpc
    client_rpc.videos('138cd9e5-3c4c-488a-9b6f-49907b55a040.webm')

# ...

if __name__ == "__main__":
    http = WSGIServer(('', 5000), app)
    http.serve_forever()

Excerpt from https://sdiehl.github.io/gevent-tutorial/#chat-server

Upvotes: 0

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