Flask, processing requests 1 by 1

Question

I have a flask application which listens for some job to do. The process is quite long (let us say 1 minute) and I would like not allow to process two requests at the same time.

I will be great if once I receive a request, I could close the port flask is listening to and open again when finish. Alternatively I could setup a semaphore but I am not sure about how flask is running concurrently.

Any advice?

from flask import Flask, request
app = Flask(__name__)

@app.route("/",methods=['GET'])
def say_hi():
    return "get not allowed"

@app.route("/",methods=['POST'])
def main_process():
    # heavy process here to run alone
    return "Done"

if __name__ == "__main__":
    app.run(debug=True,host='0.0.0.0')

Answer 1

You could use a semaphore for this:

import threading
import time
sem = threading.Semaphore()

@app.route("/",methods=['POST'])
def main_process():
    sem.acquire()
    # heavy process here to run alone
    sem.release()
    return "Done"

The semaphore usage is to control the access to a common resource.

You can see more info about semaphore in here

This SO question can help you as well here

EDIT:

As Georg Schölly wrote in comment, The above mentioned solution is problematic in a situation of multiple services.

Although, you can use the wsgi in order to accomplish your goal.

@app.route("/",methods=['POST'])
def main_process():
    uwsgi.lock()
    # Critical section
    # heavy process here to run alone
    uwsgi.unlock()
    return "Done"

uWSGI supports a configurable number of locks you can use to synchronize worker processes

For more info, read here