gunicorn, threads, workers. How can I use the same object across all threads when using Flask-Socket #1989

noahra · 2023-05-15T17:39:15Z

noahra
May 15, 2023

Hello,

I have been following this guide and modified the code to conform with my project:
https://blog.miguelgrinberg.com/post/add-a-websocket-route-to-your-flask-2-x-application

My goal is to create a web app where you can control a quadcopter-drone with the Crazyswarm library.

Here is my app.py

from flask import Flask, render_template
from flask_sock import Sock
from pycrazyswarm import Crazyswarm


swarm = Crazyswarm()
cf = swarm.allcfs.crazyflies[0]

app = Flask(_name_)
sock = Sock(app)

@app.route('/')
def index():
    return render_template('index.html')
@sock.route('/action')
def echo(sock):
    while True:
        data = sock.receive()
        if data == 'left':
            print("LEFT")
            cf.goto([-1.,0,0],0,0,True,1.)
        if data == 'right':
            cf.goto([1.,0,0],0,0,True,1.)
        if data == 'forward':
            cf.goto([0.,1.,0],0,0,True,1.)
        if data == 'back':
            cf.goto([0.,-1.,0],0,0,True,1.)
        if data =='take off':
            print('take off....')
            cf.takeoff(targetHeight=1.0,duration=2.5)
        if data =='land':
            print('...landing, please wait')
            cf.land(targetHeight=1.0,duration=2.5)

index.html:

<!doctype html>
<html>
  <head>
    <title>Drone</title>
  </head>
  <script>
     
    
     var socket = new WebSocket('wss://' + location.host + '/action');


     function sendText(button) {
     var text = button.value; // Get the value of the button
     socket.send(text); // Send the text to the server
    // socket.onopen = () => conn.send(text);
   }
   </script>
  <body>
    <h1>Drone Controller</h1>
    <div id="log"></div>
    <br>
    <button value="left" onclick="sendText(this)">Left</button>
  <button value="right" onclick="sendText(this)">Right</button>
  <button value="back" onclick="sendText(this)">Back</button>
  <button value="forward" onclick="sendText(this)">Forward</button>
  <button value="land" onclick="sendText(this)">Land</button>
  <button value="take off" onclick="sendText(this)">Take off</button>
   
  </body>
</html>

The issue lies within using this command

$ gunicorn -b 0.0.0.0:5000 --workers 4 --threads 100 app:app

As multiple threads are run, this line in app.py is called multiple times:
cf = swarm.allcfs.crazyflies[0]

This creates a conflict, as this drone already has been instantiated in a different thread.

This leads to my program terminating.

If I only use one thread, the program will die rather quickly so I need the threads.

Is there a solution for only using on object across all threads? Because clearly, each thread instantiates that object..

I appreciate all answers, please ask for more details if needed. Sorry if I am being unclear.

Warm regards,
Noah

miguelgrinberg · 2023-05-15T17:52:08Z

miguelgrinberg
May 15, 2023
Maintainer

I think you are confusing threads with processes here. You are running 4 processes, so your global variables will be created four times. Change the --workers 4 to --workers 1 and you'll have only one process. Within that process, it is okay to have 100 threads, all those share the same context and will use the same set of global variables.

6 replies

miguelgrinberg May 16, 2023
Maintainer

Maybe the client went away? That would also trigger an OSError. I would have expected a "broken pipe" error in that case, but "bad file descriptor" can also occur if you try to send something that the other side closed the connection.

noahra May 16, 2023
Author

Do you mean that somehow the frontend-part disconnects?

miguelgrinberg May 16, 2023
Maintainer

Yes, that is what I mean.

noahra May 16, 2023
Author

Ok but then is it an issue related to my index.html code?

miguelgrinberg May 16, 2023
Maintainer

I have no way to know that. The client may have closed the browser voluntarily, or maybe there are networking issues. If you suspect your client is closing a connection when it shouldn't, then you should debug your client and obtain more information.

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