[caldeveloper-l] How to avoid time conflict or overlap for CalDAV?

[Helge Heß]

Hi Thomas,

the functionality Kim is asking for a very common one for business calendaring systems (not have the same person booked twice etc). So it would indeed by nice to be able to tell the server to guarantee the availability check for the client (so that you wouldn’t be able to schedule a meeting if there is an attendee conflict).

I think in the CalDAV world there are two parts to this:

a) First the client is supposed to perform a freebusy query to check
   whether a user is available. And then show a conflict warning or
   whatever seems appropriate.

This is how many systems, including btw Exchange work. Siri also does this kind of conflict detection (“hey, you already have an event at the time, shall I still create the conflicting one, master?”)

b) But in a reasonable system you actually need to guarantee that 
  the information isn’t outdated at PUT time. I.e. that no second 
  client has scheduled the same attendee/resource.

I think in CalDAV you can accomplish that by testing the sync-token or the ctag using an `If` header on the PUT. I.e. let the PUT only succeed if the whole underlying collection didn’t change. And if it did (the PUT will fail with a conflict), redo the freebusy, then try again.

I don’t think that there is a reliable way to do this in CalDAV cross collections (calendars), that is, if the availability of a resource changed because it got booked in a different calendar, the targeted sync collection won’t usually change its sync tag and the PUT would run through.
The bad thing about CalDAV is that PUTs are not idempotent. Otherwise you could do the PUT, recheck whether it still has no conflicts, and if so drop it after the fact.

hh

> On Nov 15, 2017, at 10:13 AM, Thomas Schäfer <thomas.schaefer at 1und1.de> wrote:
> 
> Hi,
>  
> There are no stupid questions.
>  
> The CalDAV Server will not have a conflict, as it is totally ok to have multiple events at the same time.
>  
> Let me try to explain this with an example:
>  
> Your Birthday might be the next week on Thursday, 23rd. So, there might be an all-day event in your calendar.
> In the US, next Thursday is Thanksgiving (bank holiday) as well. This might also end up as an all-day event in your calendar. Both are interesting to you and need to be there. They happen at the same time, but don’t necessarily imply a conflict.
>  
> That is from calendaring view point true for all events. Only you can determine, if events really conflict with each other and decide what to do. Availability information only help people trying to schedule events with you, that there are conflicting and not conflicting timeslots in your calendar, but this does not mean, that non-conflicting timeslots in your calendar are also good for you. Maybe you do not have all your obligations in one calendar (office vs. family) or others.
>  
> CalDAV server don’t answer that questions to you and store all events in your calendar and you as a person have to decide what to make out of it.
>  
> Does this help you?
> Thomas
>  
> From: caldeveloper-l <caldeveloper-l-bounces at lists.calconnect.org> on behalf of Won-Sik Kim <tgi01054 at gmail.com>
> Date: Tuesday, 14. November 2017 at 16:08
> To: "caldeveloper-l at lists.calconnect.org" <caldeveloper-l at lists.calconnect.org>
> Subject: [caldeveloper-l] How to avoid time conflict or overlap for CalDAV?
>  
> Hello. 
>  
> I read RFC 4791 for CalDAV.
> So I have question for time conflict or overlap between two event in same
> calendar.
>  
> Let me explain by instance. I made an event PM1 ~ PM6 in calendar. And
> then I try to made another event PM2~7 in same calendar. It is time
> conflict or overlap.
>  
> How does CalDav server resolve this conflict? Does server make error
> response when second event make?
>  
> Thanks for reading.
>  
> P.S. I am beginer for CalDAV. So my question may be stupid. Also my
> primary language is not English. Please understand two points.
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