I am building REST API with Flask-restplus. One of my endpoints takes a file uploaded from client and run some analysis. The job uses up to 30 seconds. I don't want the job to block the main process. So the endpoint will return a response with 200 or 201 right away, the job can still be running. Results will be saved to database which will be retrieved later.

It seems I have two options for long-running jobs.

1. Threading
2. Task-queue

Threading is relatively simpler. But problem is, there is a limit of thread numbers for Flask app. In a standalone Python app, I could use a queue for the threads. But this is REST api, each request call is independent. I don't know if there is a way to maintain a global queue for that. So if the requests exceed the thread limit, it won't be able to take more requests.

Task-queue with Celery and Redis is probably better option. But this is just a proof of concept thing, and time line is kind of tight. Setting up Celery, Redis with Flask is not easy, I am having lots of trouble on my dev machine which is a Windows. It will be deployed on AWS which is kind of complex.

I wonder if there is a third option for this case?

I would HIGHLY recommend using Celery as you have already mentioned in your post. It is built exactly for this use case. Their docs are really informative and there are no shortage of examples online that can get you up and running quickly.

Additionally, I would say THIS would be an excellent first resource for you to start with.