Handle Flask requests concurrently with threaded=True

Question

What exactly does passing threaded = True to app.run() do?

My application processes input from the user, and takes a bit of time to do so. During this time, the application is unable to handle other requests. I have tested my application with threaded=True and it allows me to handle multiple requests concurrently.

Answer 1

As of Flask 1.0, the WSGI server included with Flask is run in threaded mode by default.

Prior to 1.0, or if you disable threading, the server is run in single-threaded mode, and can only handle one request at a time. Any parallel requests will have to wait until they can be handled, which can lead to issues if you tried to contact your own server from a request.

With threaded=True requests are each handled in a new thread. How many threads your server can handle concurrently depends entirely on your OS and what limits it sets on the number of threads per process. The implementation uses the SocketServer.ThreadingMixIn class, which sets no limits to the number of threads it can spin up.

Note that the Flask server is designed for development only. It is not a production-ready server. Don't rely on it to run your site on the wider web. Use a proper WSGI server (like gunicorn or uWSGI) instead.