14.3.55. crate_anon.crateweb.consent.tasks

crate_anon/crateweb/consent/tasks.py

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Copyright (C) 2015, University of Cambridge, Department of Psychiatry. Created by Rudolf Cardinal (rnc1001@cam.ac.uk).

This file is part of CRATE.

CRATE is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

CRATE is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with CRATE. If not, see <https://www.gnu.org/licenses/>.

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Celery tasks for the CRATE consent-to-contact system.

See also crate_anon.crateweb.consent.celery, which defines the app.

If you get a “received unregistered task” error:

1. Restart the Celery worker. That may fix it.

2. If that fails, consider: SPECIFY ABSOLUTE NAMES FOR TASKS. e.g. with @shared_task(name="myfuncname"). Possible otherwise that if this module is imported in different ways (e.g. absolute, relative), you’ll get a “Received unregistered task” error. https://docs.celeryq.org/en/latest/userguide/tasks.html#task-names

Acknowledgement/not doing things more than once:

• https://docs.celeryproject.org/en/latest/userguide/tasks.html

• default is to acknowledge on receipt of request, not after task completion; that prevents things from happening more than once. If you can guarantee your function is idempotent, you can acknowledge after completion.

• https://docs.celeryproject.org/en/latest/faq.html#faq-acks-late-vs-retry

• We’ll stick with the default (slightly less reliable but won’t be run more than once).

Circular imports:

• https://stackoverflow.com/questions/17313532/django-import-loop-between-celery-tasks-and-my-models

• The potential circularity is:

  • At launch, Celery must import tasks, which could want to import models.

  • At launch, Django loads models, which may use tasks.

• Simplest solution is to keep tasks very simple (as below) and use delayed imports here.

Race condition:

• Django:

  1. existing object

  2. amend with form

  3. save()

  4. call function.delay(obj.id)

  Object is received by Celery in the state before save() at step 3.

• https://celery.readthedocs.org/en/latest/userguide/tasks.html#database-transactions

• https://stackoverflow.com/questions/26862942/django-related-objects-are-missing-from-celery-task-race-condition

• https://code.djangoproject.com/ticket/14051

• https://github.com/aaugustin/django-transaction-signals

• SOLUTION: https://docs.djangoproject.com/en/dev/topics/db/transactions/#django.db.transaction.on_commit

  from django.db import transaction
  transaction.on_commit(lambda: blah.delay(blah))
  

  Requires Django 1.9. As of 2015-11-21, that means 1.9rc1