[horde] Database cleanup horde_histories

Michael J Rubinsky mrubinsk at horde.org
Thu May 2 13:28:39 UTC 2019

Quoting Jens Wahnes <wahnes at uni-koeln.de>:

> Michael J Rubinsky wrote:
>> The history data doesn't really have anything to do with the  
>> *display* of entries. If the entry in question still exists in the  
>> application's backend storage, the entry will display fine. In  
>> general, the history data is used for synchronization of things  
>> like ActiveSync clients on a mobile device, or *DAV clients. If you  
>> don't care about synchronization it is generally safe to delete  
>> this information. Even if you do care about synchronization, it  
>> would generally be safe to delete REALLY old entries - basically as  
>> long as any clients you care about have synchronized since the  
>> history event date, you should be fine.
> Thank you for explaining.
> I think the trouble really comes in when there are in fact entries  
> of the kind I mentioned previously (e.g. monthly meeting entered  
> into a group calendar by someone who has long left the workgroup).  
> When the history records are incomplete, and there are members  
> joining and leaving the workgroup all the time, there will be issues  
> with syncing these recurring events (e.g. to mobile devices with  
> reminders supposed to be popping up there).

I'm curious what kind of issues with syncing you see in this  
situation. With regards to syncing, the history data is ONLY used to  
detect if an entry has changed between two sync points. The actual  
data for the event entry is stored in Kronolith, outside of the  
history system. As long as the user has enough permissions to  
synchronize the shared calendar, there should be no issue. When a new  
user is given permissions to a calendar to sync, the complete calendar  
is synchronized at that time, and the "sync point" of that state is  
saved in association with that sync client. This is greatly  
simplified, but basically during sync the history system is consulted  
to see if any entries have been added for that calendar since the last  
sync-point the client knows about and the current sync-point.   
Anything outside of this window of time is irrelevant for this user.  
So, basically, you can delete any history entries that are older than  
the oldest sync-point a sync client is associated with.

>> Note that history data might have auditing value in your  
>> organization, so that is also something to keep in mind as well.
> We're pretty much in the opposite situation. We must not keep  
> records of personal data according to GDPR statutes, at least when  
> these individuals are no longer affiliated with our organization.  
> It's hard to meet these different goals at the same time, but still  
> we strive to do our best -- any help how to improve this appreciated.
> Jens
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