How to initialize MongoClient in a multiprocess Flask application?

Question

I have a Flask application which I'm running with uWSGI. The application initializes a MongoClient(). When running uWSGI with multiple workers I'm getting the MongoClient opened before fork warning.

I saw that one way to overcome this problem is to initializethe MongoClient inside an init method decorated with uWSGI's @postfork

The problem is that after the change my tests fails because pytest can't find uwsgi

#15 1.880 usr/local/lib/python3.9/site-packages/uwsgidecorators.py:10: in <module>
#15 1.880     import uwsgi
#15 1.880 E   ModuleNotFoundError: No module named 'uwsgi'

Questions:

• Is there any other easy way to initialize a MongoClient for each process?
• If not, how can I test my app.py?

Answer 1

Is there any other easy way to initialize a MongoClient for each process?

Wherever you use that client, use a function to acquire it, like below.

The function has guards to ensure each process gets their own client.

_mongo_client = None
_mongo_client_pid = None


def get_mongoclient():
    global _mongo_client, _mongo_client_pid
    if os.getpid() != _mongo_client_pid:
        _mongo_client = MongoClient(...)
        _mongo_client_pid = os.getpid()
    return _mongo_client

If not, how can I test my app.py?

You could use a guard like

try:
    from uwsgi import postfork
except ImportError:
    postfork = lambda f: f  # no-op decorator

to make your app work with and without uwsgi. The @postfork-marked code won't be called outside uwsgi.