Making an asynchronous task in Flask

Question

I am writing an application in Flask, which works really well except that WSGI is synchronous and blocking. I have one task in particular which calls out to a third party API and that task can take several minutes to complete. I would like to make that call (it's actually a series of calls) and let it run. while control is returned to Flask.

My view looks like:

@app.route('/render/<id>', methods=['POST'])
def render_script(id=None):
    ...
    data = json.loads(request.data)
    text_list = data.get('text_list')
    final_file = audio_class.render_audio(data=text_list)
    # do stuff
    return Response(
        mimetype='application/json',
        status=200
    )

Now, what I want to do is have the line

final_file = audio_class.render_audio()

run and provide a callback to be executed when the method returns, whilst Flask can continue to process requests. This is the only task which I need Flask to run asynchronously, and I would like some advice on how best to implement this.

I have looked at Twisted and Klein, but I'm not sure they are overkill, as maybe Threading would suffice. Or maybe Celery is a good choice for this?

Answer 1

I would use Celery to handle the asynchronous task for you. You'll need to install a broker to serve as your task queue (RabbitMQ and Redis are recommended).

app.py:

from flask import Flask
from celery import Celery

broker_url = 'amqp://guest@localhost'          # Broker URL for RabbitMQ task queue

app = Flask(__name__)    
celery = Celery(app.name, broker=broker_url)
celery.config_from_object('celeryconfig')      # Your celery configurations in a celeryconfig.py

@celery.task(bind=True)
def some_long_task(self, x, y):
    # Do some long task
    ...

@app.route('/render/<id>', methods=['POST'])
def render_script(id=None):
    ...
    data = json.loads(request.data)
    text_list = data.get('text_list')
    final_file = audio_class.render_audio(data=text_list)
    some_long_task.delay(x, y)                 # Call your async task and pass whatever necessary variables
    return Response(
        mimetype='application/json',
        status=200
    )

Run your Flask app, and start another process to run your celery worker.

$ celery worker -A app.celery --loglevel=debug

I would also refer to Miguel Gringberg's write up for a more in depth guide to using Celery with Flask.