Designing a Calendar system like Google Calendar [closed]

Answer 1

You should have a start date, end date, and expiration date. Single day events would have the same start date and end date, and allows you to do partial day events as well. As for re-occuring events, then the start and end date would be for the same day, but have different times, then you have an enumeration or table that specifies the repeat frequency (daily, weekly, monthly, etc).

This allows you to say "this event appears every day" for daily, "this event appears on the 2nd day of every week" for weekly, "this event appears on the 5th day of every month" for monthly, "this event appears on the 215th day of every year" for yearly as long as the date is less than the expiration date.

Answer 2

I'm tackling exactly this problem, and I had completely spaced iCalendar (rfc 2445) up until reading this thread, so I have no idea how well this will or won't integrate with that. Anyway the design I've come up with so far looks something like this:

• You can't possibly store all the instances of a recurring event, at least not before they occur, so I simply have one table that stores the first instance of the event as an actual date, an optional expiration, and nullable repeat_unit and repeat_increment fields to describe the repetition. For single instances the repition fields are null, otherwise the units will be 'day', 'week', 'month', 'year' and increment is simply the multiple of units to add to start date for the next occurrence.
• Storing past events only seems advantageous if you need to establish relationships with other entities in your model, and even then it's not necessary to have an explicit "event instance" table in every case. If the other entities already have date/time "instance" data then a foreign key to the event (or join table for a many-to-many) would most likely be sufficient.
• To do "change this instance"/"change all future instances", I was planning on just duplicating the events and expiring the stale ones. So to change a single instances, you'd expire the old one at it's last occurrence, make a copy for the new, unique occurrence with the changes and without any repetition, and another copy of the original at the following occurrence that repeats into the future. Changing all future instances is similar, you would just expire the original and make a new copy with the changes and repition details.

The two problems I see with this design so far are:

1. It makes MWF-type events hard to represent. It's possible, but forces the user to create three separate events that repeat weekly on M,W,F individually, and any changes they want to make will have to be done on each one separately as well. These kind of events aren't particularly useful in my app, but it does leave a wart on the model that makes it less universal than I'd like.
2. By copying the events to make changes, you break the association between them, which could be useful in some scenarios (or, maybe it would just be occasionally problematic.) The event table could theoretically contain a "copied_from" id field to track where an event originated, but I haven't fully thought through how useful something like that would be. For one thing, parent/child hierarchical relationships are a pain to query from SQL, so the benefits would need to be pretty heavy to outweigh the cost for querying that data. You could use a nested-set instead, I suppose.

Lastly I think it's possible to compute events for a given timespan using straight SQL, but I haven't worked out the exact details and I think the queries usually end up being too cumbersome to be worthwhile. However for the sake of argument, you can use the following expression to compute the difference in months between the given month and year an event:

(:month + (:year * 12)) - (MONTH(occursOn) + (YEAR(occursOn) * 12))

Building on the last example, you could use MOD to determine whether difference in months is the correct multiple:

MOD(:month + (:year * 12)) - (MONTH(occursOn) + (YEAR(occursOn) * 12), repeatIncrement) = 0

Anyway this isn't perfect (it doesn't ignore expired events, doesn't factor in start / end times for the event, etc), so it's only meant as a motivating example. Generally speaking though I think most queries will end up being too complicated. You're probably better off querying for events that occur during a given range, or don't expire before the range, and computing the instances themselves in code rather than SQL. If you really want the database to do the processing then a stored procedure would probably make your life a lot easier.

Answer 3

The brute-force-ish but still reasonable way would be to create a new row in your single events table for every instance of the recurring event, all pointing not to the event preceding it in the series but to the first event in the series. This simplifies selecting and/or deleting all elements in a particular series, since you can select based on parent id. It also allows users to delete individual items from a series without affecting the rest of them.

This query gets you the series that starts on element 3:

SELECT * FROM events WHERE id = 3 OR parentid = 3

To get all items for this month, assuming you'd have a start date and an end date in your events table, all you'd have to do is:

SELECT * FROM events WHERE startdate >= '2008-08-01' AND enddate <= '2008-08-31'

Handling the creation/modification of series programmatically wouldn't be very difficult, but it really would depend on the feature set you want to provide and how you think it'll be used. If you want to differentiate between series and events, you could have a separate series table and a nullable series_id on your events, allowing you the freedom to muck about with individual events while still retaining control over your series.

Answer 4

Derek Park, I would be creating each and every instance of an event in a table, and this table would be regenerated every month (so any event that was set to reoccurr 'forever' would be regenerated one month in advance using a windows service or maybe at the sql server level). The polling won't only be done every 15 minutes, that might only be for polls related to email notifications. When someone wants to view their events for a month, I will have to fetech all their events, and re-occuring events and figure out which events to display (since a re-occuring event might have been created 6 months ago, but relates to a month the user is viewing).

Zack, i'm not too concerned with having a perfectly normalized database, the fact that I'm thinking of creating a secondary table is already breaking one of the rules hehe. My core database tables are following 'the rules', but I don't mind creating secondary tables/columns at times when it benefits things performance wise.

Answer 5

Attempting to store each instance of every event seems like it would be really problematic and, well, impossible. If someone creates an event that occurs "every thursday, forever", you clearly cannot store all the future events.

You could try to generate the future events on demand, and populate the future events only when necessary to display them or to send notification about them. However, if you are going to build the "on-demand" generation code anyway, why not use it all the time? Instead of pulling from the event table, and then having to use on-demand event generation to fill in any new events that haven't been added to the table yet, just use the on-demand event generation exclusively. The end result will be the same. With this scheme, you only need to store the start and end dates and the event frequency.

I don't see any way that you can avoid having on-demand event generation, so I can't see the utility in the event table. If you want it for the sake of caching, then I think you're taking the wrong approach. First, it's a poor cache because you can't avoid on-demand event generation anyway. Second, you should probably be caching at a higher level anyway. If you want to cache, then cache generated pages, not events.

As far as making your polling more efficient, if you are only polling every 15 minutes, and your database and/or server can't handle the load, then you are already doomed. There's no way your database will be able to handle users if it can't handle much, much more frequent polling without breaking a sweat.

Answer 6

I would say start with the ical standard. If you use it as your model, then you'll be able to do everything that google calendar, outlook, mac ical (the program), and get virtually instant integration with them.

From there, time to bone up on your ajax and javascript cuz you can't have a flashy web ui with drag drop and multiple calendars without a ton of ajax and javascript.