On calendar servers and caldav compatibility

Links and contact info

calendar-cli
https://github.com/tobixen/calendar-cli

python caldav client library
https://github.com/python-caldav/caldav

Tobias Brox
tobias@redpill-linpro.com

Redpill Linpro
https://www.redpill-linpro.com

http://calendarfest.tobixen.no/presentation.html

Exception handling

vobject
Bomb

icalendar
Thrashbin

Ironing icalendar data

(Steam iron photo by Lombroso, CC BY-SA 4.0)

s/COMPLETED:(\d+)\s', 'COMPLETED:\g<1>T120000Z'/;
s/CREATED:00001231T000000Z/CREATED:19700101T000000Z/;
s/(DTSTAMP:[^\n]+\n.*\n)DTSTAMP:[^\n]+\n/\1/;
s/ *$//;
s/\\+('\")/\1/;

Most normal caldav incompatibilities

RFC4791: "Support for the CALDAV:free-busy-query REPORT is REQUIRED."
RFC6833 not supported
Recurring events - search and expandation
Date search for tasks
Journal support

That’s all trivial mendable problems with the icalendar representation. The problems with the CalDAV implementations are less trivial to mend. Particularly those problems are recurring over many server implementations …

No support for the freebusy report. Note that there are two kinds of "freebusy" requests, the RFF6833 specifies another way to do it, but unfortunately …
Few calendar servers supports the RFC 6833 scheduling extension fully - even servers that support scheduling and iTIP often don’t support it over the caldav standard.
Search for recurring events with expansion of recurring events - hard to implement, even Google gets it wrong.
Date search for tasks - now, this is a bit exotic, and what does one expect to get from such a date search? Should events without DTSTART be included? How should tasks without DURATION nor DUE be handled? The RFC is not crystal clear on those questions … and then there are recurring tasks as well …
Journal support. Does anyone use journals? I don’t … but why not, I think it makes sense to store historical data in a calendar. It shouldn’t be hard to implement, but it’s probably not at the top of the priority list.

Table 1. Some compatibility flags
Flag	Description	Servers
no expand	Server may throw errors when asked to do a expanded date search	sogo, xandikos
no recurring	Server is having issues with recurring events/todos, like recurrances not found in date search	bedework, icloud, xandikos

Table 2. Some more compatibility flags
no recurring expandation	Server will not expand recurring events	google, radicale
no scheduling	RFC6833 is not supported	radicale, xandikos, more
non existing calendar found	Server will not yield a 404 on a random calendar URL (perhaps the calendar will be automatically created on access)	radicale, zimbra

Table 3. Even more compatibility flags
no freebusy rfc4791	Server does not support a freebusy-request as per RFC4791	icloud, radicale, sogo
no journal	Server does not support journal entries	bedework, davical, icloud, sogo, zimbra
duplicate in other calendar with same uid is lost	Fetch an event from one calendar, save it to another … and the duplicate will be ignored	bedework, zimbra

Table 4. There are too many compatibility flags
propfind allprop failure	The propfind test fails … possibly the test code is wrong	bedework, icloud
no todo	Support for VTODO (tasks) apparently missing	bedework, google, icloud
vtodo datesearch nodtstart task is skipped	date searches for todo-items will not find tasks without a dtstart	davical, sogo

There are really too many compatibility flags - I’ve maybe shown half of them.

Table 5. Support of non-standard stuff
calendar order	Server supports (nonstandard) calendar ordering property	radicale, zimbra, baikal, more?
calendar color	Server supports (nonstandard) calendar color property	radicale, zimbra, baikal, more?

Servers tested

Xandikos
Radicale
Zimbra
DAViCal
iCloud
Google
SOGo
Baikal
Bedework

Servers that I haven’t tested

NextCloud
Microsoft Exchange
Office 365
Others?

Xandikos

Version 0.2.3:

RFC6833 is not supported
Server may throw errors when asked to do a expanded date search
Server is having issues with recurring events and/or todos. date searches covering recurrances may yield no results, and events/todos may not be expanded with recurrances

The two latter should be solved in higher versions, but higher versions freezes when I’m running tests. I believe it’s a bug in Xandikos.

Radicale

The given user starts without an assigned default calendar - without pre-defined calendars at all
Server will automatically create calendars on access
Server does not support a freebusy-request as per RFC4791
Server will not expand recurring events
RFC6833 is not supported
Server supports (nonstandard) calendar ordering property
Server supports (nonstandard) calendar color property

Zimbra

Server will not yield a 404 when accessing a random calendar URL
Server does not support journal entries
The display name of a calendar cannot be set/changed (in zimbra, display name is given from the URL)
Fetch an event from one calendar, save it to another … and the duplicate will be ignored
Server will not accept calendar object resources without an UID
Tasklists can be created, but a normal calendar does not support tasks

(… to be continued)

A GET towards a valid calendar object resource URL will yield 404
RFC6578 is not supported, things will break if we try to do a sync-token report
Recurring events are supported, but not recurring todos
Server supports (nonstandard) calendar ordering property
Server supports (nonstandard) calendar color property

Baïkal

Date search on todo items fails
Server supports (nonstandard) calendar ordering property
Server supports (nonstandard) calendar color property

(Disclaimer: hasn’t been tested for a while)

DAViCal

Date searches for todo-items will not find tasks without a dtstart
Issues with simple sync requests?
Server does not support journal entries

Google

See https://developers.google.com/calendar/caldav/v2/guide

mkcalendar is not supported
events cannot be edited
Support for VTODO (tasks) apparently missing
Server will not expand recurring events

iCloud

Needs a new and unique calendar ID for every test execution
Don’t allow copying from one calendar to another
Deleting a calendar does not delete the events on the calendar
Server does not support journal entries
Support for VTODO (tasks) apparently missing
Server does not support a freebusy-request as per RFC4791
Server is having issues with recurring events and/or todos
The propfind test fails … possibly the test code is wrong
calendar.object_by_uid(uid) does not work
… and more

Bedework

Server does not support journal entries
Support for VTODO (tasks) apparently missing
The propfind test fails … possibly the test code is wrong
Server is having issues with recurring events and/or todos.
Duplication of an event in the same calendar not allowed (even with different uid)
Fetch an event from one calendar, save it to another … and the duplicate will be ignored

SOGo

Server does not support journal entries
Server does not support a freebusy-request as per RFC4791
The sync token is typically a time stamp, and we need to sleep a second in the test code to get things right
Server may throw errors when asked to do a expanded date search
date searches for todo-items will not find tasks without a dtstart

Help wanted

I need servers to test against!
With a valid username and password
or preferably three

Links and contact info

calendar-cli
https://github.com/tobixen/calendar-cli

python caldav client library
https://github.com/python-caldav/caldav

Tobias Brox
tobias@redpill-linpro.com

Redpill Linpro
https://www.redpill-linpro.com

http://calendarfest.tobixen.no/presentation.html