How to best port flask apps to v3 ?

[paketb0te]

Hey all,

with the recent release of connexion 3.0, apps now need to be run with an ASGI server.
I appreciate this, but have a question regarding the use of FlaskApp:

we have a bunch of flask apps where we do not use any async functions (which works ok in our current setup with WSGI). Our current setup ist to run the app in Apache, using uWSGI (where parallel request handling is achieved by multiple processes / threads).

How does that translate to running the app in an ASGI server?

Is there some mechanism that takes care of handling multiple parallel requests (e.g. dispatching requests to separate threads, like asyncio.to_thread), or do I need to take care of that myself by passing such parameters to the webserver running the app?

Assuming that we cannot (in the short term) port all apps to be actually async, what is the correct way to run blocking functions in connexion 3 / ASGI ?

I'm thankful for any pointers, I could not find anything related in the Running your application section of the docs.

[RobbeSneyders]

Hi @paketb0te,

For processes, you can set the number of workers in the ASGI server.

Threads depends on the Connexion Application you use:

• FlaskApp: Since Flask is a WSGI app instead of ASGI, we execute any requests passed to the FlaskApp in a thread pool (of 10 workers) automatically. If you define an async routes in the FlaskApp, you're just using Flask async routes within a thread.
• AsyncApp: The AsyncApp implements ASGI, so here threads are completely replaced by asyncio.

[RobbeSneyders]

A short term fix could be to wrap Connexion with an ASGI-to-WSGI middleware, although I'd advice against that.

[paketb0te]

Thanks for clarifying, @RobbeSneyders !

We currently use FlaskApp, so it's good to know that Connexion uses a thread pool automatically. Is my conclusion correct, that this means a single ASGI server process can handle 10 requests in parallel?

And regarding a possible transition to AsyncApp: Is my understanding correct, that it is not helpful to just use the same blocking code inside an AsyncApp?