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Scheduled event system [a little OT]

by Cody Pendant (Prior)
on Jun 19, 2009 at 05:58 UTC ( #772934=perlquestion: print w/ replies, xml ) Need Help??
Cody Pendant has asked for the wisdom of the Perl Monks concerning the following question:

I'm considering a kind of system where a script finds events which are due to happen and fires them at the right time.

I think I understand the basics of how cron would call a perl script, which would talk to a database ... but I'm trying to think how the database would be set up and work.

For any given event, "December 31st at 9 PM, do X" it's not that hard -- I guess the system would fire at 9, select the event based on its time being less-than-or-equal-to the current time (and its status as not completed), do whatever it was, then mark it as having completed.

But a truly useful system would have something where users could put in things like "every second Friday", "the second Wednesday of every month", "every Tuesday in March" and also need to have spans of time, i.e. "every second Friday, ending on January 31st" -- and I think the system would be even better if it could allow for exceptions, like "every Friday, but NOT Friday the 26th of June".

Is there anything pre-written out there? Some kind of object-mapping? What are the normal design patterns for a thing like this -- populate a database with multiple dates when a recurring event is added? Have a database of exceptions to events which are allowed to over-rule the normal events?

As I said in the title, it's not exactly a perl question but I know the monks will have interesting things to say.



Nobody says perl looks like line-noise any more
kids today don't know what line-noise IS ...

Comment on Scheduled event system [a little OT]
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Re: Scheduled event system [a little OT]
by Anonymous Monk on Jun 19, 2009 at 06:12 UTC
      That's great, thanks. The Recurrence module looks like it will take me quite a long way.


      Nobody says perl looks like line-noise any more
      kids today don't know what line-noise IS ...
Re: Scheduled event system [a little OT]
by Plankton (Priest) on Jun 19, 2009 at 06:49 UTC
    Some projects where I work use Dollar Universe. I don't think it is open source though.
Re: Scheduled event system [a little OT]
by bart (Canon) on Jun 19, 2009 at 11:56 UTC
    What are the normal design patterns for a thing like this -- populate a database with multiple dates when a recurring event is added?

    Well, the way I would do it, is to calculate when the next event will occur, and then schedule that. When that event is triggered, you can calculate the next occurrence. So there is never a need to store more than one date.

    As for the exceptions list... You can use cron as the base, but instead of a command line, you store a symbolic name... or an ID. Specify the associated command line separately. You can have more entries for one name, and give each a precedence, pretty much just like the filter rules for an email/usenet client. Specify what you want to happen: run, or suppress. The first rule that triggers specifies what will happen at that specified time. And when the verdict is to suppress, you'll have to calculate the next occurrence after that time...

Re: Scheduled event system [a little OT]
by gloryhack (Deacon) on Jun 20, 2009 at 09:45 UTC
    You might have a look at remind. Kids these days, always reinventing wheels... :-)
      Thank you. Looks interesting.


      Nobody says perl looks like line-noise any more
      kids today don't know what line-noise IS ...

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