numegil / django-async

A simple asynchronous execution Django application.

Django Async is an asynchronous execution queue for Django with proper database transaction management.

Building a database backed task queue is a fairly trivial thing, but getting the database transactions exactly right is no simple matter.

Using Django Async

Installation is very simple, just add the async application to your Django applications in settings.py.

To run a job asynchronously just use the schedule function:

from async import schedule
schedule('my.function', args=(1, 2, 3), kwargs=dict(key='value'))

Tasks can be run by executing the management command flush_queue:

python manage.py flush_queue

flush_queue will run once through the jobs that are scheduled to run at that time, but will exit early if any job throws an exception. Normally you would use it from an external script that simply keeps re-running the command.

while [ true ] ; do ( python manage.py flush_queue && sleep 10 ) ; done

Jobs are executed in priority order first (higher numbers execute earlier), then by the scheduled time (unscheduled jobs will go last, but of course only jobs whose scheduled time has arrived will run) and finally by their ID order (which should be the order they were added). A failed task will be re-scheduled for later execution.

##async.schedule##

schedule(function, args = None, kwargs = None, run_after= None, meta = None, priority = 5)

Returns a Job instance that is used to record the task in the database. The job has a method execute which will attempt to run the job. Don't do this directly until you've fully understood how transactions are handled

• function Either the fully qualified name of the function that is to be run, or the function itself.
• args A tuple or list of arguments to be given to the function.
• kwargs A dict containing key word arguments to be passed to the function.
• run_after The earliest time that the function should be run.
• meta Parameters for controlling how the function is to be executed.
• priority Jobs with higher numbers are always executed before jobs with lower numbers.

##async.api.deschedule##

deschedule(function, args = None, kwargs = None)

Marks all jobs in the queue that match the given function and arguments as executed.

De-scheduling does not guarantee that the de-scheduled tasks will never run unless the de-scheduling is done within a task executed in the job queue. This is because the task might have already started executing within the queue whilst it is de-scheduled from outside the queue.

##async.api.health##

info = health()

Returns a dict containing basic information about the queue which can be used for monitoring.

Transaction handling

Database transactions are hard to get right, and unfortunately Django doesn't make them much easier. Firstly, you really want to be using a proper transactional database system.

Django has two major flaws when it comes to transaction handling:

1. The Django transaction functionality fails to create composable transactions.
2. The [https://docs.djangoproject.com/en/dev/topics/db/transactions/](Django documentation) makes a very poor recommendation about where to put the django.middleware.transaction.TransactionMiddleware.

The first problem is not going to get fixed in Django, but the second can be handled by putting the middleware in the right place -- that is, as early as possible. The only middleware that should run before the transaction middleware is any whose functionality relies on it being first.

Within the async task execution each task is executed decorated by django.db.transaction.commit_on_success. This means that you cannot execute a task directly from within a page request if you are using the transaction middleware.

Doing development

This project uses git flow. Don't forget to do git flow init -d

To create virtual environments for running the tests you can execute test-projects/make-virtual-environments. To run the tests execute runtests.