38 lines
1.8 KiB
Markdown
38 lines
1.8 KiB
Markdown
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---
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layout: post
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title: "Recurring events are hard"
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date: 2018-07-19 13:22:00
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tags: [development]
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published: true
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author:
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name: Gergely Polonkai
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email: gergely@polonkai.eu
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---
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It was almost a month ago when I
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[announced]({% post_url 2018-06-26-please-welcome-calendar-social %}) the development of
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Calendar.social. Since then I’m over some interesting and some less interesting stuff; (web)
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development, after all, is just a recurrence of patterns. Speaking of recurrence, I arrived to a
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really interesting topic: recurring events.
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My initial thought was like “oh, that’s easy! Let’s insert all future occurences as a separate
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`Event` object, linking to the original one for the details. That makes handling exceptions easy,
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as I just have to update/delete that specific instance.” Well, not really. I mean, an event
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repeating daily *forever* would fill up the database quickly, isn’t it? That’s when I decided to
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look how other projects do it.
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As it turns out, my first thought is about the same as everyone else has their mind, with about
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the same reasons. Then, they usually turn down the idea just like I did. And instead, they
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implement recurrence patterns and exception patterns.
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My favourite is
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[this article](https://github.com/bmoeskau/Extensible/blob/master/recurrence-overview.md) so far.
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The author suggests to use the recurrence patterns specced by
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[RFC2445](http://www.ietf.org/rfc/rfc2445.txt) (the spec for the iCalendar format). The
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interesting part in this solution is how to query recurring events: you simply store the timestamp
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of the last occurence of the events (or, if the event repeats forever, the greatest timestamp your
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database supports.)
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Choosing the maximum date seemed to be the tricky one, but it turned out both Python and popular
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SQL backends support dates up to the end of year 9999.
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