Wednesday, August 11, 2004

If this background material is familiar to you, scroll to the bottom for the call to arms.

I've been following calendaring standards for six years now. In that time, iCalendar has become a fairly widely accepted standard for expressing events, todos and other calendar information in a MIME text document. Many calendaring applications can import iCalendar files, or export to them, or generate an email with an invitation formatted as an iCalendar document. However, there are some problems with iCalendar's complexity, particularly in expressing recurrances, and the companion standards for conveying iCalendar files (iMIP and iTIP) have their own problems.

Calendar interoperability testing has happened sporadically in these years. The first calendar interoperability event was held in 2000, a virtual event in 2002 and another in-person event this summer at Berkeley, organized by the newly revitalized CalConnect consortium. Still, interoperability hasn't improved as much as we'd like because at some point you need to go back and fix the standard.

Also during these six years, the CalSch working group has worked and bickered over the proposal to standardize how clients access a calendar stored on a server -- this protocol, CAP, would be to calendars what IMAP is to email folders. I've never liked the design of CAP, down to the very core model of how it works. So six months ago I made a counter-proposal, CalDAV, and threw it out there as an internet-draft. Finally I'm getting more than just nibbles on CalDAV, in fact several groups have told me their intent to implement CalDAV starting this year. And at the same time, other folks are getting invigorated about revising iCalendar and bringing it to draft standard.

This is all great news. Calendaring interoperability has languished except for that burst of productivity back in 1998. People are locked into one calendar application depending on what server technology they have available, since there's no common calendar access standard. Invitations work, kinda, but in practice the problems with recurrances mean that people must follow up the machine-readable text with human-readable text in case a mistake was made between two different vendors' software.

Good news, but nowhere near done yet -- this is just the beginning. Now we need volunteers. We need people to write specification text, review text, and manage the issues. We need people simply to pay attention to the work being done and provide their experience, or simply opinions, to bring issues to resolution.

Here's where to start -- two new mailing lists, one to take iCalendar to draft standard, one to develop and standardize CalDAV. Tell your friends! Subscribe to one, subscribe to both! We've got good work to do and good people to do it but we need your help.


Anonymous said...

I'm very excited to see the interest in CalDAV, as I share your enthusiasm for a calendar protocol standard that can actually be used in the real world. Thanks for all your work on this!

Mike Shaver
(rushing off to join the caldav list)

Unknown said...

Have you talked w/the folks over at the Center for Document Engineering at Berkeley? In particular Allison Bloodworth is heading up the "Berkeley Campus-Wide Event Calendar," which should be of interest. (
-Scott McMullan

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