I am developing a library to edit contacts on a CardDAV Server and I wonder what is the proper way to sync contacts.
So when I find an etag for a specific contact changed: How do I sync both?
Do I just combine the changed data, e.g. phone numbers? Or must one side (Server or client) win? And how to I detect if a number changed or was added?
The Building a CardDAV client document explains all this very well.
But to address your questions:
So when I find an etag for a specific contact changed: How do I sync both?
You load the vCard from the server. Then it depends on the logic of your client. Do you want to auto merge? Do you want to prompt the user whether he wants to merge? Etc.
Usually you want to auto-merge. So do this. After you have the merged vCard, PUT that again to the server, but make sure to use the If-Match header to ensure that it didn't change again on the server side.
Do I just combine the changed data, e.g. phone numbers?
What you consider useful is entirely up to your application. But just combining fields may not be what you want. For example you wouldn't be able to detect deletes. So in most cases this is going to be a three-way merge:
Or must one side (Server or client) win?
Some clients do it like that, but this is not required. However, if you modify after a change, you need to be VERY careful with sync cycles!
And how to I detect if a number changed or was added?
You store the old copy you know and diff.
In general it is a good idea to store the (last known) opaque server copy locally and just pick out the fields your client cares about. Then when uploading the item again, you just patch the ones again. (and preserve the rest of what the server sent you).
Summary: A proper vCard diff and local cache is non-trivial. Many clients fail on that and loose or dupe user data. So unless you plan to put the necessary work and testing into this, an easier way is to detect the changes and ask the user what he wants to do (let server win, force user copy, merge).
