CardDAV: What is Turba’s true data model?

Turba Addressbook imports, exports and syncs to and from many formats. I discussed all the formats in a previous article.

You can even customize the addressbooks, make them contain extra fields or omit common fields. Saving and loading data from the addressbook backend is in some way one more such conversion. Turba can actually read and write to SQL and LDAP backends, to the Preferences System and to an outdated version of the Kolab Groupware Server and IMSP Servers. It can derive Addressbooks from Horde Groups, Favourite Recipients (usually from IMP) and stored search queries (virtual addressbooks).

This diversity of backends and configuration options comes at a cost. It is sometimes not clear what is the data model and who owns the data. This complicates synchronisation scenarios like CardDAV.

Let’s look at this in detail. LDAP backends might be considered the owner of data and Turba would be just a view to that data or a client with edit permissions. LDAP would be able to change data, add new contacts, delete contacts or change contact details without turba even knowing. This is less of a problem with the SQL driver as the SQL schema of Turba should never be consumed directly, rather through turba’s API or exchange formats. Groups are a primary culprit of not telling Turba any changes.

On the other hand, sync protocols may contain data the current turba configuration or backend does not recognize. CardDAV and its data format vCard may contain a wealth of properties and allow custom extensions fields. CardDAV servers are supposed not to simply drop/forget fields they do not recognize because that could make the client forget fields in the data it actually supports. On the other hand, a client may send an update of a contact missing a property which is stored in a previous version of the contact. We need to understand if the client simply cannot handle that property or if it is supposed to remove that property. Well-behaved clients should not strip properties they do not understand and well-behaved servers should not do that either.

Turba may support contact information which cannot be mapped to CardDAV/vCard but I am currently unaware of any. As vCard allows custom fields, at the moment I consider vCard the lingua franca.

Another tough scenario are references to other contacts or resources. CardDAV/vCard support multiple ways to indicate these and the backend may support a completely different model of referencing. How should we react on references to contacts which we do not (yet) have? Many of these problems also affect CalDAV sync in the kronolith calendar and nag tasks app.

I am ignoring ActiveSync and SyncML because I do no longer possess any syncml capable device and I have limited understanding of the ActiveSync code. It seems to have a static mapping between turba internal field names and ActiveSync field names.

To resolve all these issues, Turba needs to have its own data model and it needs to store synced data its backend is not interested in. It must not lose or forget any information when syncing back to the client. This is currently not the case.

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