A small demonstration of getting Celery 2.4.0 working with Flask 1.0.2 (with logging).
Flask and Celery work well together once you work through the tedious details of getting everything set up just right. Googling for help is liable to turn up partial answers that don't work together, in part due to recent revisions in Celery. Hence this example.
If you have VirtualBox and Vagrant installed, vagrant up will get you a VM.
If you don't, see the steps in the script block at the top of Vagrantfile.
You'll need Redis running. If you built the VM, it'll be there.
If you've built the VM, vagrant ssh into it twice
(or once and use tmux get get multiple shells).
In one shell,
venv/bin/celery -A demo.celery --loglevel=info worker
In another shell,
FLASK_APP=demo.py venv/bin/flask run --host='0.0.0.0'
(or use the convenient run script, which does the same,
plus turning on FLASK_DEBUG)
Then browse to http://localhost:5000/
Behind the scenes, but visible in the logs, the handler for / on the
Flask app side creates a task and then waits for it to complete.
On the celery side, a worker that's been waiting for tasks to run gets it,
processes it, and stores the answer.
You can also initiate tasks form the command line. From a third shell,
FLASK_APP=demo.py venv/bin/flask add 40 2
will initiate a task, wait for the result, and print it. A trivial example, but imagine instead launching a task that scrapes a web site and populates local database tables using models and code that are shared with the Flask app.
If you're using this as a starting point, note that both celery and flask
start with demo.py. If you need to any interesting app setup on the Flask
side, you might want to make a separate worker.py, which would still call
create_app(), but can forego the extra app setup.
For scheduled tasks, there are two options: The first is to use the command-line trick above from a cron(1) job. The second is to run a separate celery "heartbeat" worker, and configure schedules via celery. Celery has some neat scheduling options, such as solar noon at a given latitude/longitude. Consult the Celery docs.
If you get close to production, consider replacing Redis with RabbitMQ as the broker.
None. There's nothing novel here worth claiming. It's also use-at-your-own-risk-ware. It works for me (on a Raspberry Pi, no less), but YMMV.