使用 Gunicorn 和 Gevent 运行 Flask 时如何使用请求发出非阻塞请求

Question

My Flask application will receive a request, do some processing, the processiong takes long time. Running Gunicorn with Gevent will allow it to handle many of these slow requests at the same time. I konw when my application process this request, it also can process other requests, But my purpose is that in this request it can response client quickly and still process image download on background。 How can I modify the example below so that the view is non-blocking?

@app.route('/do', methods = ['POST'])
def do():
    # here download many picture, it will tasks long time, and block。
    return 'ok'

Answer 1

Flask + gevent isn't the right tool for this. Gevent uses monkey patching to run regular code on the event loop- but in the process, you don't have the ability to run regular event loop stuff (like scheduling an async task). I am not a gevent expert (so maybe there is a technique I am not aware of), but I don't think so.

To accomplish this, I would do one of two things.

1. Set up an async task system. I have used celery a ton, but there are other alternatives. The downside of this is you usually need another piece of infrastructure- Redis, rabbitmq, something like that. It really is the best solution though.

2. Use an explicitly async web framework. I use Sanic for this. It lets you schedule coroutines directly from a view so that they can continue to run even after the view coroutine has completed. This means that ALL your code has to be async though, which is a learning curve.

There is a less good solution to use threading or multiprocessing to start your work and just not have anything monitoring it to do cleanup. Finally, uwsgi has some stuff built in for background tasks, though I haven't tried it.