Publish CalDav events for site members and manage participants

Question

I'm running PHP site with a user database and a shared calendar (events) moderated database.

I want to publish these events in iCal/CalDAV so that members can:

• Get all calendar events, in sync, with authentication (private URL...)
• Join an event
• (But not create or modify an event)

The app shall leverage the site existing user & calendar database.

I've looked at the CalDav implementations in PHP & Python, they all seem too complicate for this simple usage:

• sabre/dav has experimental shared calendar support, and leveraging the existing user database seems tricky.
• DAViCal seems to have its own user database, complex ACL schemes, etc.
• Etc.

How to do that?

• Use an CalDAV/iCal library that is adapted? Any idea of a library that would do the job?
• Write my own CalDAV/iCal server, with the risk of poor client support.

EDIT: Useful links to use sabre/dav with an existing website database:

• Using external databases

EDIT: Improvements:

• Although it works as expected, sabre/DAV is way too slow in creating a server-side event with invites for ~100 local people ("principals"). And it generates 99x useless copies of the event for each invitee. It took like 8 seconds to process the invites (30+ seconds with xdebug profiling on and it crashed it!).

• Regarding using a shared calendar, there's only two possibilities in the standard: either give the invitee read-write acces, so he can join the event (but also change/delete it!) or give him read-only access... but he can't join it. So I've played with Sabre\CalDAV\Backend\PDO::updateCalendarObject() to restrict event edits to the participant PARTSTAT.

But whatever the solution, there's an issue with conflicts:

• When a participant A modifies a calendar entry saying it participates (PARTSTAT ACCEPTED), Sabre/DAV processes this information by updating all calendar objects, in all user calendars. When doing so, SabreDAV increments the SYNC token of all these calendars.

• Therefore, when a participant B tries to join an event, but has not synchronized the information that participant A joined the event in the meantime, there's a conflict and participant B join information is LOST.

Conflict solution:

• RFC 6638 defines a way to avoid such conflicts by using a schedule-tag. This is only in Sabre/DAV roadmap for v3.3. I've developed experimental support for sabre/dav here but still, all Android clients I've tested don't support this RFC, only iOS does! Anyway, even with schedule-tag, if the organizer changes the event data or when using shared calendars, some participant join information will be lost.

• So I've made my own sabre/dav implementation to mostly never loose participant join information (except when the event is reschedule): sabre/dav partstat-only-calendar. Basically, it works by into account the conflicting EVENT sent by the user and records only the participant PARSTAT info in the server event, instead of normally pushing a PreconditionFailed exception. I still need to publish an exemple of server run script that leverage all the functionalities.

Answer 1

(The question seems a little broad for StackOverflow.)

SabreDAV seems to be the choice when implementing a Cal/CardDAV server in PHP. It very likely makes sense to use it.

a) Get all calendar events, in sync, with authentication (private URL...)

Well, that should be covered.

b) Join an event

I assume your use case is that you have an event, but the user is not proactively invited to it. But you still want the user user to 'add' himself to the event. This is not a use case the CalDAV standard currently supports. There are shared calendars in CalDAV and there are server side scheduled events. I think the latter might work for you.

You could synthesize this 'add-me' behavior. Dynamically add the user as a participant to the event when presenting it to a client. He can then Accept/Maybe/Deny it. If he does, persist that fact in the master event - he is now a regular invitee.

c) (But not create or modify an event)

This is the default behavior for iTIP/CalDAV server scheduled events. Only the ORGANIZER can modify events except for stuff like alarms etc. So that seems to fit your use case.