问题
How can I run a celery task at a given time, but only once?
I read the documentation and couldn't find any example of this.
回答1:
If you insist on using Celery
To run a task at a specified time, in Celery you would normally use a periodic task, which conventionally is a recurring task.
However, you may create a periodic task with a very specific schedule and condition that happens only once so effectively it runs only once.
Unfortunately we can only specify so much, e.g. we can specify hour, minute, day_of_month and month_of_year but we can't specify year
However with that, your task would run at most 1 time for per year, so below are some workarounds:
Unschedule it after it is ran
It should be relatively easy to unschedule it once it is ran (you have 1 year to do so!)
Use a "DONE" flag when the task completes
With a flag written somewhere (disk or DB), you can first check if the task has run before or not, i.e. if done: exit
Exit if not proper year
or you want to be safe, just add code into the task that checks the year, e.g. if year != 2017: exit.
Simple cron/Os level scheduler works too
You may also skip Celery altogether and use some OS level facility like cron for UNIX-like systems, more on that here.
The general idea remains the same.
回答2:
You can use paramater eta when calling the task. Example:
from datetime import datetime, timedelta
@app.task()
def hello(self):
return 'hello world'
tomorrow = datetime.utcnow() + timedelta(days=1)
hello.apply_async(eta=tomorrow)
Documentation: http://docs.celeryproject.org/en/latest/userguide/calling.html#eta-and-countdown
Alternatively, when you want to call hello multiple times and to be sure it is executed only one at the time, you can use locking - more about it in the documentation: http://docs.celeryproject.org/en/latest/tutorials/task-cookbook.html#ensuring-a-task-is-only-executed-one-at-a-time
来源：https://stackoverflow.com/questions/41568305/celery-run-task-once-at-a-specified-time