periodiccallback of asynchronous function

Question

Does PeriodicCallback ensure no parallel executions? Is the callback triggered while a previous triggered one is still running or does it ensure that there is only one callback running at a time? As far as I can see the later one is true, but I want to be sure! I should control that time of asynchronous function is no longer than next periodiccallback scheduler.

For example:

If I have a periodiccallback every 5 seconds and my function time sometimes (because makes http request etc) is longer (for example 7 second), how can I jump the "10 second" and pass to 15? Only when it happens.

Answer 1

The tornado docs says

If the callback runs for longer than callback_time milliseconds, subsequent invocations will be skipped to get back on schedule.

So if I'm understanding your question correctly, PeriodicCallback already does exactly what you want if your callback is a synchronous function!

If your callback isn't synchronous, then you will need to use locks to check yourself if something is already running or not. Something like:

from tornado import gen, locks

lock = locks.Lock()
running = False
@gen.coroutine
def task():
    global running
    with (yield lock.acquire()):
        if running:
            return
        running = True
    try:
        yield do_something()
    finally:
        with (yield lock.acquire()):
            running = False

http://www.tornadoweb.org/en/stable/ioloop.html#tornado.ioloop.PeriodicCallback