Seeking ideas for a cron implementation

[Sean DiZazzo]

On Aug 22, 1:30 pm, Karthik Gurusamy <kar1... at gmail.com> wrote:
> Hi,
>
> I'm working on a cron like functionality for my application.
> The outer loops runs continuously waking every x seconds (say x=180,
> 300, ..).
> It needs to know what events in cron has expired and for each event do
> the work needed.
>
> It's basically like unix cron or like a calendar application with some
> restrictions. The outer loop may come back a lot later and many events
> might have missed their schedule -- but this is okay.. We don't have
> to worry about missed events (if there were n misses, we just need to
> execute call back once).
>
> Let's take some examples [Let e denotes an event]
> e1: hour=1  min=30                             # Run every day once at
> 1:30 AM
> e2: wday=0, hour=1  min=0                   # run every Monday at 1 AM
> e3: month=10, day=10, hour=10 min=0  # run on October 10th, 10 AM
> every year
>
> class Cron_Event (object):
>     def __init__ (year=None, month=None, day=None, hour=None ..etc)
>       #  do init
>
> class Cron (object):
>     def __init__ ():
>         # do init
>     def event_add (e):
>         # add an event
>     def execute()
>         # see if any events has "expired" .. call it's callback
>         # I'm looking for ideas on how to manage the events here
>
> From outer loop
> cron = Cron()
> # create various events like
> e1 = Cron_Event(hour=1)
> cron.event_add(e1)
> e2 = Cron_Event(wday=0, hour=1)
> cron.event_add(e2)
>
> while True:
>     sleep x seconds (or wait until woken up)
>     cron.execute()
>     # do other work.. x may change here
>
> If I can restrict to hour and minute, it seems manageable as the
> interval between two occurrences is a constant. But allowing days like
> every Monday or 1st of every month makes things complicated. Moreover
> I would like each constraint in e to take on multiple possibilities
> (like every day at 1AM,  2 AM and 4 AM do this).
>
> I'm looking for solutions that can leverage datetime.datetime
> routines.
> My current ideas include for each e, track the next time it will fire
> (in seconds since epoch as given by time.time()). Once current time
> has passed that time, we execute the event. e.g.>>> datetime.datetime.now()
>
> datetime.datetime(2008, 8, 22, 13, 19, 54, 5567)>>> time.time()
>
> 1219436401.741966    <--- compute event's next firing in a format like
> this
>
>
>
> The problem seems to be how to compute that future point in time (in
> seconds since epoch)  for a generic Cron_Event.
>
> Say how do I know the exact time in future  that will satisfy a
> constraint like:
>  month=11, wday=1, hour=3, min=30    # At 3:30 AM on a Tuesday in
> November
>
> Thanks for your thoughts.
>
> Karthik

I only scanned your message, but maybe datetime.timedelta() will
help..

>>> import datetime
>>> now = datetime.datetime.now()
>>> print now
2008-08-22 13:48:49.335225
>>> day = datetime.timedelta(1)
>>> print day
1 day, 0:00:00
>>> print now + day
2008-08-23 13:48:49.335225

~Sean