Flask/RabbitMQ: consume messages on a separate thread than the app

1. Pika is not thread safe. You should avoid sharing the connection object across Flask's contexts. Writing your own Flask plugin wouldn't take that much boilerplate though. It would be very similar to the documentation example plugin. Otherwise, you could do a quick search with flask pika on a search engine and you'll find some existing plugins for this purpose. I have not tried them and they don't seem really popular, but maybe you should give them a go?

2. I don't see why it wouldn't be possible. Flask knows how to deal with this. However, I reckon it would severly degrade performances. Moreover, you might hit some corner-cases if the plugins you use are not perfectly written.

3. Just like you would declare any lock for threading. Nothing much. You put it at the module level (not in Flask's context) so that it is global, that's it.

That being said, I think you shouldn't proceed this way. You should rather run the update-job in a different process from the Web Server (using Flask CLI or whatever if you need to re-use some functions). It will be better performance-wise, it's easier to reason about, it's more loosely coupled.

Also, you should avoid running into locking headaches as long as possible. Believe me, it's a real source of problems. It's a nightmare to test properly, to debug, to maintain and quite risky when it comes to real-production use-cases. And if you really, really need a lock, don't hold it for one minute, it's way too long.

I don't know your exact requirements, but there surely is a solution that is OK and that does not involve such complexity.